#036 Inference

How to test for the presence of functions in order to branch to arbitrary functions by name.

Notes

In shell scripts, it is often useful to branch to a specific function given a parameter that matches the function name in some way.

A naive approach is to encode all the options with a case statement. For example:

function run_a() {}
function run_b() {}
name=$1
case ${name} in
a|b)
  run_${name}
*}
echo "no match"
;;
esac

But that is a bit tedious, because each new function not only needs to be defined, but also added to the case statement.

The Trick

The type builtin command can be used to test for shell functions (among other things). For example:

if type  -t "${function_name}" 2>/dev/null | grep -q 'function'
then
  echo "calling the function now we know it exists.."
  $function_name
else
  echo "there is no function by that name"
fi

Test Code

The test.sh script demonstrates the technique in a bit more detail. Here’s a console transcript of it in action:

Credits and References

• type BASH builtin command
• Using case statements - Bash Guide for Beginners
• Determine if a function exists in bash - stackoverflow
• Test for function’s existence that can work on both bash and zsh? - stackexchange