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| #acl GreyCat:read,write,revert,admin,delete Lhunath:read,write,revert pgas:read,write,revert All:read [[BashGuide/CommandsAndArguments|Commands and Arguments ->]] ---- |
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'''You are invited to make additions or modifications so long as you can keep them accurate. Please test any code samples you write.''' |
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| ["BASH"] is a BourneShell compatible shell, which adds many new features to its ancestor. Most of them are available in the 'KornShell', too. | -------- <<Anchor(About)>> = About This Guide = |
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| [[TableOfContents]] | This guide aims to aid people interested in learning to work with [[BASH]]. It aspires to teach good practice techniques for using, and to educate them about the internal operation of [[BASH]]. |
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| ----- | This guide is targeted at beginning users. It assumes no advanced knowledge -- just the ability to login to a Unix-like system and open a command-line (terminal) interface. It will help if you know how to use a text editor; we will not be covering editors, nor do we endorse any particular editor choice. Familiarity with the fundamental Unix tool set, or with other programming languages or programming concepts, is not required, but those who have such knowledge may understand some of the examples more quickly. |
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[[Anchor(about)]] == About This Guide == This guide aims to become a point of reference for people interested in learning to work with ["BASH"]. It aspires to teach its readers good practice techniques in developing scripts for the ["BASH"] interpreter and educate them about the internal operation of ["BASH"]. This guide is targetted at beginning users. It assumes no basic knowledge, but rather expects you to have enough common sense to put two and two together. If something is unclear to you, you should report this so that it may be clearified in this document for future readers. |
If something is unclear to you, you are invited to report this (use BashGuideFeedback, or the `#bash` channel on `irc.freenode.org`) so that it may be clarified in this document for future readers. |
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| The primary maintainer(s) of this document: * -- [[Lhunath]] (primary author) * -- GreyCat The guide is also available in [[http://stuff.lhunath.com/BashGuide.pdf|PDF format]]. Alternatively, you can just hit print after going to [[FullBashGuide]]. That guarantees you'll be printing the latest version of this document. -------- <<Anchor(Definition)>> = A Definition = [[BASH]] is an acronym for '''''B'''ourne '''A'''gain '''Sh'''ell''. It is based on the ''Bourne'' shell and is mostly compatible with its features. Shells are command interpreters. They are applications that provide users with the ability to give commands to their operating system interactively, or to execute batches of commands quickly. In no way are they required for the execution of programs; they are merely a layer between system function calls and the user. Think of a shell as a way for you to speak to your system. Your system doesn't need it for most of its work, but it is an excellent interface between you and what your system can offer. It allows you to perform basic math, run basic tests and execute applications. More importantly, it allows you to combine these operations and connect applications to each other to perform complex and automated tasks. [[BASH]] is '''not''' your operating system. It is not your window manager. It is not your terminal (but it oftens runs ''inside'' your terminal). It does not control your mouse or keyboard. It does not configure your system, activate your screensaver, or open your files when you double-click them. It is generally not involved in launching applications from your window manager or desktop environment. It's important to understand that [[BASH]] is only an interface for you to execute statements (using [[BASH]] syntax), either at the interactive [[BASH]] prompt or via [[BASH]] scripts. |
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| [[Anchor(definition)]] == A Definition == ["BASH"] is an acronym for '''''B'''ourne '''A'''gain '''Sh'''ell''. It is based on the ''Bourne'' shell and is mostly compatible with its features. Shells are applications that provide users with the ability to give commands to their operating system interactively, or to allow them to execute batch processes quickly. In no way are they required for execution of processes, they are merely a layer between system function calls and the user. |
-------- . '''In The Manual: [[http://www.gnu.org/software/bash/manual/bashref.html#Introduction|Introduction]]''' ---- . ''Shell'': A (possibly interactive) command interpreter, acting as a layer between the user and the system. <<BR>> ''[[BASH]]'': The Bourne Again Shell, a ''Bourne'' compatible shell. |
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| '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC1 Introduction]''' -------- |
<<Anchor(Using_Bash)>> = Using Bash = |
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| Most users that think of [[BASH]] think of it as a prompt and a command line. That is [[BASH]] in ''interactive mode''. [[BASH]] can also run in ''non-interactive mode'', as when executing scripts. We can use scripts to automate certain logic. Scripts are basically lists of commands (just like the ones you can type on the command line), but stored in a file. When a script is executed, all these commands are (generally) executed sequentially, one after another. | |
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| We'll start with the basics in an ''interactive shell''. Once you're familiar with those, you can put them together in scripts. | |
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| [[Anchor(using)]] == Using Bash == Most users that think of ["BASH"] think of it as a prompt and a commandline. That is ["BASH"] in ''interactive mode''. ["BASH"] can also run in ''non-interactive mode'' through scripts. We can use scripts to automate certain logic. Scripts are basically lists of commands that you can type on the commandline. When such a script is executed, all these commands are executed sequentially; one after another. We'll start with the basics in an ''interactive shell''. Once you're familiar with those, you can put them together in scripts. ----- [[Anchor(basics)]] = The Basics = [[Anchor(commands)]] == Commands And Arguments == ["BASH"] takes commands on the commandline. Commands can be different things. They can be application executables, aliases, function names, etc. * '''Application Executables''': ["BASH"] keeps a variable that tells it where to find the executables for certain applications. This variable is called `PATH`, and it usually contains `/bin:/usr/bin`. This is a string of pathnames separated by colons. Each path can contain executables. When a command is specified in ["BASH"] without a pathname (e.g. `ls`), ["BASH"] searches these paths for the executable for this command. * '''Aliases''': ["BASH"] can use aliases to make it easier to quickly execute complex commands. An alias is a ''name'' that is mapped to a certain ''string''. Whenever that ''name'' is used as a command in bash, it is replaced by the ''string''. * '''Functions''': Functions in ["BASH"] are much like aliases. When a command is executed by the name of a function, the code of that function is executed instead. Each command can be followed by arguments. It is very important that you understand how this works exactly. If you don't grasp these concepts well, the quality of your code will degrade significantly and you will introduce very dangerous bugs. So, pay close attention in the next few chapters. |
'''Important! <<BR>> You should make yourself familiar with the `man` and `apropos` commands on the shell. They will be vital to your self-tutoring.''' |
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| $ ls a b c |
$ man man $ man apropos |
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| `ls` is a command that lists files in the current directory. | In this guide, the `$` at the beginning of a line represents your [[BASH]] prompt. Traditionally, a shell prompt either ends with `$`, `%` or `#`. If it ends with `$`, this indicates a shell that's compatible with the Bourne shell (such as a POSIX shell, or a Korn shell, or [[BASH]]). If it ends with `%`, this indicates a ''C shell'' (csh or tcsh); this guide does not cover C shell. If it ends with `#`, this indicates that the shell is running as the system's superuser account (`root`), and that you should be extra careful. Your actual [[BASH]] prompt will probably be much longer than `$`. Prompts are often highly individualized. The `man` command stands for "manual"; it opens documentation (so-called "man pages") on various topics. You use it by running the command `man [topic]` at the [[BASH]] prompt, where `[topic]` is the name of the "page" you wish to read. Note that many of these "pages" are considerably longer than one printed page; nevertheless, the name persists. Each command (application) on your system is likely to have a man page. There are pages for other things too, such as system calls or specific configuration files. In this guide, we will only be covering commands. Note that if you're looking for information on [[BASH]] built-ins (commands provided by [[BASH]], not by external applications) you should look in `man bash` instead. [[BASH]]'s manual is extensive and detailed. It is an excellent reference, albeit more technical than this guide. [[BASH]] also offers a `help` command which contains brief summaries of its built-in commands (which we'll discuss in depth later on). |
