What is a function exactly Post: 302346429

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Top Forums UNIX for Dummies Questions & Answers What is a function exactly Post 302346429 by ilikecows on Saturday 22nd of August 2009 12:49:38 AM

08-22-2009

Registered User

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:

Code:

spacecheck()
{
echo -n "You are in: "
pwd
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:

Code:

spacecheck()
{
...
}

if [[ conditions_are_true ]]
then
spacecheck
fi

ls -l
pwd
spacecheck

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.

ilikecows

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LEARN ABOUT LINUX

rbash

RBASH(1)						      General Commands Manual							  RBASH(1)

NAME

       rbash - restricted bash, see bash(1)

RESTRICTED SHELL

       If  bash  is  started with the name rbash, or the -r option is supplied at invocation, the shell becomes restricted.  A restricted shell is
       used to set up an environment more controlled than the standard shell.  It behaves identically to bash with the exception that the  follow-
       ing are disallowed or not performed:

       o      changing directories with cd

       o      setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV

       o      specifying command names containing /

       o      specifying a file name containing a / as an argument to the .  builtin command

       o      specifying a filename containing a slash as an argument to the -p option to the hash builtin command

       o      importing function definitions from the shell environment at startup

       o      parsing the value of SHELLOPTS from the shell environment at startup

       o      redirecting output using the >, >|, <>, >&, &>, and >> redirection operators

       o      using the exec builtin command to replace the shell with another command

       o      adding or deleting builtin commands with the -f and -d options to the enable builtin command

       o      using the enable builtin command to enable disabled shell builtins

       o      specifying the -p option to the command builtin command

       o      turning off restricted mode with set +r or set +o restricted.

       These restrictions are enforced after any startup files are read.

       When a command that is found to be a shell script is executed, rbash turns off any restrictions in the shell spawned to execute the script.

SEE ALSO

       bash(1)

GNU Bash-4.0							    2004 Apr 20 							  RBASH(1)

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