mirror of
https://github.com/flokoe/bash-hackers-wiki.git
synced 2024-11-25 07:43:42 +01:00
89 lines
2.7 KiB
Plaintext
89 lines
2.7 KiB
Plaintext
====== 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
|
|
|
|
[[ $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
|
|
</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
|
|
[[ $arg = -executable ]] && args+=(-perm /u+x) || args+=("$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 "${args[@]}"
|
|
}
|
|
fi
|
|
find "$@"
|
|
}
|
|
</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
|
|
|
|
</code> |