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| $ mkdir d $ cd d $ ls }}} `mkdir` is a command that creates a new directory. We specified the argument `d` to that command. This way, the application `mkdir` is instructed to create a directory called `d`. After that, we use the application `cd` to change the current directory to `d`. `ls` shows us that the current directory (which is now `d`) is empty, since it doesn't display any filenames. -------- '''Tip: [[BR]] You can use the `type` command to figure out the type of a command. [[BR]] For example:''' {{{ $ type rm rm is hashed (/bin/rm) $ type cd cd is a shell builtin }}} ---- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC16 Simple Commands]''' -------- [[Anchor(splitting)]] == Commandline Argument Splitting == Commands in ["BASH"] can take multiple arguments. These arguments are used to tell the command exactly what it's supposed to do. In ["BASH"], you separate these arguments by whitespace (spaces, tabs and newlines). {{{ $ ls $ touch a b c $ ls a b c }}} `touch` is an application that changes the 'Last Modified'-time of a certain file to the current time. If the filename that it's given does not exist yet, it simply creates that file, as a new and empty file. In this example, we passed three arguments. `touch` creates a file for each argument. `ls` shows us that three files have been created. {{{ $ rm * $ ls $ touch a b c $ ls a b c }}} `rm` is an application that removes all the files that it was given. ''*'' is a ''glob''. It basically means ''all files in the current directory''. You will read more about this later on. Now, did you notice that there are several spaces between `a` and `b`, and only one between `b` and `c`? Also, notice that the files that were created by `touch` are no different than the first time. You now know that the amount of whitespace between arguments does not matter. This is important to know. For example: {{{ $ echo This is a test. This is a test. $ echo This is a test. This is a test. }}} In this case, we provide the `echo` command with four arguments. 'This', 'is', 'a' and 'test.'. `echo` takes these arguments, and prints them out one by one with a space in between. In the second case, the exact same thing happens. The extra spaces make no difference. To protect the whitespace properly, we need to pass the sentence as one single argument. We can do this by using quotes: {{{ $ echo "This is a test." This is a test. }}} Quotes group everything together and pass it as a single argument. This argument is 'This is a test.', properly spaced. `echo` prints this single argument out just like it always does. Be very careful to avoid the following: {{{ $ ls The secret voice in your head.mp3 secret $ rm The secret voice in your head.mp3 rm: cannot remove `The': No such file or directory rm: cannot remove `voice': No such file or directory rm: cannot remove `in': No such file or directory rm: cannot remove `your': No such file or directory rm: cannot remove `head.mp3': No such file or directory $ ls The secret voice in your head.mp3 }}} You need to make sure you quote filenames properly. If you don't you'll end up deleting the wrong things! `rm` takes filenames as arguments. If you do not quote filenames with spaces, `rm` thinks that each argument is another file. Since ["BASH"] splits your arguments at the spaces, `rm` will try to remove each word. Please have a good look at http://bash-hackers.org/wiki/doku.php?id=syntax:words if all this isn't very clear to you yet. -------- '''Good Practice: [[BR]] You should ''always'' quote sentences or strings that belong together; even if it's not absolutely necessary. This will keep you alert and reduce the risk of human error in your scripts. [[BR]] For example, you should always quote arguments to the `echo` command.''' -------- [[Anchor(globs)]] == Globs == Globs are a very important concept in ["BASH"], if only for their incredible convenience. Properly understanding globs will benefit you in many ways. Globs are basically patterns that can be used to match filenames or other strings. Globs are composed of normal characters and meta characters. Meta characters are characters that have a special meaning. These are the basic meta characters: * '''*''': Matches any string, including the null string. * '''?''': Matches any single character. * '''[...]''': Matches any one of the enclosed characters. Here's an example of how we can use glob patterns to expand to filenames: {{{ $ ls a abc b c $ echo * a abc b c $ echo a* a abc }}} ["BASH"] sees the glob, for example `a*`. It ''expands'' this glob, by looking in the current directory and matching it against all files there. Any filenames that match the glob, are enumerated and replaced by the glob. As a result, the statement `echo a*` is replaced by the statement `echo a abc`, and is then executed. ["BASH"] will always make sure that whitespace and special characters are escaped properly when expanding the glob. For example: {{{ $ touch "a b.txt" $ ls a b.txt $ rm * $ ls }}} Here, `rm *` is expanded into `rm a\ b.txt`. This makes sure that the string `a b.txt` is passed as a single argument to `rm`, since it represents a single file. It is important to understand that using globs to enumerate files is nearly '''always''' a better idea than using `ls` for that purpose. Here's an example with some more complex syntax which we will cover later on, but it will illustrate the problem very well: {{{ $ ls a b.txt $ for file in `ls`; do rm "$file"; done rm: cannot remove `a': No such file or directory rm: cannot remove `b.txt': No such file or directory $ for file in *; do rm "$file"; done $ ls }}} Here we use the `for` command to go through the output of the `ls` command. The `ls` command results in a string `a b.txt`. The `for` command splits that string into arguments over which it iterates. As a result, for iterates over `a` and `b.txt`. Naturally, this is '''not''' what we want. The glob however expands in the proper form. It results in the string `a\ b.txt`, which `for` takes as a single argument. ["BASH"] also supports a feature called `Extended Globs`. These globs are more powerful in nature. This feature is turned off by default, but can be turned on with the `shopt` command, which is used to toggle '''sh'''ell '''opt'''ions: {{{ $ shopt -s extglob }}} * '''?(list)''': Matches zero or one occurrence of the given patterns. * '''*(list)''': Matches zero or more occurrences of the given patterns. * '''+(list)''': Matches one or more occurrences of the given patterns. * '''@(list)''': Matches one of the given patterns. * '''!(list)''': Matches anything except one of the given patterns. The list inside the parentheses is a list of globs separated by the `|` character. Here's an example: {{{ $ ls names.txt tokyo.jpg california.bmp $ echo !(*jpg|*bmp) names.txt }}} Our glob now expands to anything that does not match the `*jpg` or the `*bmp` pattern. Only the text file passes for that, so it is expanded. Then, there is Brace Expansion. Brace Expansion technically does not fit in the category of Globs, but it is similar. Globs only expand to actual filenames, where brace expansion will expand to any permutation of the pattern. Here's how they work: {{{ $ echo th{e,a}n then than $ echo {/home/*,/root}/.*profile /home/axxo/.bash_profile /home/lhunath/.profile /root/.bash_profile /root/.profile $ echo {1..9} 1 2 3 4 5 6 7 8 9 $ echo {0,1}{0..9} 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 |
$ help $ help read |
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| '''Good Practice: [[BR]] You should always use globs in favor of `ls` (or similar) to enumerate files. Globs will always expand safely and minimize the risk for bugs. [[BR]] You can sometimes end up with some very weird filenames. Generally speaking, scripts aren't always tested against all the odd cases that it may end up being used with.''' |
. '''In the FAQ: <<BR>> [[BashFAQ/061|Is there a list of which features were added to specific releases (versions) of Bash?]]''' |
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| '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC35 Pattern Matching]''' |
. ''Interactive mode'': A mode of operation where a prompt asks you for one command at a time. . ''Script'': A file that contains a sequence of commands to execute one after the other. |
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| = Contents = | |
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| The guide has been divided into sections, which are intended to be read roughly in the order presented. If you skip ahead to a specific section, you might find yourself missing some background information from previous sections. (Links to relevant sections are not always provided when a topic is mentioned.) | |
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| [[Anchor(characters)]] == Special Characters == |
