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Commands and Arguments

BASH reads commands from its input (which is usually either a terminal or a file). Each line of input that it reads is treated as a command -- an instruction to be carried out. (There are a few advanced cases, such as commands that span multiple lines, but we won't worry about that just yet.)

Bash divides each line into words at each whitespace character (spaces and tabs). The first word it finds is the name of the command to be executed. All the remaining words become arguments to that command (options, filenames, etc.).

Assume you're in an empty directory. (If you want to try this code out, you can create and go into an empty directory called test by running: mkdir test; cd test.)

$ ls                # List files in the current directory (no output: no files).
$ touch a b c       # Create files 'a', 'b', and 'c'.
$ ls                # List files again; this time outputs: 'a', 'b', and 'c'.
a  b  c

The ls command prints out the names of the files in the current directory. The first time we run ls we get no output, because there are no files yet.

The # character, when it is at the beginning of a word, indicates a comment. Comments are ignored by the shell; they are meant only for humans to read. Everything from the # to the end of the current line is considered a comment, and ignored. If you're running these examples in your own shell, you don't have to type the comments; but even if you do, things will still work.

touch is an application that changes the Last Modified time of a file to the current time. If the filename that it is 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 *              # Remove all files in the current directory.
$ ls                # List files in the current directory (no output: no files).
$ touch a   b c     # Create files 'a', 'b' and 'c'.
$ ls                # List all files again; this time the output shows 'a', 'b' and 'c'.
a  b  c

rm is an application that removes all the files that it was given. * is a glob. It basically means all and in this case means all files in the current directory. You will read more about globs later.

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. 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.

echo is a command that writes its arguments to standard output (which in our case is the terminal). In this example, 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. If we actually want the extra whitespace, 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 inside them into a single argument. This argument is 'This    is    a    test.', properly spaced. Note that the quotes are not part of the argument; BASH removes them before handing the argument to echo. echo prints this single argument out just like it always does.

Be very careful to avoid the following:

$ ls                                          # There are two files in the current directory.
The secret voice in your head.mp3  secret
$ rm The secret voice in your head.mp3        # Executes rm with 6 arguments; not 1!
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                                          # List files in the current directory: It is still there.
The secret voice in your head.mp3             # But your file 'secret' is now gone!

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 your filenames have spaces and you do not quote them, BASH thinks each word is a separate argument. BASH hands each argument to rm separately, like individually wrapped slices of processed cheese. rm treats each argument as a separate file.

In the above example, rm tried to delete a file for each word in the filename of the song, instead of keeping the filename intact. That caused our file secret to be deleted, and our song to remain behind!

This is what we should have done:

$ rm "The secret voice in your head.mp3"

Arguments are separated from the command name and from each other by white space. This is important to remember. For example, the following is wrong:

$ [-f file]

You want the [ command name (a synonym for test) to be separated from the arguments -f, file and ]. If you do not separate [ and -f from each other with whitespace, BASH will think you are trying to execute the command name [-f and look in PATH for a program named [-f. Additionally, the arguments file and ] need to be separated by spaces. The [ command expects its last argument to be ]. The correct command separates all arguments with spaces:

$ [ -f file ]

(We'll cover the [ command in more detail later. We see a lot of people who are confused by it, and think that they can omit the white space between it and its arguments, so we need to present this particular example very early.)

And of course, if your filename contains whitespace or other special characters, it should be quoted:

$ [ -f "my file" ]

NOTE: Please have a good look at Arguments, Quotes, WordSplitting and http://wiki.bash-hackers.org/syntax/words if all this isn't very clear to you yet. It is important that you have a good grasp of how the shell interprets the statements you give it before you continue this guide.

• Good Practice:
  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.
  For example, you should always quote arguments to the echo command.

• In the FAQ:
  I'm trying to put a command in a variable, but the complex cases always fail!
  How can I handle command-line arguments (options) to my script easily?

• Arguments: These are the optional additional words you can specify when running commands. They are given after the command's name ('ls -l foo' executes ls with two arguments).

• Quotes: The two forms of quotes (' and ") are used to protect certain special characters inside them from being interpreted as special by Bash. The difference between ' and " will be discussed later.

Strings

The term string refers to a sequence of characters which is treated as a single unit. The term is used loosely throughout this guide, as well as in almost every other programming language.

In BASH programming, almost everything is a string. When you type a command, the command's name is a string, and each argument is a string. Variable names are strings, and the contents of variables are strings as well. A filename is a string, and most files contain strings. They're everywhere!

An entire command can also be considered a string. This is not normally a useful point of view, but it illustrates the fact that parts of strings can sometimes be considered strings in their own right. A string which is part of a larger string is called a substring.

