What is the difference between `grep`, `egrep`, and `fgrep`?

  • Can anyone tell me the technical differences between grep, egrep, and fgrep and provide suitable examples?

    When do I need to use grep over egrep and vice versa?

    superuser has a great illustration of the differences between grep, egrep, fgrep and pgrep: http://superuser.com/questions/508881/what-is-the-difference-between-grep-pgrep-egrep-fgrep

  • Caleb

    Caleb Correct answer

    9 years ago
    • egrep is 100% equivalent to grep -E
    • fgrep is 100% equivalent to grep -F

    Historically these switches were provided in separate binaries. On some really old Unix systems you will find that you need to call the separate binaries, but on all modern systems the switches are preferred. The man page for grep has details about this.

    As for what they do, -E switches grep into a special mode so that the expression is evaluated as an ERE (Extended Regular Expression) as opposed to its normal pattern matching. Details of this syntax are on the man page.

    -E, --extended-regexp
    Interpret PATTERN as an extended regular expression

    The -F switch switches grep into a different mode where it accepts a pattern to match, but then splits that pattern up into one search string per line and does an OR search on any of the strings without doing any special pattern matching.

    -F, --fixed-strings
    Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched.

    Here are some example scenarios:

    • You have a file with a list of say ten Unix usernames in plain text. You want to search the group file on your machine to see if any of the ten users listed are in any special groups:

      grep -F -f user_list.txt /etc/group
      

      The reason the -F switch helps here is that the usernames in your pattern file are interpreted as plain text strings. Dots for example would be interpreted as dots rather than wild-cards.

    • You want to search using a fancy expression. For example parenthesis () can be used to indicate groups with | used as an OR operator. You could run this search using -E:

      grep -E '^no(fork|group)' /etc/group
      

      ...to return lines that start with either "nofork" or "nogroup". Without the -E switch you would have to escape the special characters involved because with normal pattern matching they would just search for that exact pattern;

      grep '^no\(fork\|group\)' /etc/group
      

    Some systems don't have `egrep` or `fgrep`; `-E` and `-F` are standard. There are actually small incompatibilities in `egrep`: it treats `{` slightly differently.

    `fgrep` uses the Aho-Corasick backend, while `grep` uses a modified version of Commentz-Walter. This means that `grep` has a worst case O(mn) complexity while `fgrep` is at worst O(m+n).

    Gilles "au contraire mon capitan", some systems (older perhaps) may not have -F option to standard grep but have a fgrep, some ppl complain about that on the original thread I found about using grep for special characters

    I recently happened upon an Android system on which `egrep` and `grep -E` were handled by different executables, BusyBox and BSD grep. The two variants had behavioral differences for non-Posix regex constructs, such as `\s`. This led to much confusion...

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Content dated before 6/26/2020 9:53 AM