February 13, 2013

A bashism a week: negative matches

Probably due to the popular way of expressing the negation of a character class in regular expressions, it is common to see negative patterns such as [^b] in shell scripts.

However, using an expression such as [^b] where the shell is the one processing the pattern will cause trouble with shells that don't support that extension. The right way to express the negation is using an exclamation mark, as in: [!b]

Big fat note: this only applies to patterns that the shell is responsible for processing. Some of such cases are:

case foo in
        echo bar
# everything but backups:
for file in documents/*[!~]; do
    echo doing something with "$file" ...

If the pattern is processed by another program, beware that most won't interpret the exclamation the way the shell does. E.g.

$ printf "foo\nbar\nbaz\n" | grep '^[^b]'
$ printf "foo\nbar\nbaz\n" | grep '^[!b]'

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