I'm studying bash scripts. I got the sample script below from the web.
#!/bin/bash
str="test"
if [ x$str == x"test" ]; then
echo "hello!"
fi
What is x on fifth line(x$str and x"test")?
Does "x" have special meaning?
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2 Answers
No special meaning, but if $str was empty, then the
if [ $str == "test" ]
would result in a substitution of nothing into the test and it would be like this
if [ == "test" ]
which would be a syntax error. Adding the X in front would resolve this, however quoting it like this
if [ "$str" == "test" ]
is a more readable and understandable way of achieving the same.
3 Comments
ewanm89
Looks like the example was taken from the testcases used to test a fresh bash compile, only reference I've found to the prepend x method was arguing that the "" method was better and whether the test case should be changed.
Soren
What is better is a matter of taste -- I have seen the x method a few places, and im sure there are places in pipelines or deep substitutions where it would actually make the code more readable -- however for simple cases like the one above, the "" is intuitively understood and would never have raised a question.
ewanm89
Yeah, I didn't say it was my argument, I said it was the argument on the GNU bash mailing list.
It's to make sure that the left side of the expression in your example is not empty. If str was not set, the condition would otherwise be [ == "test" ] which would give an error.
Instead of using an single letter to make it not-empty, you could also put the variable inside double quotation characters, and skip the x completely ([ "$str" == "test" ]).
answered Nov 5, 2011 at 13:35
Some programmer dude
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