What is a function exactly

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# 1  
Old 08-21-2009
What is a function exactly

I'm reading about function from my book on bash ("learning the bash shell"), But I'm not exactly sure what a function is -- sometimes the definitions are quite dry and or unclear. What is the difference between a function and a shell script?

And by the way, does anyone know of any other books on the bash shell that may not be as dry as the one I'm currently reading?

Follow up -- can someone also tell me what is a built in -- sometimes for example I'll type, say man cd and I'll get a page on all the builtin commands, which says this:

Shell builtin commands are commands that can be executed within the running shell's process.

Exactly what does that mean?
# 2  
Old 08-21-2009
A function is just a routine that can be called multiple times within the same program or script in this case.

Like "real" programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure, then consider using a function.
Advanced Bash Scripting Guide
# 3  
Old 08-22-2009
A built-in is simply a command that is part of the bash shell and not a separate program. For example the 'ls' command is a separate program that is executed as a child process. The export command is a built-in command that is part of bash and will not spawn a child process when invoked. Probably the simplest explanation is a built-in is a command that is part of the bash executable file while a "non built-in" command has a separate executable file. A good way to think of a function is a script within a script or a script within your interactive session. For example here is a function to check disk usage in your current directory:

echo -n "You are in: "
echo -n "It is using: "
du -s .
echo "Filesystem usage: "
df -k .

If you were to type whats in code tags into your terminal and then type spacecheck, bash would run all the commands between the {}. If you were to include this in a script the commands in the {} would be executed whenever you use spacecheck as a command in the script. For example:


if [[ conditions_are_true ]]

ls -l

As you can see once a function is defined in either a script or interactive session it is used just like any other command.

I hope this clarifies it for you.
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