* [[/CommandsAndArguments|Commands and Arguments]] * Types of commands; argument splitting; writing scripts. * [[/SpecialCharacters|Special Characters]] * [[/Parameters|Parameters]] * Variables; special parameters; parameter types; parameter expansion. * [[/Patterns|Patterns]] * Globs; filename matching; extended globs; brace expansion; regular expressions. * [[/TestsAndConditionals|Tests and Conditionals]] * Exit status; {{{&&}}} and {{{||}}}; if, test and {{{[[}}}; while, until and for; case and select. * [[/Arrays|Arrays]] * Arrays; associative arrays. * [[/InputAndOutput|Input and Output]] * Redirection; here documents; here strings; pipes; process substitution. * [[/CompoundCommands|Compound Commands]] * Subshells; command grouping; arithmetic evaluation; functions; aliases. * [[/Sourcing|Sourcing]] * Reading commands from other files. * [[/JobControl|Job Control]] * [[/Practices|Practices]] * Choosing your shell; quoting; readability; debugging. |
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| There are several special characters in ["BASH"] that have a non-literal meaning. When we use these characters, ["BASH"] evaluates these characters and their meaning, but usually does not pass them on to the underlying commands. Here are a few of those special characters, and what they do: * '''"text"''': Double quotes. Double quotes protect the text inside from being split into multiple words or arguments. They also prevent the special meaning of single quotes inside. * ''''text'''': Single quotes. Single quotes protect the text inside from any kind of expansion by the shell and keeps it from being split into multiple words or arguments. They also prevent the special meaning of all special characters inside. * '''# text''': Comment character. Any text that follows until the first newline is not processed as shell commands or arguments. * ''';''': Command separator. The colon is used to separate multiple commands from each other if the user chooses to keep them on the same line. It's ''basically'' the same thing as a newline. * '''\''': Escape character. The escape character protects the next character from being used in any special sort of way. * '''>''' or '''<''': Redirection character. These characters are used to modify (redirect) the input and/or output of a command. * '''[[ expression ]]''': Test expression. This evaluates the conditional expression. * '''{ commands; }''': Command Group. This executes the commands inside the braces as though they were only one command. It is convenient for places where ["BASH"] syntax requires only one command to be present. * '''`command`''', '''$(command)''': Command substitution (The latter form is '''highly''' preferred). Command substitution executes the command inside the substitution form first, and replaces itself by that command's output. * '''(command)''': Subshell Execution. This executes the command in a new bash shell, instead of in the current. * '''((expression))''': Arithmetic Evaluation. Inside the parentheses, operators such as +, -, * and / are seen as mathematical operators. * '''$((expression))''': Arithmetic Expansion. Comparable to the above, however this expression is replaced the result of its arithmetic evaluation. * '''$''': Expansion character. This character is used for any form of parameter expansion. More about this later. Some examples: {{{ $ echo "I am $USER" I am lhunath $ echo 'I am $USER' I am $USER $ # boo $ echo An open\ \ \ space An open space $ echo "My computer is $(hostname)" My computer is Lyndir $ echo boo > file $ echo $(( 5 + 5 )) 10 $ (( 5 > 0 )) && echo "Five is bigger than zero." Five is bigger than zero. }}} -------- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC6 Shell Syntax]''' -------- [[Anchor(parameters)]] == Parameters and Variables == Parameters should be seen as a sort of named space in memory where you can store your data. Generally speaking, they will store string data, but can also be used to store integers or arrays. Let's get your vocabulary straight before we get into the real deal. There are parameters and variables. Variables are actually just a kind of parameters. Parameters that are denoted by a name. I'm sure you'll understand things better with a few examples: {{{ $ # Some parameters that aren't variables: $ echo My shell is $0, and was started with these options: $- My shell is -bash, and was started with these options: himB $ # Some parameters that ARE variables: $ echo I am $USER, and I live at $HOME. I am lhunath, and I live at /home/lhunath. }}} '''Please note: Unlike PHP/Perl/... parameters do NOT start with a $-sign. The $-sign you see in the examples merely causes the parameter that follows it to be ''expanded''. Expansion basically means that the shell replaces it by its content. As such, `USER` is the parameter (variable), that contains your username. `$USER` will be replaced with its content; which in my case, is `lhunath`.''' I think you've got the drift now. Here's a summary of most non-variable parameters: * '''Positional Parameters''': 0, 1, 2, ...; They contain the n'th argument that was passed to the current script. * '''Special Parameters''': * '''*''': Expands to a string enumerating all positional parameters. * '''@''': Depending on the context in which it is used, expands to either a string enumerating all positional parameters, or several strings; one for each parameter. * '''#''': Expands to the amount of positional parameters that are currently set. * '''?''': Expands to the exit code of the most recently completed foreground application. * '''$''': Expands to the ["PID"] of the current shell. * '''!''': Expands to the ["PID"] of the application most recently executed in the background. * '''_''': Expands to the last argument of the last command that was executed. And here are some examples of variables that the shell initializes for you: * '''BASH_VERSION''': Contains a string describing the version of ["BASH"]. * '''HOSTNAME''': Contains the hostname of your computer, I swear. * '''PPID''': Contains the ["PID"] of the process that started this shell. * '''PWD''': Contains the current directory. * '''RANDOM''': Each time you expand this variable, a random number between 0 and 32767 is generated. * '''UID''': The integer ID of the current user. * '''COLUMNS''': The amount of characters fit on one line in your terminal. (The width of your terminal in characters.) * '''LINES''': The amount of lines that fit in your terminal. (The height of your terminal in lines.) * '''HOME''': The current user's home directory. * '''PATH''': A colon-separated list of paths that will be searched to find the executable for a command that is executed, if it is not an alias or a function (or absolutely referenced). * '''PS1''': Contains a string that describes the format of your shell prompt. * '''TMPDIR''': Contains the directory that is used to store temporary files (by the shell). Of course, you aren't restricted to only these variables. Feel free to define your own: {{{ $ country=Canada $ echo "I am $USER and I currently live in $country." I am lhunath and I currently live in Canada. }}} Notice what we did to assign the value `Canada` to the variable `country`. Remember that you are '''NOT allowed to have any spaces before or after that equals sign'''! {{{ $ language = PHP -bash: language: command not found $ language=PHP $ echo "I'm far too used to $language." I'm far too used to PHP. }}} Remember that ["BASH"] is not Perl or PHP. You need to be very well aware of how ''expansion'' works to avoid '''big''' trouble. If you don't, you'll end up creating very dangerous situations in your scripts, especially when making this mistake with `rm`: {{{ $ ls no secret secret $ file='no secret' $ rm $file rm: cannot remove `no': No such file or directory }}} Imagine we have two files, `no secret` and `secret`. The first contains nothing useful, but the second contains the secret that will save the world from impending doom. Unthoughtful as you are, you forgot to '''quote''' your parameter expansion of `file`. ["BASH"] expands the parameter and the result is `rm no secret`. ["BASH"] splits the arguments up by their whitespace as it normally does, and `rm` is passed two arguments; 'no' and 'secret'. As a result, it fails to find the file `no` and it deletes the file `secret`. You doomed the world, you should be proud. -------- '''Good Practice: [[BR]] You should always keep parameter expansions well quoted. This prevents the whitespace or the possible globs inside of them to give you gray hair or unexpectedly wipe stuff off your computer. The only good PE, is a quoted PE.''' |
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| '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC23 Shell Parameters], [http://www.gnu.org/software/bash/manual/bashref.html#SEC60 Shell Variables]''' -------- [[Anchor(conditionals)]] == Tests and Conditionals == Sequential execution of applications is one thing, but to achieve a sort of logic in your scripts or your commandline one-liners, you'll need variables and conditionals. Conditionals are used to determine the execution flow of a script. [[Anchor(exitcode)]] === Exit Status === Every application results in an exit code whenever it terminates. This exit code is used by whatever application started it to evaluate whether everything went OK. This exit code is like a return value from functions. It's an integer between 0 and 255 (inclusive). Convention dictates that we use 0 to denote success, and any other number to denote failure of some sort. The specific number is entirely application-specific, and is used to hint as to what exactly went wrong. For example, the `ping` command sends ICMP packets over the network to a certain host. That host normally responds to this packet by sending the exact same one right back. This way, we can check whether the remote host can receive our packets. `ping` has a range of exit codes which can tell us what went wrong, if anything did: '''From the `ping` manual: If ping does not receive any reply packets at all it will exit with code 1. If a packet count and deadline are both specified, and fewer than count packets are received by the time the deadline has arrived, it will also exit with code 1. On other error it exits with code 2. Otherwise it exits with code 0. This makes it possible to use the exit code to see if a host is alive or not.''' The parameter `?` shows us the exit code of the last foreground process that terminated. Let's play around a little with ping to see it's exit codes: {{{ $ ping God ping: unknown host God $ echo $? 2 $ ping -c 1 -W 1 1.1.1.1 PING 1.1.1.1 (1.1.1.1) 56(84) bytes of data. --- 1.1.1.1 ping statistics --- 1 packets transmitted, 0 received, 100% packet loss, time 0ms $ echo $? 1 }}} -------- '''Good Practice: [[BR]] You should make sure that your scripts always return a non-zero exit code if something unexpected happened in its execution. You can do this with the `exit` builtin:''' {{{ rm file || { echo "Could not delete file!"; exit 1; } }}} |
