We want to create a simple script with 2 commands:

1. greet-eng: will print "Hello!"
2. greet-ita: will print "Ciao!"

Anything else has to print out an error message explaining what are the accepted commands.

We end up with the following code:

#!/bin/sh

if [ $1 = "greet-eng" ] ; then
        echo "Hello!"
elif [ $1 = "greet-ita" ] ; then
        echo "Ciao!"
else
        echo "Supported commands are 'greet-ita' and 'greet-eng'"
fi

In production it runs fine for some time until a user tells us that he got the following error:

./script.sh: 3: [: =: unexpected operator
./script.sh: 5: [: =: unexpected operator
Supported commands are 'greet-ita' and 'greet-eng'

What happened?

Explanation

The problem here is in the two if statements: we put the quote on the wrong side! String literals don't need to be quoted in bash (unless they contain wildcards).

When the script is executed, in

if [ $1 = "greet-eng" ] ; then

the $1 is replaced with its current value. If it is empty, the statement becomes:

if [ = "greet-eng" ] ; then

resulting in a syntax error.

Solution

If we move the quotes, everything works as expected:

#!/bin/sh

if [ "$1" = greet-eng ] ; then
        echo "Hello!"
elif [ "$1" = greet-ita ] ; then
        echo "Ciao!"
else
        echo "Supported commands are 'greet-ita' and 'greet-eng'"
fi

Bash

If you can use bash instead of sh, you can use the [[ operator:

#!/bin/bash

if [[ $1 = greet-eng ]] ; then
        echo "Hello!"
elif [[ $1 = greet-ita ]] ; then
        echo "Ciao!"
else
        echo "Supported commands are 'greet-ita' and 'greet-eng'"
fi