College of Engineering & CS
Wright State University
Dayton,
Ohio 45435-0001
|
|
CEG 333: Introduction to UnixPrabhaker Mateti
|
Grep is a tool for searching for text in files. Given a regular expression, grep
displays every matching line in a file (or standard input).
| Option | Meaning |
grep PATTERN [FILENAME...] |
Print all the lines matching a SMRE PATTERN in the given files. If no files are given, search stdin. |
-i |
Make the search case-insensitive ("a" will match both "a", and "A"). |
-r |
Recurse through the given directories. |
-v |
Invert the search: match every line not containing PATTERN. |
-A NUM |
Show NUM lines of context after every matching line. |
-B NUM |
Show NUM lines of context before every matching line. |
-C NUM |
Show NUM lines of context before and after every matching line. |
-c |
Print only a count of matching lines, not the matches themselves. |
-E |
Use extended regular expressions (allows for more complex
patterns). Equivalent to the egrep command. |
Example 1: find all lines in either files under the "CEG333" directory or the ./README file which contain "Unix" or "Linux", with any capitalization.
"(uni|linu)x" is a regular expression meaning "either uni or linu followed by an x", so:
$ grep -ri (uni|linu)x CEG333 README
-bash: syntax error near unexpected token `('
Oops. Many of the special characters in regular expressions also have special meaning to the shell. When the shell sees them, it tries to interpret them and fails. So most regular expressions should be quoted (enclosed in quotation marks) so that the shell knows to just pass them along to grep.
Note: double (") and single (') quotation marks have slightly different meanings (see "man bash"). Usually, single quotes should be used.
$ grep -ri '(uni|linu)x' CEG333 README
This time, grep prints some lines, but they aren't the ones the
pattern should match. Notice the -E option in the
above table? By default, grep assumes that the pattern is a basic
regular expression, in which fewer characters have a special
meaning. To correctly execute the regular expression, either put
grep into extended mode:
$ grep -Eri '(uni|linu)x' CEG333 README
or escape the special syntax (with backslashes) to tell grep it should retain its meaning:
$ grep -ri '\(uni\|linu\)x' CEG333 README
Example 2: show a ps listing without kernel
processes (those that have their names enclosed in square
brackets).
$ ps aux | grep -v '\[.*\]'
Since it's likely large, it may be better to view it one screen-full at a time ("less" is a pager. Another, simpler, pager is "more"):
$ ps aux | grep -v '\[.*\]' | less