[[BashGuide/CommandsAndArguments|Commands and Arguments ->]] |
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| '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC52 Exit Status]''' -------- [[Anchor(operators)]] === Control Operators === Now that we know what exit codes are, and that an exit code of '0' means the command's execution was successful, we'll learn to use this information. The easiest way of performing a certain action depending on the success of a previous command is through the use of 'control operators'. These operators are `&&` and `||`, which respectively represent a logical AND and a logical OR. These operators are used inbetween two commands, and they are used to control whether the second command should be executed depending on the success of the first. Let's put that theory in practice: {{{ $ mkdir d && cd d }}} This simple example has two commands, `mkdir d` and `cd d`. You could easily just use a semi-colon there to separate both commands and execute them sequentially; but we want something more. In the above example, ["BASH"] will execute `mkdir d`, then `&&` will check the result of the `mkdir` application as it finishes. If the `mkdir` application resulted in a success (exit code 0), then `&&` will execute the next command, `cd d`. If `mkdir d` failed, and returned a non-0 exit code, `&&` will skip the next command, and we will stay in the current directory. Another example: {{{ $ rm /etc/some_file.conf || echo "I couldn't remove the file!" rm: cannot remove `/etc/some_file.conf': No such file or directory I couldn't remove the file! }}} `||` is much like `&&`, but it does the exact opposite. It only executes the next command if the first '''failed'''. As such, the message is only echoed if the `rm` command was unsuccessful. You can make a sequence with these operators, but you have to be very careful when you do. Remember what exit code the operator is '''really''' going to be checking against! Here's an example that might cause confusion: {{{ $ false && true || echo "Riddle, riddle?" Riddle, riddle? $ true && false || echo "Riddle, riddle?" Riddle, riddle? }}} `true` is obviously always going to be successful. `false` is obviously always going to be unsuccessful. Can you guess why the `echo` statement is executed in both occasions? The key to understanding how to sequence these operators properly is by evaluating exit codes from left to right. In the first example, `false` is unsuccessful, so `&&` does not execute the next command (which is `true`), but the next `||` gets a shot too. `||` still sees that the last exit code was that from `false`, and `||` executes the next command when the previous was unsuccessful. As a result, the `echo` statement is executed. The same for the second statement again. `true` is successful, so the `&&` executes the next statement. That is `false`; the last exit code now becomes unsuccessful. After that, `||` is evaluated, which sees the unsuccessful exit code from `false` and executes the `echo` statement. It's all easy with `true`s and `false`s; but how about real commands? {{{ $ rm file && touch file || echo "File not found!" }}} All seems well with this piece of code, and when you test it, I'm sure you'll see that it actually does what it's supposed to. It tries to delete a file, and if it succeeds, it creates it again as a new and empty file; if something goes wrong we get the error message. What's the catch? Perhaps you guessed, perhaps not, but here's a hint: Imagine we're in a directory where we don't have permission to create a file? It won't stop us from deleting the file if the file happens to be ours. `rm` will succeed in deleting our file, but `touch` will fail to create it anew because of permission issues. As a result, we get a strange error message saying that the file wasn't found while we were actually trying to '''create''' it. What's up with that? -------- '''Good Practice: [[BR]] It's best not to get overcourageous when dealing with conditional operators. They can make your script hard to understand, especially for a person that's assigned to maintain it and didn't write it himself.''' ---- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC18 List of Commands]''' -------- [[Anchor(if)]] === If-statements === `if` is an application that executes the command that it receives as argument, and checks that command's exit code to see whether its execution was successful. Depending on that exit code, `if` executes a specific block of code. {{{ $ if true > then echo "It was true." > else echo "It was false!" > fi It was true. }}} Here you see the basic outline of an ''if-statement''. We start by calling `if` with the argument `true`. `true` is a sort of built-in application, running it is like running an application that always ends successfully. `if` runs that application, and once the application's done, it checks the exit code. Since `true` always exits successfully, `if` continues to the `then`-block, and executes its code. Should the `true` application have failed somehow, and returned an unsuccessful exit code, the `if` statement would have skipped the `then` code, and executed the `else` code block instead. There are commands that can help us a lot in doing conditional checks. They are `[` (also named `test`) and `[[`. `[` is a normal application that reads its arguments and does some checks with them. `[[` is much like `[`, however, it is not an application. It's a built-in and it offers far more versatillity. Let's get practical: {{{ $ if [ a = b ] > then echo "a is the same as b." > else echo "a is not the same as b." > fi a is not the same as b. }}} `if` executes the command `[` with the arguments 'a', '=', 'b' and ']'. `[` uses these arguments to determine what must be checked. It then checks whether the string 'a' is identical to the string 'b', and if this is the case, it will exit successfully. However, since we know this is not the case, `[` will not exit successfully (it's exit code will be 1). `if` sees that `[` terminated unsuccessfully and executes the code in the `else` block. Now, to see why `[[` is so much more interesting and trustworthy than `[`, let us highlight some possible problems with `[`: {{{ $ if [ my dad = my dog ] > then echo "I have a problem." > fi -bash: [: too many arguments }}} Can you guess what caused the problem? [[BR]] `[` was executed with the arguments 'my', 'dad', '=', 'my', 'dog' and ']'. `[` doesn't understand what test it's supposed to execute, because it expects the second argument to be the operator. In our case, the operator is the third argument. Yet another reason why '''quotes''' are so terribly important. Whenever we type whitespace in bash that belongs together with the words before or after it, '''we need to quote the whole string''': {{{ $ if [ 'my dad' = 'my dog' ] > then echo "I have a problem." > fi }}} This time, `[` sees an operator (`=`) in the second argument and it can continue with its work. Now, this may be easy to see and avoid, but it gets just a little trickier when we put the strings in variables, rather than literally in the statement: {{{ $ dad='my dad'; dog='my dog' $ if [ $dad = $dog ] > then echo "I have a problem." > fi -bash: [: too many arguments }}} How did we mess up this time? [[BR]] Here's a hint: ["BASH"] takes our ''if-statement'' and expands all the parameters in it. The result is `if [ my dad = my dog ]`. Boom, game over. Here's how it's supposed to look like: {{{ $ if [ "$dad" = "$dog" ] > then echo "I have a problem." > fi }}} To help us out a little, ["BASH"] introduced a new style of conditional test. Original as the ["BASH"] authors are, they called it `[[`. `[[` was loaded with several very interesting features which are missing from `[`. One of them helps us in dealing with parameter expansions: {{{ $ if [[ $dad = $dog ]] > then echo "I have a problem." > fi $ if [[ I want $dad = I want $dog ]] > then echo "I want too much." > fi -bash: conditional binary operator expected -bash: syntax error near `want' }}} This time, $dad and $dog didn't need to be quoted. Since `[[` isn't an application (while `[` is), but a built-in, it has special magical powers. It parses its arguments before they are expanded by bash and does the expansion itself; taking the result as a single argument, even if that result contains whitespace. ''However'', be aware that simple strings still have to be quoted properly. `[[` can't know whether your literal whitespace in the statement is intentional or not; so it splits it up just like ["BASH"] normally would. Let's fix our last example: {{{ $ if [[ "I want $dad" = "I want $dog" ]] > then echo "I want too much." > fi }}} Now that you've got a decent understanding of quoting issues that may arise, let's have a look at some of