Strings do not have any intrinsic meaning. Their meaning is defined by how and where they are used.

Let's try another example. With your favorite editor, write a shopping list and save it with the file name "list", and use cat to show it:

$ cat list
shampoo
tissues
milk (skim, not whole)

We typed a command: cat list. The shell reads this command as a string, and then divides it into the substrings cat and list. As far as the shell is concerned, list has no meaning. It's just a string with four characters in it. cat receives the argument list, which is a string that it interprets as a filename. The string list has become meaningful because of how it was used.

The file happens to contain some text, which we see on our terminal. The entire file content, taken as a whole, is a string, but that string is not meaningful. However, if we divide the file into lines (and therefore treat each line as a separate string), then we see each individual line has meaning.

We can divide the final line into words, but these words are not meaningful by themselves. We can't buy (skim at the store, and we might get the wrong kind of milk. Dividing the lines into words is not a useful thing to do in this example. But the shell doesn't know any of this -- only you do!

So, when you are dealing with commands, and data, and variables -- all of which are just strings to the shell -- you have all the responsibility. You need to be sure everything that needs to be separated can be separated properly, and everything that needs to stay together stays together properly.

We'll touch on these concepts repeatedly as we continue.

Types of Commands

Bash understands several different types of commands: aliases, functions, builtins, keywords, and executables.

• Aliases: Aliases are a way of shortening commands. They are only used in interactive shells, not in scripts. (This is one of the very few differences between a script and an interactive shell.) An alias is a name that is mapped to a certain string. Whenever that name is used as a command name, it is replaced by the string before executing the command. So, instead of executing:

   $ nmap -Pn -A --osscan-limit 192.168.0.1

  You could use an alias like this:

   $ alias nmapp="nmap -Pn -A --osscan-limit"
   $ nmapp 192.168.0.1

  Aliases are limited in power; the replacement only happens in the first word. If you want more flexibility, use a function. Aliases are only useful as simple textual shortcuts.

• Functions: Functions in Bash are somewhat like aliases, but more powerful. Unlike aliases, they can be used in scripts. A function contains shell commands, and acts very much like a small script; they can even take arguments and create local variables. When a function is called, the commands in it are executed. Functions will be covered in depth later in the guide.

• Builtins: Bash has some basic commands built into it, such as cd (change directory), echo (write output), and so on. You can think of them as functions that are provided already.

• Keywords: Keywords are quite like builtins, but the main difference is that special parsing rules apply to them. For example, [ is a bash builtin, while [[ is a bash keyword. They are both used for testing stuff. [[ is a keyword rather than a builtin and and is therefore able to offer an extended test:

   $ [ a < b ]
   -bash: b: No such file or directory
   $ [[ a < b ]]

  The first example returns an error because bash tries to redirect the file b to the command [ a ] (See File Redirection). The second example actually does what you expect it to. The character < no longer has its special meaning of File Redirection operator when it's used in a [[ command.

• Executables: The last kind of command that can be executed by bash is an executable, also called an external command or application. Executables are commonly invoked by typing only their name. This is because a pre-defined variable makes known to bash a list of common, file paths of residing executables. This variable is called PATH, and it is a set of directory names separated by colons -- for example, /usr/bin:/bin. When a command is specified in Bash without a pathname (e.g. myprogram, or ls), and it isn't an alias, function, builtin or keyword, bash searches through the directories in PATH, in order from left to right, to see whether they contain an executable of the name you typed. If the executable is outside a known path, the executables file path will need to be defined: if the executable is in the current directory, use ./myprogram; if it's in the directory /opt/someprog/bin, use /opt/someprog/bin/myprogram.

• Tip:
  You can use the type command to figure out the type of a command.
  For example:

   $ type rm
   rm is hashed (/bin/rm)
   $ type cd
   cd is a shell builtin

• In The Manual: Simple Commands

• In the FAQ:
  What is the difference between test, [ and [[ ?
  How can I make an alias that takes an argument?

• Alias: A name that is mapped to a string. Whenever that name is used as a command, it is replaced by the string it has mapped.

• Function: A name that is mapped to a set of commands. Whenever that name is used as a command, the function is called with the arguments provided after the function's name on the command line. Functions are the basic method of making new commands.

• Builtin: Certain commands have been built into Bash. These are handled internally whenever they are executed on the command line (and do not create a new process).

• Application: A program that can be executed by referring to its pathname (/bin/ls), or simply by its name if its location is in your PATH variable. Running an application creates a new process.

Scripts

A script is basically a sequence of commands in a file. Bash reads the file and processes the commands in order. It only moves on to the next command when the current one has ended, unless the current one has been executed asynchronously (in the background). Don't worry too much about the latter case yet -- you'll learn about how that works later on.

