For the following cmds, the test.file contains:

1 cat barn food
2 dog house kibble
3 fish water food
4 cow pasture grass

grep

“searches the named input FILE for lines containing a match to PATTERN”

* Good for searching a file for a certain pattern

user@tim:~$ grep --help
Usage: grep [OPTION]... PATTERN [FILE]...
Search for PATTERN in each FILE or standard input.
  // snip //
  
user@tim:~$ grep cat test.file 
1 cat barn food

user@tim:~$ grep food test.file 
1 cat barn food
3 fish water food

awk

“pattern scanning and processing language”

* good for printing columnar data, note that the field separator (between the columns) can be anything (space, tab, colon, etc) and can be specified with -F

user@tim:~$ awk '{print $2}' test.file
cat
dog
fish
cow

sed

“stream editor for filtering and transforming text”

user@tim:~$ sed s/cat/lion/ test.file
1 lion barn food
2 dog house kibble
3 fish water food
4 cow pasture grass

(note that the input file //test.file// did NOT change)

user@tim:~$ cat test.file
1 cat barn food
2 dog house kibble
3 fish water food
4 cow pasture grass

user@tim:~$ sed s/cat/lion/ test.file > new.file
(save into //new.file//)

user@tim:~$ cat new.file
1 lion barn food
2 dog house kibble
3 fish water food
4 cow pasture grass

user@tim:~$ cat test.file
1 cat barn food
2 dog house kibble
3 fish water food
4 cow pasture grass

diff

“Compare files”

user@tim:~$ diff --help
Usage: diff [OPTION]... FILES
Compare FILES line by line.
  // snip //
  
user@tim:~$ diff test.file new.file 
1c1
< 1 cat barn food
---
> 1 lion barn food