the other features that `[` and `[[` were blessed with: * Tests supported by `[` (also known as `test`): * '''-e FILE''': True if file exists. * '''-f FILE''': True if file is a regular file. * '''-d FILE''': True if file is a directory. * '''-h FILE''': True if file is a symbolic link. * '''-r FILE''': True if file is readable by you. * '''-s FILE''': True if file exists and is not empty. * '''-t FD ''': True if FD is opened on a terminal. * '''-w FILE''': True if the file is writable by you. * '''-x FILE''': True if the file is executable by you. * '''-O FILE''': True if the file is effectively owned by you. * '''-G FILE''': True if the file is effectively owned by your group. * '''FILE -nt FILE''': True if the first file is newer than the second. * '''FILE -ot FILE''': True if the first file is older than the second. * '''-z STRING''': True if the string is empty (it's length is zero). * '''-n STRING''': True if the string is not empty (it's length is not zero). * '''STRING = STRING''': True if the first string is identical to the second. * '''STRING != STRING''': True if the first string is not identical to the second. * '''STRING < STRING''': True if the first string sorts before the second. * '''STRING > STRING''': True if the first string sorts after the second. * '''EXPR -a EXPR''': True if both expressions are true (logical AND). * '''EXPR -o EXPR''': True if either expression is true (logical OR). * '''INT -eq INT''': True if both integers are identical. * '''INT -ne INT''': True if the integers are not identical. * '''INT -lt INT''': True if the first integer is less than the second. * '''INT -gt INT''': True if the first integer is greater than the second. * '''INT -le INT''': True if the first integer is less than or equal to the second. * '''INT -ge INT''': True if the first integer is greater than or equal to the second. * Additional tests supported only by `[[`: * '''STRING = (or ==) PATTERN''': Not string comparison like with `[` (or `test`), but ''pattern matching'' is performed. True if the string matches the glob pattern. * '''STRING =~ REGEX''': True if the string matches the regex pattern. * '''( EXPR )''': Parantheses can be used to change the evaluation precedence. * '''EXPR && EXPR''': Much like the '-a' operator of `test`, but does not evaluate the second expression if the first already turns out to be false. * '''EXPR || EXPR''': Much like the '-o' operator of `test`, but does not evaluate the second expression if the first already turns out to be true. * '''! EXPR''': Inverses the result of the expression. You want some examples? Sure: {{{ $ test -e /etc/X11/xorg.conf && echo "Your Xorg is configured!" Your Xorg is configured! $ test -n "$HOME" && echo "Your homedir is set!" Your homedir is set! $ [[ boar != bear ]] && echo "Boars aren't bears!" Boars aren't bears! $ [[ boar != b?ar ]] && echo "Boars don't look like bears!" $ [[ $DISPLAY ]] && echo "Your DISPLAY variable is not empty, you probably have Xorg running." Your DISPLAY variable is not empty, you probably have Xorg running. $ [[ ! $DISPLAY ]] && echo "Your DISPLAY variable is not not empty, you probably don't have Xorg running." }}} -------- '''Good Practice: [[BR]] Whenever you're making a ["BASH"] script, you should always use `[[` (unless if for some reason you need very specific and rare functionality from `[`). [[BR]] Whenever you're making a Shell script, which may end up being used in an environment where ["BASH"] is not available, you should use `[`, because it is far more compatible (it's an application, not built into ["BASH"], like `[[`).''' ---- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC20 Conditional Constructs]''' -------- [[Anchor(loops)]] === Conditional Loops === You've learned how to code some basic logic flow for your scripts. It's important that you understand a thing or two about keeping scripts healthy first. ["BASH"] scripts, much like any other kind of scripts, should never be overrated. Although they have great potential once you fully understand its features; they aren't always the right tool for the job. At the same time, when you make scripts, you should remember to keep them light, both in length and in complexity. Very long and/or very complex scripts are most often also very bad scripts. Those that aren't yet soon will be; because they are always very difficult to maintain and adapt/extend. A technique that we can use to try and keep code length and complexity down is loops. There are two kinds of loops. Using the correct kind of loop correctly will help you keep your scripts readable and healthy. ["BASH"] supports `while` loops and `for` loops. The `for` loops can appear in three different forms. Here's a summary: * '''`while [command]`''': Repeat so long as command is executed successfully (exit code: 0). * '''`for [variable] in [words]`''': Repeat the loop for each word after putting it into the variable. * '''`for (( [expression]; [expression]; [expression] ))`''': Starts by evaluating the first expression, repeats the loop so long as the second expression is valid and at the end of each loop evaluates the third expression. Let's put that in practice; here are some examples to illustrate the differences but also the similarities between the loops: {{{ $ while true > do echo "Infinite loop! > done $ (( i=10 )); while (( i > 0 )) > do echo "$i empty cans of beer." > (( i-- )) > done $ for (( i=10; i > 0; i-- )) > do echo "$i empty cans of beer." > done $ for i in {10..0} > do echo "$i empty cans of beer." > done }}} The last three loops achieve exactly the same result; just in a different syntax. You'll encounter this many times in your shell scripting experience. There will nearly always be multiple approaches to solving a problem. The test of your skill soon won't be about solving a problem as much as about how best to solve it. You need to learn to pick the best angle of approach for the job. Usually, the main factors to keep into account will be the simplicity and flexibility of the resulting code. My personal favorite is the latter of the examples. In the example I used ''Brace Expansion'' to generate the words; but there are other ways, too. Let's take a closer look at that last example, because although it looks the easier of both `for`s, it can often be the trickiest too; if you don't know exactly how it works. As I mentioned before; it takes one word from a list of words and puts each in the variable, one at a time, then loops through the code with it. The tricky part is how ["BASH"] decides what the words are. Let me explain myself by expanding the braces from that previous example: {{{ $ for i in 10 9 8 7 6 5 4 3 2 1 0 > do echo "$i empty cans of beer." > done }}} ["BASH"] takes the characters between `in` and the end of the statement, at splits them up into words. You shouldn't confuse the splitting that happens here with the splitting that happens with ''Commandline Arguments''; even though they look exactly the same at first sight. Commandline arguments are split at ''spaces'', ''tabs'' and ''newlines''; while the splitting in this `for` statement happens at ''spaces'' by default. This default behaviour can be changed. The way `for` determines what delimiter to use for the splitting is by looking into the `IFS` variable; and taking the first character there. `IFS` is an acronym for ''Internal Field Separator''; and by default it contains a ''space'', a ''tab'' and a ''newline''. Since the ''space'' is the first character there, `for` uses it to split up the words in our sequence; and feeds each word to the variable `i`; one at a time. '''As a result; be VERY careful not to make the following mistake:''' {{{ $ ls The best song in the world.mp3 $ for file in $(ls *.mp3) > do rm "$file" > done rm: cannot remove `The': No such file or directory rm: cannot remove `best': No such file or directory rm: cannot remove `song': No such file or directory rm: cannot remove `in': No such file or directory rm: cannot remove `the': No such file or directory rm: cannot remove `world.mp3': No such file or directory }}} You should already know to quote the `$file` in the `rm` statement; but what's going wrong here? Right. ["BASH"] expands the command substitution (`$(ls *.mp3)`), replaces it by its output, and as a result executes `for file in The best song in the world.mp3`. ["BASH"] splits that up in words by using ''spaces'' and tries to `rm` each word. ''Boom, you are dead''. You want to quote it, you say? Let's add another song: {{{ $ ls The best song in the world.mp3 The worst song in the world.mp3 $ for file in "$(ls *.mp3)" > do rm "$file" > done rm: cannot remove `The best song in the world.mp3 The worst song in the world.mp3': No such file or directory }}} Quotes will indeed protect the whitespace in your filenames; but it will do more than that. The quotes will protect '''all the whitespace''' from the output of `ls`. There is no way ["BASH"] can know which parts of the output of `ls` represent filenames; it's not psychic. The output of `ls` is a simple string, and ["BASH"] treats it as that for lack of better. The `for` puts the whole quoted output in `i` and runs the `rm` command with it. ''Damn, dead again''. So what do we do? As suggested earlier; globs are your best friend: {{{ $ for file in *.mp3 > do rm "$file" > done }}} This time, ["BASH"] '''does''' know which are filenames, and it '''does''' know what the filenames are and as such it can split them up nicely. The result of expanding the glob is this: `for file in "The best song in the world.mp3" "The worst song in the world.mp3"`. Problem resolved. Let's talk about changing that delimiter. Say, you've got