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| = Guide to BASH Operation and Scripting = 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. If a question is not strictly shell specific, but rather related to Unix, it may be in the UnixFaq. [[TableOfContents]] == Introduction == 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. == 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 interact with their operating system on an interactive level, 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. == 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. == The Basics == BASH takes commands on the commandline. Commands can be different things. They can be application executables, aliasses, 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. * '''Aliasses''': ["BASH"] can use aliasses 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 aliasses. 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. == 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 Accessed'-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 inbetween. 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` things 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. == Globs == Globs are a very important concept in ["BASH"], if only for their increadible 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 paranthesis 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. |
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