Virtually any example that you see in this guide can be used in a script just as well as on the command line.

Making a script is easy. Just make a new file, and put this in it at the top:

#!/usr/bin/env bash

This header is called a interpreter directive (it is also called a hashbang or shebang). It makes sure that whenever your script is executed, Bash will be used as its interpreter. The way it works is: when the kernel executes a non-binary file, it reads at the first line of the file; if the line begins with #!, the kernel uses the line to determine the interpreter that the code should be passed to. (There are other valid ways to do this, as well -- see below.) The #! must be at the very start of the file, with no spaces or blank lines before them. Your script's commands should appear on separate lines below this.

Please do not be fooled by examples on the Internet that use /bin/sh as interpreter. sh is not bash. Even though sh's syntax and bash's look very much alike and even though most bash scripts will run in sh, a lot of the examples in this guide only apply to bash and will just break or cause unexpected behaviour in sh.

Also, please refrain from giving your scripts that stupid .sh extension. It serves no purpose, and it's completely misleading (since it's going to be a bash script, not an sh script).

And by the way, it's perfectly fine if you use Windows to write your scripts, but if at all possible, avoid using Notepad for writing scripts. Microsoft Notepad can only make files with DOS-style line-endings. That means that each line you make in notepad will be ended by two characters: a Carriage Return and a Newline character. Bash reads lines as terminated by Newline characters only. As a result, the Carriage Return character will cause you incredible headache if you don't know it's there (very weird error messages). If at all possible, use a decent editor like Vim, Emacs, kate, GEdit, GVIM or xemacs. If you don't, then you will need to remove the carriage returns from your scripts before running them.

Once your script file has been made, you can call it like this:

$ bash myscript

In this example, we execute bash and tell it to read our script. When we do this, the #! line is just a comment. Bash does not do anything at all with it.

Alternatively, you can give your script executable permissions. When you do this, you can actually execute the script as an application instead of calling Bash manually:

$ chmod +x myscript
$ ./myscript

When executed in this way, the #! line tells the operating system (OS) what interpreter to use. The OS runs /usr/bin/env, which in turn runs bash, which reads our script. BASH itself ignores the # header line, just like last time.

Some people like to keep their scripts in a personal directory. Others like to keep their scripts somewhere in the PATH variable. Most like to do both at once. Here's what I suggest you do:

$ mkdir -p "$HOME/bin"
$ echo 'PATH="$HOME/bin:$PATH"' >> "$HOME/.bashrc"
$ exec bash

The first command will make a directory called bin in your home directory. It is traditional for directories that contain commands to be named bin, even when those commands are scripts and not compiled ("binary") programs. The second command will add a line to your .bashrc file which adds the directory we just made to the beginning of the PATH variable. Every new instance of Bash will now check for executable scripts in your bin directory. Finally, the third line replaces our current instance of Bash with a new one, which reads the .bashrc file.

Changes to DotFiles (such as .bashrc) never have an immediate effect. You have to take some step to re-read the files. In the example above, we used exec bash to replace the running shell. If you wanted, you could close your existing terminal and open a new one. Bash would then initialize itself again by reading .bashrc (and possibly other files). Or, you could just execute that line of code on the command line (PATH="$HOME/bin:$PATH") or manually process your .bashrc file in the running shell by running source "$HOME/.bashrc" .

In any case, we can now put our script in our bin directory and execute it as a normal command (we no longer need to prepend our script's name with its path, which was the ./ part in the previous examples):

$ mv myscript "$HOME/bin"
$ myscript

• Tip:
  While you're defining the interpreter in your header, you might want to take the time to explain your script's function and expected arguments a little too:

  #! /usr/bin/env bash
  #
  #   scriptname argument [argument] ...
  #
  # A short explanation of your script's purpose.
  #
  # Copyright [date], [name]

• Tip:
  You can use this header to specify up to one word of optional arguments that you want to pass to the interpreter. For example, the following arguments will turn on some verbose debugging:

  #! /bin/bash -xv

• But that requires knowing where Bash is installed, and it's not always in /bin. Unfortunately, you can't use this:

  #! /usr/bin/env bash -xv

• because that would require two words of arguments in the header line. Unix doesn't allow that. You can do this instead:

  #! /usr/bin/env bash
  set -xv

• For more hints, see Debugging.

• Header: The header of a script determines the application that will function as its interpreter (e.g. bash, sh, perl, ...). Colloquially, this is also called a shebang -- a slang term derived by combining hash (#) and bang (!). You will probably see the word shebang used more often than header, particularly since header has several other meanings in different contexts, while shebang only means one thing.

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