yourself a nice cooking recipe, and you want to write a script that tells you how to use it. Sure, let's get right at it: {{{ $ recipe='2 c. all purpose flour > 6 tsp. baking powder > 2 eggs > 2 c. milk > 1/3 c. oil' $ for ingredient in $recipe > do echo "Take $ingredient; mix well." > done }}} Can you guess what the result will look like? I recommend you run the code if you can't and ponder the reason first. It will help you understand things. Yes, as explained earlier, `for` splits its stuff up in words by using the delimiter from `IFS`. To read the recipe correctly, we need to split it up by newlines instead of by spaces. Here's how we do that: {{{ $ recipe='2 c. all purpose flour > 6 tsp. baking powder > 2 eggs > 2 c. milk > 1/3 c. oil' $ IFS=$'\n' $ for ingredient in $recipe > do echo "Take $ingredient; mix well." > done Take 2 c. all purpose flour; mix well. Take 6 tsp. baking powder; mix well. Take 2 eggs; mix well. Take 2 c. milk; mix well. Take 1/3 c. oil; mix well. }}} Beautiful. '''Note: This delimiter is only used when the words consist of an expansion. Not when they're literal. Literal words are always split at spaces:''' {{{ $ PATH=/bin:/usr/bin $ IFS=: $ for i in $PATH > do echo "$i" > done /bin /usr/bin $ for i in $PATH:/usr/local/bin > do echo "$i" > done /bin /usr/bin /usr/local/bin $ for i in /bin:/usr/bin:/usr/local/bin > do echo "$i" > done /bin:/usr/bin:/usr/local/bin }}} Lets focus a little more on the `while` loop. It promises even more simplicity than this `for` loop; so long as you don't need any `for` specific features. The `while` loop is very interesting for its capacity of executing commands and basing the loop's progress on the result of them. Here are a few examples of how `while` loops are very often used: {{{ $ # The sweet machine; hand out sweets for a cute price. $ while read -p $'The sweet machine.\nInsert 20c and enter your name: ' name > do echo "The machine spits out three lollipops at $name." > done $ # Check your email every five minutes. $ while sleep 5m > do kmail --check > done $ # Wait for a host to come back online. $ while ! ping -c 1 -W 1 "$host" > do echo "$host is still unavailable." > done; echo -e "$host is available again!\a" }}} -------- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC19 Looping Constructs]''' -------- [[Anchor(io)]] == Input And Output == This basic principle of computer science applies just as well to applications started through ["BASH"]. ["BASH"] makes it fairly easy to play around with the input and output of commands, which gives us great flexibility and increadible opportunities for automation. [[Anchor(fds)]] === File Descriptors === Input and output from and to processes always occurs via so called ''File Descriptors'' (in short: FDs). FDs are kind of like pointers to sources of data. When something reads from or writes to that FD, the data is being read from or written to the FD's data source. FDs can point to regular files, but they can also point to more abstract data sources, like the input and output source of a process. By default, every new process has three FDs. They are referred to by the names ''Standard Input'', ''Standard Output'' and ''Standard Error''. In short, they are respectively called `stdin`, `stdout` and `stderr`. The ''Standard Input'' is where the characters you type on your keyboard usually come from. The ''Standard Output'' is where the program sends most of its normal information to so that the user can see it, and the ''Standard Error'' is where the program sends its error messages to. Be aware that GUI applications work in the same way; but the actual GUI doesn't work via these FDs. GUI applications can still read and write from and to the standard FDs, but they usually don't. Usually, they do all the user interaction via that GUI; making it hard to control for ["BASH"]. As a result, we'll stick to simple console applications. Those we can easily feed data on the "Standard Input" and read data from on its "Standard Output" and "Standard Error". Let's make these definitions a little more concrete. Here's a demonstration of how "Standard Input" and "Standard Output" work: {{{ $ read -p "What is your name? " name; echo "Good day, $name. Would you like some tea?" What is your name? lhunath Good day, lhunath. Would you like some tea? }}} `read` is a command that reads information from `stdin` and stores it in a variable. We specified `name` to be that variable. Once `read` has read a line of information from `stdin`, it finished and lets `echo` display a message. `echo` uses `stdout` to send its output to. `stdin` is connected to your terminal's input device; which is probably going to be your keyboard. `stdout` is connected to your terminal's output device; which I assume is a computer monitor. As a result; you can type in your name and are then greeted with a friendly message on your monitor, offering you a cup of tea. So what is `stderr`? Let's demonstrate: {{{ $ rm secrets rm: cannot remove `secrets': No such file or directory }}} Unless if you had a file called `secrets` in your current directory; that `rm` command will fail and show an error message explaining what went wrong. Error messages like these are by convention displayed on `stderr`. `stderr` is also connected to your terminal's output device, just like `stdout`. As a result, error messages display on your monitor just like the messages on `stdout`. However, this separation makes it easy to keep errors separated from the application's normal messages. Some people like to use wrappers to make all the output on `stderr` red, so that they can see the error messages more clearly. This is not generally advisable, but it is a simple example of the many options this separation provides us with. -------- '''Good Practice: [[BR]] Remember that when you create scripts, you should send your custom error messages to the `stderr` FD. This is a convention and it is very convenient when applications follow the convention. As such, so should you! You'll about redirection soon, but let me show you quickly how it's done:''' {{{ echo "Uh oh. Something went really bad.." >&2 }}} -------- [[Anchor(redirection)]] === Redirection === The most basic form of input/output manipulation in ["BASH"] is ''Redirection''. ''Redirection'' is used to change the data source or destination of an application's FDs. That way, you can send the application's output to a file instead of the terminal, or have the application read from a file instead of from the keyboard. Redirection, too, comes in different shapes. There's ''File Redirection'', ''File Descriptor manipulation'', ''Heredocs'' and ''Herestrings''. -------- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC37 Redirections]''' -------- ==== File Redirection ==== ''File Redirection'' is probably the most basic form of redirection. I'll start with this so you can grasp the concept of redirection well. {{{ $ echo "The story of William Tell. > > It was a cold december night. Too cold to write." > story $ cat story The story of William Tell. It was a cold december night. Too cold to write. }}} As a result; the `echo` command will not send its output to the terminal, but the `> story` operation '''changes the destination of the `stdout` FD''' so that it now points to a file called `story`. Be aware that before the `echo` command is executed, ["BASH"] first checks to see whether that file `story` actually exists. If it doesn't, it is created as an empty file, so that the FD can be pointed to it. This behaviour can be toggled with ''Shell Options'' (see later). We then use the application `cat` to print out the contents of that file. `cat` is an application that reads the contents of all the files you pass it as arguments. It then outputs each file one after another on `stdout`. In essence, it con'''cat'''enates the contents of all the files you pass it as arguments. '''Warning:''' Far too many code examples and shell tutorials on the Internet tell you to use `cat` whenever you need to read the contents of a file. '''This is highly ill-adviced!''' `cat` only serves well to contatenate contents of multiple files together, or as a quick tool on the shell prompt to see what's inside a file. You should '''NOT''' use `cat` to read from files in your scripts. There will almost always be far better ways to do this. Please keep this warning in mind. Useless usage of `cat` will merely result in an extra process to create, and often results in poorer read speed because `cat` cannot determine the context of what it's reading and the purpose for that data. When we use `cat` without passing any kind of arguments, it obviously doesn't know what files to read the content for. In this case, `cat` will just read from `stdin` instead of from a file (much like `read`). Since `stdin` is normally not a regular file, starting `cat` without any arguments will seem to do nothing: {{{ $ cat }}} It doesn't even give you back your shell prompt! What's going on? `cat` is still reading from `stdin`, which is your keyboard. Anything you type now will be sent to `cat`. As soon as you hit the ''Enter'' key, `cat` will do what it normally does; it will display what it reads on `stdout`, just the same way as when it displayed our story on `stdout`: {{{ $ cat test? test? }}} Why does it say `test?` twice now? Well, as you type, your terminal shows you all the characters that you send to `stdin` before sending them there. That results in the first `test?` that you see. As soon as you hit ''Enter'', `cat` has read a line from `stdin`, and shows it on `stdout`, which is also your terminal; hence, resulting in the second line: `test?