Can anyone tell me the technical differences between
fgrep and provide suitable examples?
When do I need to use
egrep and vice versa?
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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.
Interpret PATTERN as an extended regular expression
-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.
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
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
egrep is the same as grep -E. fgrep is the same as grep -F. Direct invocation as either egrep or fgrep is deprecated, but is provided to allow historical applications that rely on them to run unmodified.
grep -F if you don't want the grepped string to be interpreted as a pattern.
grep -E if you need to use an extended regexp.
fgrep are basically equivalent to
grep -E and
grep -F (respectively):
-E, --extended-regexp Interpret PATTERN as an extended regular expression (ERE, see below). (-E is specified by POSIX.) -F, --fixed-strings Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched. (-F is specified by POSIX.)
Error messages might differ though.
From "man grep":
three variant programs egrep, fgrep and rgrep are available. egrep is the same as grep -E. fgrep is the same as grep -F. rgrep is the same as grep -r. Direct invocation as either egrep or fgrep is deprecated, but is provided to allow historical applications that rely on them to run unmodified.
If you search for a verbatim string, and want to be sure that the string you passed is interpreted verbatim (i.e. without risking that say a dot or question mark will be interpreted as something else) use fgrep or egrep -F.
Btw. I really encourage you to give ack a try - it supports PREGs, and has more useful defaults (i.e. coloring, skipping probably-not-what-you-want directories like .svn, ability to follow symlinks, and it's a bit easier to type ack than grep -E).