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Full Discussion: What is a function exactly
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
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:

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.
 

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SCRIPT(1)                                                          User Commands                                                         SCRIPT(1)

NAME
script - make typescript of terminal session SYNOPSIS
script [options] [file] DESCRIPTION
script makes a typescript of everything displayed on your terminal. It is useful for students who need a hardcopy record of an interactive session as proof of an assignment, as the typescript file can be printed out later with lpr(1). If the argument file is given, script saves the dialogue in this file. If no filename is given, the dialogue is saved in the file type- script. OPTIONS
-a, --append Append the output to file or to typescript, retaining the prior contents. -c, --command command Run the command rather than an interactive shell. This makes it easy for a script to capture the output of a program that behaves differently when its stdout is not a tty. -e, --return Return the exit code of the child process. Uses the same format as bash termination on signal termination exit code is 128+n. -f, --flush Flush output after each write. This is nice for telecooperation: one person does `mkfifo foo; script -f foo', and another can supervise real-time what is being done using `cat foo'. --force Allow the default output destination, i.e. the typescript file, to be a hard or symbolic link. The command will follow a symbolic link. -q, --quiet Be quiet (do not write start and done messages to standard output). -t[file], --timing[=file] Output timing data to standard error, or to file when given. This data contains two fields, separated by a space. The first field indicates how much time elapsed since the previous output. The second field indicates how many characters were output this time. This information can be used to replay typescripts with realistic typing and output delays. -V, --version Display version information and exit. -h, --help Display help text and exit. NOTES
The script ends when the forked shell exits (a control-D for the Bourne shell (sh(1)), and exit, logout or control-d (if ignoreeof is not set) for the C-shell, csh(1)). Certain interactive commands, such as vi(1), create garbage in the typescript file. script works best with commands that do not manipulate the screen, the results are meant to emulate a hardcopy terminal. It is not recommended to run script in non-interactive shells. The inner shell of script is always interactive, and this could lead to unexpected results. If you use script in the shell initialization file, you have to avoid entering an infinite loop. You can use for example the .profile file, which is read by login shells only: if test -t 0 ; then script exit fi You should also avoid use of script in command pipes, as script can read more input than you would expect. ENVIRONMENT
The following environment variable is utilized by script: SHELL If the variable SHELL exists, the shell forked by script will be that shell. If SHELL is not set, the Bourne shell is assumed. (Most shells set this variable automatically). SEE ALSO
csh(1) (for the history mechanism), scriptreplay(1) HISTORY
The script command appeared in 3.0BSD. BUGS
script places everything in the log file, including linefeeds and backspaces. This is not what the naive user expects. script is primarily designed for interactive terminal sessions. When stdin is not a terminal (for example: echo foo | script), then the session can hang, because the interactive shell within the script session misses EOF and script has no clue when to close the session. See the NOTES section for more information. AVAILABILITY
The script command is part of the util-linux package and is available from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils /util-linux/>. util-linux June 2014 SCRIPT(1)
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