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]]

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.

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 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

Commands And Arguments

["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 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 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.

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 double quotes 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 for forms of redirecting data from and to processes from and to files.

• expression: Test expression. This evaluates the conditional expression.

• {command}: 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 paranthesis 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.