2.9 KiB
====== 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.
#!/bin/bash
[[ $1 = -v || $1 = --verbose ]] && 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
===== 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!
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
[[ $arg = -executable ]] && args+=(-perm /u+x) || args+=("$arg")
done
command find "${args[@]}"
}
elif [[ $(command find "$0" -perm +u+x 2> /dev/null) ]]; then
find() {
typeset arg args
for arg do
[[ $arg = -executable ]] && args+=(-perm +u+x) || args+=("$arg")
done
command find "${args[@]}"
}
else # Last resort
find() {
typeset arg args
for arg do
[[ $arg = -executable ]] && args+=(-exec test -x {} ; -print) || args+=("$arg")
done
command find "${args[@]}"
}
fi
find "$@"
}
#!/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