SC3014 – ShellCheck Wiki

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In POSIX sh, == in place of = is undefined.

(or "In dash, ... is not supported." when using dash)

Problematic code:

#!/bin/sh
if [ $1 == "-n" ]
then
  dry_run=1
fi

Correct code:

#!/bin/sh
if [ $1 = "-n" ]
then
  dry_run=1
fi

Rationale:

== is used for equality comparison in C-like languages, and is also supported by bash and ksh in [ .. ] and test expressions.

sh and dash only supports = for equality comparison, so use that instead.

Exceptions:

If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.

You can use # shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.

Related resources:

• Greg's Wiki
  • Bash Pitfall #20: [ bar == "$foo" ]
  • Bashism - Conditionals

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