How to match a substring using Posix Shell?

Question

What's an idiomatic way to test if a string contains a substring in a Posix Shell?

Basically this, but Posix:

[[ ${my_haystack} == *${my_needle}* ]]

Non-Posix Example

I'm looking for the equivalent of this, but that works in a Posix / Almquist / dash / ash shell:

#!/bin/bash

set -e
set -u

find_needle() {
    my_haystack="${1}"
    my_needle="${2}"

    if [[ ${my_haystack} == *${my_needle}* ]]; then
        echo "'${my_haystack}' contains '${my_needle}'"
    else
        echo "'${my_haystack}' does NOT contain '${my_needle}'"
    fi
}

find_needle "${1:-"haystack"}" "${2:-"a"}"

(that doesn't work in sh)

My ideal solution would be one that doesn't require the use of a subshell or pipe, and that doesn't exit on failure in strict mode.

Workaround

This works, but I'm wondering if there's another way to test a substring without echoing and piping to grep.

#!/bin/sh

set -e
set -u

find_needle() {
    my_haystack="${1}"
    my_needle="${2}"
    if echo "${my_haystack}" | grep -q "${my_needle}"; then
        echo "'${my_haystack}' contains '${my_needle}'"
    else
        echo "'${my_haystack}' does NOT contain '${my_needle}'"
    fi
}

find_needle "${1:-"haystack"}" "${2:-"a"}"

Or maybe this is the most idiomatic way?

Debatable if it's idiomatic, but case $haystack in (*$needle*) echo yes;; (*) echo no;; esac is POSIX. The ( are optional, but I like them. Note grep treats 'needle' as a regexp (BRE) with . special but ? literal not special as in shell pattern including bash [[, also leading ^ and trailing $ special, and [charlist] similar but not identical. — dave_thompson_085
– dave_thompson_085, Commented Aug 19, 2022 at 7:22
Are you asking for a way without piping or for the most idiomatic way? My go-to for POSIX is with grep, but see this thread. — root
– root, Commented Aug 22, 2022 at 0:16
How did you get the nab @root as a username? That's awesome. I'm root on npm and I tried to get it on github. I envy you. :) Yes, I'm asking for both. — root-aj
– root-aj, Commented Aug 22, 2022 at 15:40

root-aj · Accepted Answer · 2022-08-22 15:41:28Z

Substring match with `case`

As @dave_thompson_085 points out [1], you can use case:

case $haystack in
    *$needle*)
        return 0
        ;;
    *)
        return 1
        ;;
esac

Example Script

✅ no subshell
✅ no pipe
🤷‍♂️ simplest, most idiomatic? maybe

#!/bin/sh

set -e
set -u

test_substring() {
    haystack="${1}"
    needle="${2}"

    case $haystack in
        *$needle*)
            return 0
            ;;
        *)
            return 1
            ;;
    esac
}

find_needle() {
    my_haystack="${1}"
    my_needle="${2}"
    if test_substring "${my_haystack}" "${my_needle}"; then
        echo "'${my_haystack}' contains '${my_needle}'"
    else
        echo "'${my_haystack}' does NOT contain '${my_needle}'"
    fi
}

find_needle haystack a
find_needle haystack x

Output:

'haystack' contains 'a'
'haystack' does NOT contain 'x'

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How to match a substring using Posix Shell?

Non-Posix Example

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Substring match with `case`

Example Script

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Substring match with `case`