Grep command for string processing (1) Processing of text in a file and other

The grep command is used in order to retrieve or search for a certain string.

Broadly speaking, the way you do the grep command depends on whether you grep the text in the file or the file name.

It's simple, but it can be applied to many things.

By the following command, you can download the files/folders for practice *1.

$ svn export

#Move to the working folder
$ cd ./grep_practice


Show lines with a certain string

#Display the file
$ cat test_01.txt

#Output the lines containing 'abc'
$ grep 'abc' test_01.txt
$ grep "abc" test_01.txt
$ cat test_01.txt | grep 'abc'

Show lines without the certain string

#Output the lines without 'abc
$ grep -v 'abc' test_01.txt 
$ cat test_01.txt | grep -v "abc"

Output lines without a certain string as an alias file

#Save the lines without 'abc' as 'test_01_v.txt'.
$ grep -v 'abc' test_01.txt > test_01_v.txt

#Display of test_01_v.txt
$ cat test_01_v.txt


-v: search for a string that does not contain or does not match

Delete blank lines

#Show the lines without blank lines
$ cat test_02.txt | grep -v '^\s*$'

#To save as an alias file
$ cat test_02.txt | grep -v '^\s*$' > test_02_r.txt


^: beginning of line

\s: empty word

^\s*: Zero or more repeated spaces from the beginning of the line.

Count the number of lines in a file

#Counting the total number of lines
$ cat test_01.txt | grep '' -c

#How not to use grep
$ cat test_01.txt | wc -l

#Counting only the number of lines containing 'abc'
$ cat test_01.txt | grep 'abc' -c


-c: Counting lines or stuffs.

Searching for strings with multiple criteria

grep for AND search

$ cat test_02.txt

#AND search
#Showing lines that contain 'AAA' and 'ABB' in the companion
$ grep 'AAA' test_02.txt | grep 'ABB'

grep for OR search

$ cat test_02.txt

#OR search (-e notation)
#Display lines containing 'AAA' or 'BBB'
$ grep -e 'AAA' -e 'BBB' test_02.txt

#OR search (regular expression)
#Display lines containing 'AAA' or 'BBB'
$ grep 'AAA\|BBB' test_02.txt


-e: Specify a search pattern

-w: Search for a pattern match across words

-x: Find a whole line that matches the pattern

-i: It's not case sensitive.

-n: Show line numbers in the search results

Processing of file names

Search for file names with a certain string

#Display of files in the directory
$ ls

#Display the .txt file only
$ ls | grep '.txt'

#counting .txt file
$ ls | grep '.txt' | wc -l

#Display the line, word and byte counts in the file
$ ls | grep 'test_01.txt' | wc
$ wc test_01.txt


-l: Targeting file names

Search including subdirectories

#Counting the number of files containing '.pdf'
$ ls | grep '.pdf' | wc -l

#Show files including subdirectories
$ du -a

#Display of files containing '.pdf', including subdirectories
$ du -a | grep '.pdf'

#Count the number of files, including subdirectories
$ du -a | grep '.pdf' | wc -l

Search and delete certain files

#Delete files that contain '.pdf' in the current directory
$ ls | grep '.pdf' | xargs rm -rf

#Delete files containing '.pdf' in the subdirectories
$ du -a | grep '.pdf' | xargs rm -rf

Here, xargs means the result of the previous command is passed to the next command rm as an argument.


Terminal Shortcuts

Move the cursor
Ctrl + b backward / backward a word
Ctrl + f move forward and one word forward
Ctrl + a move to the beginning of the line
Ctrl + e move to end of line
Ctrl + w delete a single word or phrase
Ctrl + k delete to end of line
Ctrl + u delete to the beginning of the line
Ctrl + d delete a character in the cursor
Ctrl + h remove one character after the cursor
previous command history
next command history
History History
Ctrl + c kill the current command
Ctrl + z pause the running command
Ctrl + d Exit, Logout
Ctrl + l Clear the screen
Ctrl + t change the cursor's character with the previous one
Ctrl + m, Ctrl + j, Ctrl + o Enter

cat : view, merge, and create for file

#Display in the text file (test.txt)
$ cat test.txt

#Add line numbers and display the file
$ cat -n test.txt

#Create a combined file from multiple files
$ cat test1.txt test2.txt > test3.txt

#Create an empty file (press Ctrl + d to exit)
$ cat > test4.txt

#All history displays, "~ (tilde)" means the home directory
$ cat ~/.bash_history
$ history

head / tail : show the beginning and end of the file
#Display 10 lines from the top of the file.
$ head -n 10 test.txt

#Display 10 lines from the end of the file
$ tail -n 10 test.txt

Terminal Commands You Should Remember
#Terminate the system
$ shutdown -f now
$ reboot

#Give all users execute privileges
$ chmod a+x test.command

#Executing commands as the Root user

$ sudo ...

Other Terminal Commands
#Display the date and time
$ date

#View 2012 Calendar
$ cal 2020

#Show Active Jobs
$ jobs

*1:To run the command, delete the "$".