How to test if a command is a shell reserved word?

Question

I am writing a bash script and I would like to verify if a string is a shell reserved word (like if, for, alias, etc...).

How can I can do this?

Answer 1

#!/bin/bash

string="$1"

if [[ $(type "$string" 2>&1) == "$string is a shell"* ]]; then
    echo "Keyword $string is reserved by shell"
fi

Answer 2

If you want only the shell keywords, then:

#!/bin/bash
string="$1"
[[ $(type -t "$string" 2>&1) == "keyword" ]] && echo reserved || echo not reserved

builtins won't pass this test (only keywords).

A way of doing this with possibility of extending in various cases:

#!/bin/bash
string="$1"
checkfor=('keyword' 'builtin')
for ((i=0;i<${#checkfor[@]};i++))
do
  [[ $(type -t "$string" 2>&1) == "${checkfor[$i]}" ]] && reserved=true && break || reserved=false
done
[[ $reserved == true ]] && echo reserved || echo not reserved

command, hash, alias, type etc.. (builtins) will pass the above test as well as keywords.

You can add other possible test conditions by adding an element into the array checkfor:

checkfor=('keyword' 'builtin' 'file' etc...)