wiki.bash-hackers.org/howto/collapsing_functions.md

====== Collapsing Functions ======

{{keywords>bash shell scripting example function collapse}}

===== What is a "Collapsing Function"? ===== A collapsing function is a function whose behavior changes depending upon the circumstances under which it's run. Function collapsing is useful when you find yourself repeatedly checking a variable whose value never changes.

===== How do I make a function collapse? ===== Function collapsing requires some static feature in the environment. A common example is a script that gives the user the option of having "verbose" output. <code> #!/bin/bash

&& verbose=1

chatter() { if $verbose ; then chatter() { echo "$@" } chatter "$@" else chatter() { : } fi }

echo "Waiting for 10 seconds." for i in {1..10}; do chatter "$i" sleep 1 done </code>

===== How does it work? ===== The first time you run chatter(), the function redefines itself based on the value of verbose. Thereafter, chatter doesn't check $verbose, it simply is. Further calls to the function reflect its collapsed nature. If verbose is unset, chatter will echo nothing, with no extra effort from the developer. ===== More examples ===== FIXME Add more examples! <code>

Somewhat more portable find -executable

FIXME/UNTESTED (I don't have access to all of the different versions of find.)

Usage: find PATH ARGS -- use find like normal, except use -executable instead of

various versions of -perm /+ blah blah and hacks

find() { hash find || { echo 'find not found!'; exit 1; }

We can be pretty sure "$0" should be executable.

if $(command find "$0" -executable 2> /dev/null) ; then unset -f find # We can just use the command find elif $(command find "$0" -perm /u+x 2> /dev/null) ; then find() { typeset arg args for arg do && args+=(-perm /u+x) || args+=("$arg") done command find "${args[@]}" } elif $(command find "$0" -perm +u+x 2> /dev/null) ; then find() { typeset arg args for arg do && args+=(-perm +u+x) || args+=("$arg") done command find "${args[@]}" } else # Last resort find() { typeset arg args for arg do && args+=(-exec test -x {} ; -print) || args+=("$arg") done command find "${args[@]}" } fi find "$@" } </code>

<code> #!/bin/bash

Using collapsing functions to turn debug messages on/off

[ "--debug" = "$1" ] && dbg=echo || dbg=:

From now on if you use $dbg instead of echo, you can select if messages will be shown

$dbg "This message will only be displayed if --debug is specified at the command line