---
tags:
  - bash
  - shell
  - scripting
  - example
  - function
  - collapse
---

# Collapsing Functions

## 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

!!! warning "FIXME"
    Add more examples!

```bash
# 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 "${args[@]}"
    }
  fi
  find "$@"
}
```

```bash
#!/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
```