Introduction

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.

["BASH"] is a BourneShell compatible shell, which adds many new features to its ancestor. Most of them are available in the 'KornShell', too.

TableOfContents

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.

You are invited to contribute to the development of this document by extending it or correcting invalid or incomplete information.

Anchor(definition)

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 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#SEC1 Introduction]

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.

    $ ls
    a  b  c

ls is a command that lists files in the current directory.

    $ 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 shell options:

    $ 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

• 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 Manual: [http://www.gnu.org/software/bash/manual/bashref.html#SEC35 Pattern Matching]

Anchor(characters)

Special Characters

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.

• 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; }

• 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 trues and falses; 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 fors, 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.

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.

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 concatenates 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 homedirs 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 >> and < becomes <<. >> and << will not empty a file, they will just append new data to the end of it!

    $ for homedir in /home/*
    > do rm "$homedir/secret"
    > done 2>> errors

Hooray!

File Descriptor Manipulation