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https://github.com/rawiriblundell/wiki.bash-hackers.org
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100 lines
3.0 KiB
Markdown
100 lines
3.0 KiB
Markdown
# Collapsing Functions
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## What is a "Collapsing Function"?
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A collapsing function is a function whose behavior changes depending
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upon the circumstances under which it's run. Function collapsing is
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useful when you find yourself repeatedly checking a variable whose value
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never changes.
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## How do I make a function collapse?
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Function collapsing requires some static feature in the environment. A
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common example is a script that gives the user the option of having
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"verbose" output.
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#!/bin/bash
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[[ $1 = -v || $1 = --verbose ]] && verbose=1
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chatter() {
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if [[ $verbose ]]; then
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chatter() {
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echo "$@"
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}
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chatter "$@"
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else
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chatter() {
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:
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}
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fi
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}
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echo "Waiting for 10 seconds."
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for i in {1..10}; do
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chatter "$i"
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sleep 1
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done
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## How does it work?
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The first time you run chatter(), the function redefines itself based on
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the value of verbose. Thereafter, chatter doesn't check \$verbose, it
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simply is. Further calls to the function reflect its collapsed nature.
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If verbose is unset, chatter will echo nothing, with no extra effort
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from the developer.
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## More examples
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FIXME Add more examples!
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# Somewhat more portable find -executable
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# FIXME/UNTESTED (I don't have access to all of the different versions of find.)
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# Usage: find PATH ARGS -- use find like normal, except use -executable instead of
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# various versions of -perm /+ blah blah and hacks
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find() {
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hash find || { echo 'find not found!'; exit 1; }
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# We can be pretty sure "$0" should be executable.
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if [[ $(command find "$0" -executable 2> /dev/null) ]]; then
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unset -f find # We can just use the command find
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elif [[ $(command find "$0" -perm /u+x 2> /dev/null) ]]; then
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find() {
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typeset arg args
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for arg do
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[[ $arg = -executable ]] && args+=(-perm /u+x) || args+=("$arg")
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done
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command find "${args[@]}"
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}
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elif [[ $(command find "$0" -perm +u+x 2> /dev/null) ]]; then
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find() {
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typeset arg args
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for arg do
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[[ $arg = -executable ]] && args+=(-perm +u+x) || args+=("$arg")
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done
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command find "${args[@]}"
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}
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else # Last resort
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find() {
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typeset arg args
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for arg do
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[[ $arg = -executable ]] && args+=(-exec test -x {} \; -print) || args+=("$arg")
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done
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command find "${args[@]}"
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}
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fi
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find "$@"
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}
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\<code\> \#!/bin/bash \# Using collapsing functions to turn debug
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messages on/off
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\[ "--debug" = "\$1" \] && dbg=echo \|\| dbg=:
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\# From now on if you use \$dbg instead of echo, you can select if
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messages will be shown
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\$dbg "This message will only be displayed if --debug is specified at
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the command line
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