grep

grep matches patterns. it uses regex to match patterns

grep expression file

Example

grep hello helloworld.txt

will searhc for hello in the file helloworld.txt

^

^ matches all the lines that begin with hello. ^ goes at the beginning of the search expression.

grep ^hello helloworld.txt

$

$ will give you all the lines that end with your search expression. $ goes at the end of the search expression.

grep hello$ helloworld.txt 

will give you all the lines that end with hello in your helloworld.txt file.

-c, –count

-c gives you count. For example:

grep -c ^hello helloworld.txt
grep -c hello$ helloworld.txt

will give you the amount of lines that start with hello and the amount of lines that end with hello.

-i, –ignore-case

By default, grep is case-sensitive. -i gives you case-insensitive results.

-v, –invert-case

select non-mathcing lines

egrep -vi 'hello|world' file.txt

will find all lines that do not contain hello or world.

Chracters

[ ] search for a character

[] let you search for a character. For example:

grep [h] in helloworld.txt

will output all the h instances in helloworld.txt. You can also search for multiple characters, like so:

grep [hpokj] hellowolrd.txt

will return all the lines that have our given characters.

^[ ] and [ ]$

will output lines that begin ^ or end $ with our specified characters. For example:

grep ^[hpokj] hellowolrd.txt
grep [hpokj]$ hellowolrd.txt

[a-z] search for a character range

grep [a-g] file.txt
grep [1-9] file.txt

-f, –file=FILE take pattern input from a file

grep -f grepinput file.txt

OR

grep -f inputFile.txt fileToSearch.txt

-l, –files-with-matches

list files that match our pattern in the file name or somewhere within in the files.

grep -lr cron /etc/

By default, grep outputs lines that have your pattern. -l is so it output file names of files that have that pattern, whether in the file name or in the file content.

-R, -r, –recursive

grep is great for parsing websites. For example, you can download a website and get only image links from it.

egrep (grep -E)

allows use of extended regex.

.* = AND

egrep -i 'hello.*world' file.txt

will search for hello world case-insesitively

| = OR

egrep -i 'hello|world' file.txt

will search for hello or world case-insesitively

You can pipe multiple grep commands to get selective results

egrep 'hello|world' file.txt | grep -v jeff

will find all the lines that contain either hello or world but do not contain jeff

fgrep (grep -F)

special characters don’t have any meaning. hello$ will search for lines with hello$ and not every line that ends with hello. The results are literally what you asked for.

Good in cases where you have to search a lot of files. since youre not using regex, there’s less cpu usage. fgrep finishes much faster.