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.

1
29
7/12/2012 10:17:30 AM

Accepted Answer

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
45
5/28/2015 7:56:06 PM

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

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