bash, dash and string comparison

Question

I am trying to compare two strings in a simple shell script. I was using /bin/sh instead of /bin/bash, and after countless hours of debugging, it turns out sh (which is actually dash) can't handle this block of code:

if [ "$var" == "string" ]
then
    do something
fi

What is a portable way to compare strings using /bin/sh? I know I can always do the opposite by using !=, but I am wondering about a cleaner, portable way.

Omaromara

You can use [[ $var == "string" ]] , which is POSIX, but optional (afaik). Or you use [ "$var" = "string" ] . Note the "" around the variable in the single-bracket edition: it's required in case $var is empty – Roche

The important part is the quotes around $var as litb mentioned. Without the quotes, [ $var = "value" ] becomes [ = "value" ] which confuses the shell pretty horrendously. You will probably see an error like "[: =: unary operator expected" when you encounter an empty variable otherwise. – Bochum

I understand about "$var" vs. $var, my problem was == vs. = – Omaromara

[[ ]] is reserved by POSIX, but not at all defined. It's just reserved because it's a Korn feature I think. – Sandhog

Answer 1

dash is a very strict POSIX shell, if it work in dash it is almost certain it would work in other POSIX shell.

Try:

if [ "$var" = "string" ]
then
    some_command
fi

Feculent

If you are thinking "what's the difference here to the syntax in the question": it's using = instead of == – Duren

How can you use wildcards in this example? I tried if [ "$OSTYPE" = "linux"* ]; then but it doesn't get linux-arm, for example... – Candracandy

Answer 2

Why is there even a possibility that your script will be run by the "wrong" shell? I would think you could make that a pre-requisite of your product by using the standard sh-bang line at the top of your script:

#!/bin/bash

Even if a user uses a different shell, the other shells are generally still there and, if not, simply complain and state that they are a pre-req.

Exactly the same way that a specific kernel level, or the existence of awk, can be a pre-req.

For your specific question, I believe both sh and bash allow the single '=' to be used for string comparisons - that is POSIX behavior:

if [ "a" = "a" ]; then
    echo yes
fi

yes

Koenraad

"Why is there even a possibility that your script will be run by the "wrong" shell? " - Is it possible to control which shell is used by Autoconf? (That's the reason I'm searching for the answer) – Carol

Answer 3

Use = instead of ==. Comparisons are handled by test(1). /usr/bin/[ is typically a link to /usr/bin/test . The only difference is that if you use [ in a shell script, the ] is required as well.

Note that bash has a built-in test/[, so it doesn't actually use /usr/bin/test.

Jacobah

Are there any official spec about the builtin test and single or double equal sign? – Gassing

Answer 4

The answers already posted are certainly correct, but it may be worthwhile to note that occasionally parameter expansion can serve the same purpose with perhaps some additional flexibility.

% p() { printf 'notvar = %b\n' "${notvar##"${string1}"}${string2}" ; }
% string1='some stuff about things\c'
% string2='some different stuff maybe'
% notvar="$string1" p
> 'some different stuff maybe'
% notvar="$string2" p
> 'some stuff about things'

Ok, so the above isn't super-useful as is, but also consider that you can use the similar methods for testing variables in here-documents, in-line variable assignments if necessary (to a degree...), or even just as a shorter (and faster!) means of writing your first statement.

[ ! "${var##"string"}" ] && _MATCH || _NOMATCH

Or even...

[ ${#var#*"${s=string}"} -lt ${#var} ] && _SUB_STRING_TEST=TRUE

Possibly even...

% p() { printf '%s is %s of %s' "$2" "${var_chk-not}" "$1"
> }<<HEREDOC
> ${in="${1##*"${2}"*}"}
> ${in:-
>     ${in="${1##"${2}"}"}
>     ${in:-${var_chk=all}
>     ${var_chk=some}
> }
> HEREDOC
%

Arundell

Not sure I see the big benefit of these techniques unless you're trying to win an obfuscation contest. – Buckeye

Wow...I need someone to ELI5 :) – Tower

Answer 5

you can use awk

awk 'BEGIN{
 string1="test"
 string2="tes1t"
 if(s1==s2){
    print "same string"
 }else{
    print "not same"
 }
}'

Lulululuabourg