Shell equality operators (=, ==, -eq)

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What is the difference between =, == and -eq in shell scripting?

Is there any difference between the following?

[ $a = $b ]
[ $a == $b ]
[ $a -eq $b ]

Is it simply that = and == are only used when the variables contain numbers?

shell bash
4 Answers

Several answers show dangerous examples. The OP's example, [ $a == $b ], specifically used unquoted variable substitution (as of the October 2017 edit). For [...] that is safe for string equality.

But if you're going to enumerate alternatives like [[...]], you must inform also that the right-hand-side must be quoted. If not quoted, it is a pattern match! (From the Bash man page: "Any part of the pattern may be quoted to force it to be matched as a string.").

Here in Bash, the two statements yielding "yes" are pattern matching, other three are string equality:

$ rht="A*"
$ lft="AB"
$ [ $lft = $rht ] && echo yes
$ [ $lft == $rht ] && echo yes
$ [[ $lft = $rht ]] && echo yes
yes
$ [[ $lft == $rht ]] && echo yes
yes
$ [[ $lft == "$rht" ]] && echo yes
$
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