`. You can press ''Ctrl+D'' to send `cat` the ''End of File'' character. That'll cause `cat` to think the file `stdin` has closed. It will stop reading from it and return you to your prompt. Let's use file redirection to attach a file to `stdin`, so that `stdin` is no longer reading from our keyboard, but instead, now reads from the file: {{{ $ cat < story The story of William Tell. It was a cold december night. Too cold to write. }}} The result of this is exactly the same as the result from our previous `cat story`; except this time, the way it works is a little different. In our first example, `cat` opened an FD to the file `story` and read its contents through that FD. In this recent example, `cat` simply reads from `stdin`, just like it did when it was reading from our keyboard. However, this time, the `< story` operation has '''modified''' `stdin` so that its data source is the file `story` rather than our keyboard. Let's summarize: * '''`command > file`''': Send the `stdout` of command to `file`. * '''`command < file`''': Use the contents of `file` when `command` reads from `stdin`. * '''`command 1> file`''': Send the `stdout` of command to `file`. * '''`command <0 file`''': Use the contents of `file` when `command` reads from `stdin`. Redirection operators can take a number. That number denotes the FD that it changes. If the number is not present, the `>` operator uses FD 1 by default, because that is the number for `stdout`. `<` uses FD 0 by default, because that is the number for `stdin`. The number for the `stderr` FD is 2. So, let's try sending the output of `stderr` to a file: {{{ $ for homedir in /home/* > do rm "$homedir/secret" > done 2> errors }}} In this example, we're looping over each file in `/home`. We then try to delete the file `secret` in each of them. Some `homedir`s may not have a secret. As a result, the `rm` operation will fail and send an error message on `stderr`. You may have noticed that our redirection operator isn't on `rm`, but it's on that `done` thing. Why is that? Well, this way, the redirection applies to all output to `stderr` made inside the whole loop. Let's see what the result of our loop was? {{{ $ cat errors rm: cannot remove `/home/axxo/secret': No such file or directory rm: cannot remove `/home/lhunath/secret': No such file or directory }}} Two error messages in our error log file. Two people that didn't have a `secret` file in their home directory. If you're writing a script, and you expect that running a certain command may fail on occasion, but don't want the script's user to be bothered by the possible error messages that command may produce, you can silence an FD. Silencing it is as easy as normal ''File Redirection''. We're just going to send all output to that FD into the system's black hole: {{{ $ for homedir in /home/* > do rm "$homedir/secret" > done 2> /dev/null }}} The file `/dev/null` is '''always''' empty, no matter what you write or read from it. As such, when we write our error messages to it, they just disappear. The `/dev/null` file remains as empty as ever before. That's because it's not a normal file, it's a ''virtual'' device. There is one last thing you should learn about ''File Redirection''. It's interesting that you can make error log files like this to keep your error messages; but as I mentioned before, ["BASH"] makes sure that the file exists before trying to redirect to it. ["BASH"] '''also makes sure the file is empty''' before redirecting to it. As a result, each time we run our loop to delete secret files, our log file will be truncated empty before we fill it up again with new error messages. What if we'd like to keep a record of any error messages generated by our loop? What if we don't want that file to be truncated each time we start our loop? The solution is achieved by doubling the redirection operator. `>` becomes `>>`. `>>` will not empty a file, it will just append new data to the end of it! {{{ $ for homedir in /home/* > do rm "$homedir/secret" > done 2>> errors }}} Hooray! -------- '''Good Practice: [[BR]] It's a good idea to use redirection whenever an application needs file data and is built to read data from `stdin`. A lot of bad examples on the Internet tell you to pipe (see later) the output of `cat` into processes; but this is nothing more than a very ''bad'' idea.''' ---- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC38 Redirecting Input], [http://www.gnu.org/software/bash/manual/bashref.html#SEC39 Redirecting Output], [http://www.gnu.org/software/bash/manual/bashref.html#SEC40 Appending Redirected Output], [http://www.gnu.org/software/bash/manual/bashref.html#SEC41 Redirecting Standard Output and Standard Error]''' -------- ==== File Descriptor Manipulation ==== Now that you know how to manipulate process input and output by sending it to and reading it from files; let's make this a little more interesting still. It's possible to change the source and desination of FDs to point to or from files, that you know. It's also possible to copy one FD to another. Let's prepare a simple testbed: {{{ $ echo "I am a proud sentence." > file }}} We've made a file called `file`, and written a proud sentence into it. It's time I introduce a new application to you. Its name is `grep`, and it's increadibly powerful. `grep` is that one thing that you need more than anything else in your household. It basically takes a ''search string'' as its first argument and one or more files as extra arguments. Just like `cat`, `grep` also uses `stdin` if you don't specify any files as extra arguments. `grep` reads the files (or `stdin` if none were provided) and searches for the ''search string'' you gave it. Most versions of `grep` even support a `-r` switch, which makes it take directories as well as files as extra arguments, and then searches all the files and directories in those directories that you gave it. Here's an example of how `grep` can work: {{{ $ ls house/ house/drawer house/closet house/dustbin house/sofa $ grep -r socks house/ house/sofa:socks }}} In this silly example we have a directory called house with several pieces of furniture in it as files. If we're looking for our `socks` in each of those files, we send grep to search the directory `house/`. `grep` will search everything in there, open each file and look through its contents. In our example, grep finds `socks` in the file `house/sofa`; presumably tucked away under a pillow. You want a more realistic example? Sure: {{{ $ grep "$HOSTNAME" /etc/* /etc/hosts:127.0.0.1 localhost Lyndir }}} Here we instruct `grep` to search for whatever `$HOSTNAME` expands to in whatever `/etc/*` expands to. It finds my hostname, which is `Lyndir` in the file `/etc/hosts`, and shows me the line in that file that contains the ''search string''. OK, now that you understand `grep`, let's continue with our ''File Descriptor Manipulation''. Remeber that we created a file called `file`, and wrote a proud sentence to it? Let's use `grep` to find where that proud sentence is now: {{{ $ grep proud * file:I am a proud sentence. }}} Good, `grep` found our sentence in `file`. It writes the result of its operation to `stdout` which is shown on our terminal. Now let's see if we can make grep send an error message, too: {{{ $ grep proud file 'not a file' file:I am a proud sentence. grep: not a file: No such file or directory }}} This time, we instruct `grep` to search for the string `proud` in the files '`file`' and '`not a file`'. `file` exists, and the sentence is in there, so `grep` happily writes the result to `stdout`. It moves on to the next file to scan, which is '`not a file`'. `grep` can't open this file to read its content, because it doesn't exist. As a result, `grep` emits an error message on `stderr` which is still connected to our terminal. Now, how would you go about silencing this `grep` statement completely? We'd like to send all the output that appears on the terminal to a file instead; let's call it `proud.log`: {{{ $ grep proud file 'not a file' > proud.log 2> proud.log }}} Does that look about right? We first use `>` to send `stdout` to `proud.log`, and then use `2>` to send `stderr` to `proud.log` as well. Almost, but not quite. If you run this command and then look in `proud.log`, you'll see there's only an error message, not the output from `stdout`. We've created a very bad condition here. After `stdout` has written its output to the log file, the error message needs to be written to the log file. The `stderr` redirection truncates the log file and writes its own information there. Things would go even more wrong if after that new information arrives on `stdout`. `stdout`'s write operation might cancel an active write operation from `stderr` to continue writing its own information. We need to prevent having two FDs working on the same destination or source. We can do this by dublicating FDs: {{{ $ grep proud file 'not a file' > proud.log 2>&1 }}} You need to remember to always read file redirections from right to left. This is the order in which ["BASH"] assigns and processes them. First, `stdout` is changed so that it points to our `proud.log`. Then, we use the `>&` syntax to dublicate FD 1 and put this dublicate in FD 2's stead. If this is hard for you to grasp, you could read this as: `stdout becomes proud.log and stderr becomes stdout (which is proud.log)`. As a result, `stdout` obviously writes its information to `proud.log`, but `stderr` writes its information to whatever `stdout` was when the `>&` redirector was used; which was `proud.log` as well. In this case, the handle that writes to `proud.log` is the same for both `stdout` and `stderr`, and no collisions occur. Be careful not to confuse