how do you use 'grep --fixed-strings'

Question

A section of grep --help is:

  -E, --extended-regexp     PATTERNS are extended regular expressions
  -F, --fixed-strings       PATTERNS are strings

There are plenty examples online for the -E but I can't find any for -F

I'm not very good with regex and avoid it if I possibly can so this -F looks like a better option.

What is the syntax to grep something and see if it contains 3 different strings?

Answer 1

The -F option or --fixed-string, as you noted, disable the regexp engine and search for literal strings. It is useful to look for strings that are valid regular expressions, but would have a different meaning as such. E.g.:

grep -Fe '[warn]' app.log

The [warn] pattern is a valid Basic Regular Expression (BRE) and so without the -F option grep will search and catch any line with at least one of w,a, r, or n. (Note that the brackets are special in the shell too, so we need to quote them on the command line.)

As an example, assume we want to find the error messages from a log file like the following with lines like this (so, the lines with the [err] tag):

[info] terrace lights on
[err] garage door control not responding

Using grep 'err' would match both lines, as there's a hit in the word "terrace". grep '[err]' would also match both lines, as both have an "e" (and an "r"). But here, grep -F '[err]' would be a simple way to limit the match to the tags only. (Alternatively, one could use the default regex match and escape the brackets: grep -e '\[err\]', or grep -e '[[]err[]]' or even grep '[[]err]'. But that's uglier.)

If you have more patterns to search for, you can use the -e option multiple times:

grep -F -e '[warn]' -e '[debug]' -e '[err]' app.log

Lines matching any of the patterns (at least one) will be matched.

The -x, --line-regex option can also be useful in this context as it means it only considers a line a match if the whole line matches the pattern. E.g. grep -x foo will match if a line is foo but not if the line is foobar. And -Fx together would only find exactly matching lines, whatever special characters they contain.

Answer 2

To get the lines in test.txt that have at least one of the strings hello or goodbye, you'd use:

grep -F -e hello -e goodbye test.txt

That is, the patterns to look for are given as arguments to the -e options, and the -F option tells to treat them as fixed strings. Well, that doesn't matter with the above, but e.g. a pattern like a.*b would look for a, a dot, an asterisk, and a b, instead of the regex interpretation which would be a and b with anything in between.

Alternatively, you could put the looked-for strings in a file, one per line, and use the -f option to give the filename:

$ grep -F -f patterns.txt  test.txt
you say hello when you arrive
goodbye when you leave
but rarely say hello and goodbye at the same time

(Note that it'd be much harder to do the "all given strings" test. It's rather awkward even with regexes.)