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I want to search for a field that only has spaces. When counting, this two commands gave me different results:

grep "^\s*$" -E -c
grep "^(\s*$)" -E -c

Another thing. What is the correct way to say "spaces only or other thing":

grep "^(\s*$|STRING)" -E -c

or

grep "(^\s*$|^STRING)" -E -c

Or they are equivalent?

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1 Answer

up vote 3 down vote accepted

Plain grep doesn't understand \s or parentheses.(*) You want grep -P if available, or grep -E/egrep and write \s out as [ ^I] (where ^I is Tab; grep without -P doesn't understand \t either). Additionally, with double quotes some shells will process the \s and leave behind just s; you should always use single quotes with regexes from the shell, except for when you actually need to interpolate a variable (e.g. something like '^\s*'"$foo"'\s+\(', switching quoting in mid-parameter).

The alternatives in your second example (again, with -P and single quoting instead of double) do the same thing. The former is technically better because grep won't backtrack as much, but practically it won't matter.

(*) pedantry: backslashed parens will work, again requiring single quoting so the shell doesn't eat the backslashes. egrep is usually easier.

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While the general advice to use single quotes is right, in this particular case, it doesn't matter: ) and | are not single-character variables, so the shell won't interpolate $) or $|; and "\s" expands to \s, the backslash inside double quotes is retained except before "$\`. – Gilles Apr 17 '11 at 18:02
@geekosaur I was using the -E option but i forgot to include it here. I will edit the post. In this case why the two options give me different results? The single vs double-quotes can influence this? – user6678 Apr 17 '11 at 18:02
@Gilles: that turns out to depend on the shell; I've just gotten 3 different behaviors from three different shells. – geekosaur Apr 17 '11 at 18:16
@user6678: see my reply to Gilles; it depends on the shell (but it appears that in bash the double quotes should be okay; it's still a bad habit that can lead to problems in the future, though). \s is not supported by grep -E in any version of GNU grep I have available (checked on OSX and Ubuntu Maverick). That said, I would expect them to behave the same (if incorrectly), and they do so here. – geekosaur Apr 17 '11 at 18:21
@geekosaur: What shells are those? On Debian, I can't find a (Bourne-style) shell that prints anything but \a$| for printf %s\\n "\a$|". POSIX is clear that the backslash must be retained; I can't find anything about $ followed by an invalid character. – Gilles Apr 17 '11 at 18:32
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