the order: {{{ $ grep proud file 'not a file' 2>&1 > proud.log }}} This would read as `stderr becomes stdout (which is the terminal) and stdout becomes proud.log`. As a result, `stdout`'s messages will be logged, but the error messages will still go to the terminal. ''Oops''. '''Note: [[BR]] For compatibility reasons with other shells, ["BASH"] also makes yet another form of redirection available to you. The `&>` redirection operator is actually just a shorter version of what we did here; redirecting both `stdout` and `stderr` to a file:''' {{{ $ grep proud file 'not a file' &> proud.log }}} This is the same as `> proud.log 2>&1`, just a bit shorter. You pick. '''''TODO: Moving FDs and Opening FDs RW.''''' -------- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC44 Dublicating File Descriptors], [http://www.gnu.org/software/bash/manual/bashref.html#SEC45 Moving File Descriptors], [http://www.gnu.org/software/bash/manual/bashref.html#SEC46 Opening File Descriptors for Reading and Writing]''' -------- ==== Heredocs And Herestrings ==== Files aren't all that. They're boring, really. Strings are so much more interesting. They're not permanent like files on a hard disk, but they're easy to work with, easy to make an easy to manipulate. ''Heredocs'' and ''Herestrings'' allow you to perform ''Redirection'' as you would with files, just by using strings instead. Let's try it out! {{{ $ grep proud <<END > I am a proud sentence. > END I am a proud sentence. }}} This is a ''Heredoc''. ''Heredocs'' aren't really useful unless if you're trying to embody long strings of several lines inside your scripts; which is '''very bad practice'''. The way they work, is by adding the `<<STRING` operator at the end of a command. That'll tell that command's `stdin` that it has to start reading input from the script (or the commandline, if you're not in a script). The input of the ''Heredoc'' stops as soon as you repeat whatever string you used to add to the end of the `<<`. In the example above, I used the string `END`; but it can really be anything (so long as you quote it if it has whitespace). Let's check out the very similar but more interesting ''Herestrings'': {{{ $ grep proud <<<"I am a proud sentence" I am a proud sentence. }}} This time, `stdin` reads its information straight from the string you put after the `<<<` operator. This is very convenient to send data that's in variables into processes: {{{ $ grep proud <<<"$USER sits proudly on his throne in $HOSTNAME." lhunath sits proudly on his throne in Lyndir. }}} -------- '''Good Practice: [[BR]] Heredocs are usually a bad idea because scripts should contain logic, not data. If you have a document that your script needs; you should ship it in a separate file along with your script. Herestrings, however, come in handy quite often; especially for sending variable content to processes like `grep` or `sed` instead of files.''' ---- '''In The Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC42 Here Documents], [http://www.gnu.org/software/bash/manual/bashref.html#SEC43 Here Strings]''' -------- |
CategoryShell |
Introduction
You are invited to make additions or modifications so long as you can keep them accurate. Please test any code samples you write.
All the information here is presented without any warranty or guarantee of accuracy. Use it at your own risk. When in doubt, please consult the man pages or the GNU info pages as the authoritative references.
About This Guide
This guide aims to aid people interested in learning to work with BASH. It aspires to teach good practice techniques for using, and to educate them about the internal operation of BASH.
This guide is targeted at beginning users. It assumes no advanced knowledge -- just the ability to login to a Unix-like system and open a command-line (terminal) interface. It will help if you know how to use a text editor; we will not be covering editors, nor do we endorse any particular editor choice. Familiarity with the fundamental Unix tool set, or with other programming languages or programming concepts, is not required, but those who have such knowledge may understand some of the examples more quickly.
If something is unclear to you, you are invited to report this (use BashGuideFeedback, or the #bash channel on irc.freenode.org) so that it may be clarified in this document for future readers.
You are invited to contribute to the development of this document by extending it or correcting invalid or incomplete information.
The primary maintainer(s) of this document:
The guide is also available in PDF format. Alternatively, you can just hit print after going to FullBashGuide. That guarantees you'll be printing the latest version of this document.
A Definition
BASH is an acronym for Bourne Again Shell. It is based on the Bourne shell and is mostly compatible with its features.
Shells are command interpreters. They are applications that provide users with the ability to give commands to their operating system interactively, or to execute batches of commands quickly. In no way are they required for the execution of programs; they are merely a layer between system function calls and the user.
Think of a shell as a way for you to speak to your system. Your system doesn't need it for most of its work, but it is an excellent interface between you and what your system can offer. It allows you to perform basic math, run basic tests and execute applications. More importantly, it allows you to combine these operations and connect applications to each other to perform complex and automated tasks.
BASH is not your operating system. It is not your window manager. It is not your terminal (but it oftens runs inside your terminal). It does not control your mouse or keyboard. It does not configure your system, activate your screensaver, or open your files when you double-click them. It is generally not involved in launching applications from your window manager or desktop environment. It's important to understand that BASH is only an interface for you to execute statements (using BASH syntax), either at the interactive BASH prompt or via BASH scripts.
In The Manual: Introduction
Shell: A (possibly interactive) command interpreter, acting as a layer between the user and the system.
BASH: The Bourne Again Shell, a Bourne compatible shell.
Using Bash
Most users that think of BASH think of it as a prompt and a command line. That is BASH in interactive mode. BASH can also run in non-interactive mode, as when executing scripts. We can use scripts to automate certain logic. Scripts are basically lists of commands (just like the ones you can type on the command line), but stored in a file. When a script is executed, all these commands are (generally) executed sequentially, one after another.
We'll start with the basics in an interactive shell. Once you're familiar with those, you can put them together in scripts.
Important!
You should make yourself familiar with the man and apropos commands on the shell. They will be vital to your self-tutoring.
$ man man
$ man aproposIn this guide, the $ at the beginning of a line represents your BASH prompt. Traditionally, a shell prompt either ends with $, % or #. If it ends with $, this indicates a shell that's compatible with the Bourne shell (such as a POSIX shell, or a Korn shell, or BASH). If it ends with %, this indicates a C shell (csh or tcsh); this guide does not cover C shell. If it ends with #, this indicates that the shell is running as the system's superuser account (root), and that you should be extra careful.
Your actual BASH prompt will probably be much longer than $. Prompts are often highly individualized.
The man command stands for "manual"; it opens documentation (so-called "man pages") on various topics. You use it by running the command man [topic] at the BASH prompt, where [topic] is the name of the "page" you wish to read. Note that many of these "pages" are considerably longer than one printed page; nevertheless, the name persists. Each command (application) on your system is likely to have a man page. There are pages for other things too, such as system calls or specific configuration files. In this guide, we will only be covering commands.
Note that if you're looking for information on BASH built-ins (commands provided by BASH, not by external applications) you should look in man bash instead. BASH's manual is extensive and detailed. It is an excellent reference, albeit more technical than this guide.
BASH also offers a help command which contains brief summaries of its built-in commands (which we'll discuss in depth later on).
$ help
$ help read
Interactive mode: A mode of operation where a prompt asks you for one command at a time.
Script: A file that contains a sequence of commands to execute one after the other.
Contents
The guide has been divided into sections, which are intended to be read roughly in the order presented. If you skip ahead to a specific section, you might find yourself missing some background information from previous sections. (Links to relevant sections are not always provided when a topic is mentioned.)
- Types of commands; argument splitting; writing scripts.
- Variables; special parameters; parameter types; parameter expansion.
- Globs; filename matching; extended globs; brace expansion; regular expressions.
Exit status; && and ||; if, test and [[; while, until and for; case and select.
- Arrays; associative arrays.
- Redirection; here documents; here strings; pipes; process substitution.
- Subshells; command grouping; arithmetic evaluation; functions; aliases.
- Reading commands from other files.
- Choosing your shell; quoting; readability; debugging.