Overview

Linux has a useful tool called grep. By making good use of this tool, you can extract useful information from text files.

• grep document
  • options
• examples
  • grep pattern -A num text
  • grep pattern -B num text
  • grep pattern -C num text
  • grep pattern -c text
  • grep -e pattern text
  • grep -f file text
  • grep -n pattern text
  • grep -v pattern word
• Combining with other commands
  • with greater-than (redirection)
  • with awk
  • with sort, uniq

grep document

$ grep -h
usage: grep [-abcdDEFGHhIiJLlMmnOopqRSsUVvwXxZz] [-A num] [-B num] [-C[num]]
	[-e pattern] [-f file] [--binary-files=value] [--color=when]
	[--context[=num]] [--directories=action] [--label] [--line-buffered]
	[--null] [pattern] [file ...]

options

• -A num --after-context: Display num additional lines after the matched line
• -B num --before-context: Display num additional lines before the matched line
• -C num --context: Display num lines before and after the matched line
• -c --count: Count the number of matched lines
• -E pattern --regexp=pattern: Output lines matching the regular expression pattern
• -f file --file=file: Read pattern from file
• -r --recursive: Recursively search subdirectories
• -n --line-number: Display line numbers
• -v pattern: Output all lines except those matching the pattern

examples

Let's assume we have a test.txt file with the following content.

$ cat test.txt

apple is sweety
banana is long
apple mango
pitch is
water melon is expensive
grapes are cheep
pear

grep pattern -A num text

$ grep apple -A 2 test.txt
apple is sweety
banana is long
apple mango
pitch is
water melon is expensive

grep pattern -B num text

$ grep apple -B 1 test.txt
apple is sweety
banana is long
apple mango

grep pattern -C num text

$ grep apple -C 1 test.txt
apple is sweety
banana is long
apple mango
pitch is

grep pattern -c text

grep apple -c  test.txt
2

grep -e pattern text

$ grep -e "ap" test.txt
apple is sweety
apple mango
grapes are cheep

grep -f file text

$ cat regex.txt
apple

$ grep -f regex.txt test.txt
apple is sweety
apple mango

grep -n pattern text

$ grep -n appl test.txt
1:apple is sweety
3:apple mango

grep -v pattern word

Output lines excluding those containing "appl".

$ grep -v appl test.txt
banana is long
pitch is
water melon is expensive
grapes are cheep
pear

Combining with other commands

with greater-than (redirection)

Save grep results to out

$ grep -v appl test.txt > out.txt
$ cat out.txt
banana is long
pitch is
water melon is expensive
grapes are cheep
pear

with awk

Print only the first word

$ grep -v appl test.txt | awk '{print $1}'
banana
pitch
water
grapes
pear

with sort, uniq

Sort the first word and remove duplicates

$ grep ap test.txt | awk '{print $1}' | sort | uniq
apple
grapes

If you want to study or test regex further, check out regexr.com.