Thursday, 28 September 2023

Linux Cheatsheet

As a DevOps engineer, you’re expected to know your way around Linux command line tools. This cheat sheet can help.

Let’s get started! 💯

⫸ Basic commands 😊

✍ mkdir

mkdir linux-basics: creates a new directory with the specified name.

mkdir dev qa prod : creates multiple directories at once.
mkdir logfolder{1..10} : creates a range of directories with sequential numbers from 1 to 10.

✍ sudo: stands for ‘superuser do’

sudo [options] [cmd]: It is used to execute commands as a root user or super user.

sudo apt-get update : updates the package list

sudo -u username touch file.txt : creates a new file “file.txt” with the ownership of “username”.

sudo -s : starts a new shell with root privileges.

✍ history

history : display all the commands which we have executed on a terminal.

✍ ls

ls : list the contents of the current working directory

ls -l : list the content in long format with permissions, owner, size, and last modified date for each directory
ls -a : list all files in the directory, including hidden files that start with a dot (.) character.

ls -R : list the content of the current directory and its sub-directories recursively.

ls -r : reverse the order of display. Older files are listed first.
ls -h : display file size in a human-readable format. eg KB, MB, or GB.

we can also combine the above options:

ls -alh : display the content of the directory including hidden files in a long format with file size displayed in a human-readable format.

✍ cd

cd <path/to/directory> : will take you to the specified directory path.

cd ~ : used to navigate to the home directory.
cd /: take you to the root directory
cd .. : move up one directory

✍ pwd

pwd: It stands for print working directory. It prints the current working directory in which the user currently is.

⫸ File Operations 📁

✍ touch

touch: This command helps to create a new file.

✍ cat

cat filename : used to display the content of the file

cat file1 file2 > newfile : concatenate file1 , file2 content in newfile.
cat file1 >> file2 : appends the file1 content to file2.
cat -n filename : display the content with line numbers.
cat -E filename : display the content with a $ sign at the end of each line.
cat -T filename : display the content of the filename with tabs.

✍ cp

cp src dest : copy a file/ directory to another location

cp file1.txt file2.txt: This will copy the content of file1 to file2 which is in the same directory.

cp -v file.txt dev : It copies the file.txt to the dev directory in verbose mode. Hence you were able to see the progress.

cp -r dev qa : copies dir and sub-dir to destination dir

✍ mv

mv file.txt file3.txt : moves or renames a file or directory.

✍ rm

rm file/directory_name : removes a file or directory

rm file.txt : delete the file
rm -r qa : recursively delete the file and folders.

rm -rf prod : forcefully deletes the directory/file, to remove a directory we need to use -r.

⫸ Text Processing 📋

✍ grep

grep [options] [pattern] [filename] : ‘globally search for a regular expression and print’. It is used for text search and filtering based on regular expressions and returns the lines which match the pattern.

Eg: log analysis, system monitoring, and code debugging.

grep "INFO" logfile.txt : this will return the search which has the INFO keyword from logfile.txt

grep "ERROR" -r -i /home/ubuntu : this will try to find the ‘error’ keyword in the path /home.

top | grep -i systemd : grep can be combined with other Linux commands and search for the exact pattern.

df -h | grep -i /dev/shm : here we combined grep with disk space command and searched for /dev/shm

✍ awk

awk <condition> {<action>} filename: awk is a programming language and has its own syntax. It is used for text processing and manipulation.

Eg: Data extraction, Report generation or Data Manipulation.

awk '/INFO/ {print $1 $2 $3 $6} logfile.txt : here, you can pass the pattern ‘INFO’ and condition to print columns 1, 2, 3, and 6 of a log file.

awk -F',' '{print $1, $3}' fruits.txt : This will split on delimiter and print the values of columns 1 and 3 in the output

awk -F',' '$2 > 75 {print $1}' marks.txt : This will split on delimiter and then the marks of students and will print their names if marks are greater than 75.

✍ find

find : used to find files and directories in a given directory. It uses the name, size, type, or modified time of the file to search.

Eg: file management, backup, and system administration.

find . -name "logfile.txt" : this will find the file by name in the current dir and its sub-directories.
find . -name "*.txt" : this will find all the files with the extension .txt

find . -type d : this finds all the directories from the current directory.
find . -type f -size +10M : this finds all the greater than 10M
find . -name "logfile.txt" -delete : deletes all the files which match the filename.

✍ sed

sed [options] [pattern] [filename] : It's used to search, transform and replace in the file/output of the command.

sed -i 's/bitter/awesome/g' logfile.txt : It will replace the old text with the new text in a file. -i stands for editing a file in place, -g stands for replacing all occurrences, and -s stands for substituting the old value with the new one.

sed -i '3s/cool/sweet/' file.txt: replace the first occurrence of old text with new text but only on line 3 of a file

sed -i ‘/awesome/d’ file.txt : delete all lines containing the word ‘awesome’ in a file.

⫸ System Information

✍ top

top : It gives the system resources usage in real-time.

✍ ps

ps : gives information about running processes.

✍ df

df : gives the disk space usage

✍ free

free : gives the system memory usage

✍ uname

uname -u : displays system name

⫸ Networking

✍ ping

ping google.com : checks the connectivity by sending packets to the host.

✍ ssh

ssh -i user@remote_host: This allows to connect to remote server via ssh

✍ scp

scp filename remotehost:localpath : these commands copy files from the remote host to the local system.

⫸ User and Group Management

These commands are used for managing user accounts, groups, and passwords.

✍ useradd

useradd username : this will add a new user

✍ passwd

passwd username : change the password of the user

cat \etc\passwd : to view all users

✍ groupadd

groupadd grpname : it will create a new group

cat \etc\group : to view all the groups

✍ gpasswd

gpasswd -a username grpname : add a single user to the group
gpasswd -M user1,user2 groupname : add multiple users to the group.
gpasswd -A username grpname : create an admin of the group.
gpasswd -d username groupname : delete the user from the group.

✍ deluser

deluser username : this will delete a user account.

⫸ File Permission

✍ chmod

Permission Classes:

Type: Type of file or directory.

Owners: They are owners of the file
Group: Permission for groups
Other: Permission for other users. eg you might have used the share button in a Word document.

There are three types of permissions: read (r), write (w), and execute (x).

Let’s take example

In the above example, the fruits.txt has

Owner : it has read and write permission (4: read) + (2: write) = 6
Group: it has read and write permission (4: read) + (2: write) = 6
Other: it has only read (4: read) = 4

Hence the current permission for a file is 664

To change the file permission to 772, we will be using chmod cmd.

The first 7 indicates the owner, to change his permission we need read(4), write(2), and execute (1) permissions, which are represented by 4+2+1=7
The group has read(4), write(2), and execute (1) permissions, which are represented by 4+2+1=7
All other users have only write permission which is represented by 0+2+0=2

chmod 772 fruits.txt : This will change the permission of the fruits.txt file from 664 to 772.