1. Chin-Chih Chang chang@cs.twsu.edu
CS 497C – Introduction to UNIX Lecture 28: - Filters Using Regular Expressions – grep and sed
Chin-Chih Chang

2. grep: Searching for a Pattern
The sample database shown on Page 434 can be found in
grep is a filter used to search a file for a pattern.
It scans a file for the occurrence of a pattern and, depending on the options used, displays:
Lines containing the selected pattern.

3. grep: Searching for a Pattern
Lines not containing the selected pattern. (-v)
Line numbers where the pattern occurs. (-n)
Number of lines containing the pattern. (-c)
Filenames where the pattern occurs. (-l)
Its syntax treats the first argument as the pattern and the rest as filenames:
grep options pattern filename(s)
$ grep sales emp.lst
When you use multiword strings as the pattern, you must quote the pattern.

4. grep Options $ grep “neil o’bryan” emp.lst
The -c (count) option counts the occurrences.
$ grep -c directory emp*.lst
The -n (number) option can be used to display the line numbers containing the pattern.
$ grep -n ‘marketing’ emp.lst

5. grep Options
The -l (list) option displays only the names of files where a pattern has been found.
$ grep -l ‘manager’ *.lst
The -i (ignore) option makes the match case-insensitive.
To look for a pattern that begins with a hyphen, use the -e option.
$ grep –e “-mtime” /var/spool/cron/crontabs/*

6. grep Options
In Linux, you can use the -e option to match multiple patterns.
$ grep -e woodhouse -e wood emp.lst
A regular expression is an ambiguous expression formed with some special and ordinary characters, which is expanded by a command to match more than one string.
grep uses a regular expression to match a group of similar patterns.

7. Regular Expressions
A regular expression uses a character class that encloses a group of characters within a pair of rectangular brackets [].
$ grep “wo[od][de]house” emp.lst
Regular expressions use the ^(caret) to negate the character class, while the shell use ! (bang).
A single nonalphabetic string is represented by [^a-zA-Z].

8. Regular Expressions
The * (asterisk) matches the zero or more occurrences of the preceding character.
$ grep “wilco[cx]k*s*” emp.lst
A . (dot) matches a single character. The shell use the ? character to indicate that.
The dot along with the * (.*) signifies any number of characters, or none.
$ grep “p.*woodhouse” emp.lst

9. Regular Expressions
A pattern can be matched at the beginning of a line with a ^, and at the end with a $.
Because it is the command that interprets these characters, a regular expression should be quoted to prevent the shell from interfering.
$ grep “^2” emp.lst
The . and * lose their meanings when placed inside the character class. Then you need to escape these characters.

10. egrep and fgrep: The Other Members
egrep extends grep’s capabilities. It uses | to delimit multiple patterns.
$ egrep ‘woodhouse|woodcock’ emp.lst
fgrep accepts only fixed strings and is faster than the other two.
Both commands support the -f (file) option to take such patterns from the file.
fgrep –f pat.lst emp.lst