Check if a variable exists in a list in Bash

Question

I am trying to write a script in bash that check the validity of a user input.
I want to match the input (say variable x) to a list of valid values.

<

Answer 1

The shell built-in compgen can help here. It can take a list with the -W flag and return any of the potential matches it finds.

# My list can contain spaces so I want to set the internal
# file separator to newline to preserve the original strings.
IFS=$'\n'

# Create a list of acceptable strings.
accept=( 'foo' 'bar' 'foo bar' )

# The string we will check
word='foo'

# compgen will return a list of possible matches of the 
# variable 'word' with the best match being first.
compgen -W "${accept[*]}" "$word"

# Returns:
# foo
# foo bar

We can write a function to test if a string equals the best match of acceptable strings. This allows you to return a 0 or 1 for a pass or fail respectively.

function validate {
  local IFS=$'\n'
  local accept=( 'foo' 'bar' 'foo bar' )
  if [ "$1" == "$(compgen -W "${accept[*]}" "$1" | head -1)" ] ; then
    return 0
  else
    return 1
  fi
}

Now you can write very clean tests to validate if a string is acceptable.

validate "blah" || echo unacceptable

if validate "foo" ; then
  echo acceptable
else 
  echo unacceptable
fi