How to escape single quotes in Bash/Grep?

问题:

I want to search with grep for a string that looks like this:

something ~* 'bla'

I tried this, but the shell removes the single quotes argh..

grep -i '"something ~* '[:alnum:]'"' /var/log/syslog

What would be the correct search?

回答1:

grep -i "something ~\* '[[:alnum:]]*'" /var/log/syslog

works for me.

• escape the first * to match a literal * instead of making it the zero-or-more-matches character:
  ~* would match zero or more occurrences of ~ while
  ~\* matches the expression ~* after something
• use double brackets around :alnum: (see example here)
• use a * after [[:alnum::]] to match not only one character between your single quotes but several of them
• the single quotes don't have to be escaped at all because they are contained in an expression that is limited by double quotes.

回答2:

If you do need to look for quotes in quotes in quotes, there are ugly constructs that will do it.

echo 'And I said, "he said WHAT?"'

works as expected, but for another level of nesting, the following doesn't work as expected:

echo 'She said, "And I said, \'he said WHAT?\'"'

Instead, you need to escape the inner single quotes outside the single-quoted string:

echo 'She said, "And I said, '\''he said WHAT?'\''"'

Or, if you prefer:

echo 'She said, "And I said, '"'"'he said WHAT?'"'"'"'

It ain't pretty, but it works. :)

Of course, all this is moot if you put things in variables.

[ghoti@pc ~]$ i_said="he said WHAT?"
[ghoti@pc ~]$ she_said="And I said, '$i_said'"
[ghoti@pc ~]$ printf 'She said: "%s"\n' "$she_said"
She said: "And I said, 'he said WHAT?'"
[ghoti@pc ~]$ 

:-)

回答3:

• character classes are specified with [[:alnum:]] (two brackets)
• [[:alnum:]] is matching only one character. To match zero or more characters [[:alnum:]]*

• you can just use " " to quote the regex:

  grep -i "something ~\* '[[:alnum:]]*'" /var/log/syslog
  

Matteo

回答4:

It seems as per your expression, that you are using first ', then ". If you want to escape the single quotes, you can either use ' and escape them, or use double quotes. Also, as Matteo comments, character classes have double square brackets Either:

grep -i "something \~\* '[[:alnum:]]+'" /var/log/syslog

or

grep -i 'something ~* \'[[:alnum:]]+\'' /var/log/syslog