Linux utilities: GREP

Introduction to GREP

grep, egrep, fgrep – prints line matching pattern (regular expression)

Features:

1. The ability to parse lines based on text and/or RegExes

2. Post-processor

3. Searches case-sensitively, by default

4. Searches for the text anywhere on the line

Examples:

1. grep ‘Linux’ grep1.txt – normal search – searches Linux in grep1.txt and not linux

2. grep -i ‘linux’ grep1.txt – case-insensitive search search

3. grep ‘^linux’ grep1.txt – uses ‘^’ to anchor searches at the beginning of lines

4. grep -i ‘linux$’ grep1.txt – uses ‘$’ to anchor searches at the end of lines

Note: Anchors are RegEx characters (meta-characters). They’re used to match at the beginning and end of lines

5. grep -a ‘myfuncion’ binfile – searches binary file as if it is a text file

6. grep ‘[0-9]’ grep1.txt – returns lines containing at least 1 number

7. grep ‘[a-z]’ grep1.txt – returns lines containing at least 1 alphabet

8. rpm -qa | grep grep – searches the package database for programs named ‘grep’

9. rpm -qa | grep -i xorg 􀀀 wc -l – returns the number of pacakges with ‘xorg’ in their names

10. grep -c ‘name’ myfile1 myfile2 – returns the count of matching lines in each file

11. grep -v sshd messages – performs and inverted search (all but ‘sshd’ entries will be returned)

13. grep -C 2 sshd messages – returns 2 lines, above and below matching line

14 grep -o ‘matchtext’ myfile – returns only the matching part and not the complete line.

Note: Most, if not all, Linux programs log linearly, which means one line after another, from the earliest to the current

Note: Use single or double quotes to specify RegExes

Also, execute ‘grep’ using ‘egrep’ when RegExes are being used