DEV Community: Shiva Prasad Gajula

Mastering the Linux Command Line: Your Ultimate Cheat Sheet for Seamless Operations

Shiva Prasad Gajula — Wed, 31 May 2023 11:04:01 +0000

As programmers, we spend most of our time writing code to build software. However, when shifting that code to production, our coding skills may not be sufficient. In such cases, we rely on commands to accomplish various tasks, such as connecting to remote servers using SSH, checking file permissions, and editing/updating files on the server.

In this blog post, we will explore some of the most commonly used commands in Linux-based systems that will make your deployment process a bit easier (although not necessarily a piece of cake 😜!).

Since this article is a bit longer, I have divided it into the following sections.

File commands
Search Commands
System commands
User commands
Permission commands
Process commands
Network and storage commands
Compression commands
SSH and File transfer commands

File commands

As the word implies, this is everything to do with files and directories such as listing directories, printing the working folder, changing the folder, etc.

pwd Stands for Print Working Directory. Prints the path of the current working directory starting from the root
ls Displays the files and directories of the current working directory.
ls Documents Displays the contents of the Documents directory
ls / Displays the contents of the root directory
ls .. Displays the contents of the parent directory(one level above)
ls ../.. Displays the contents of the parent directory(two levels above)
ls ~ Displays the contents of the user’s home directory (/home/user)
ls -d */ Displays all the folders of the current working directory
ls * Displays the contents along with the contents of subdirectories
ls -R Displays the files and directories in the current working directory, including their corresponding sub-directories down to the last file.
ls -s Displays the content with the size.
ls -l Displays the detailed content information, including file and directory permissions, the number of links pointing to the content, the user and group associated with the content, as well as the last modified date and time.
ls -lh Similar to the above command, It represents the size of files and directories in a readable format, such as 100B for bytes or 4.0K for kilobytes.
ls -a Displays the contents including hidden files
ls -l -a or ls -a -l or ls -la or ls -al Displays the contents including hidden information
ls -t Displays the contents sorted by last modified date in descending order.
ls -tr Displays the contents sorted by last modified date in ascending order
ls -S Displays the contents sorted by size in descending order
ls -Sr Displays the contents sorted by size in ascending order
ls > output.txt Displays contents and directories and writes to output.txt file (Note: You can use any of the above-discussed flags and the ls command. such as -s, -R -t etc.)
mkdir test Creates a test directory in the current folder
cd test Changes the current working directory to test
cd .. Move up to one directory
cd / Takes to the root directory.
cd Takes to the home directory
cd ~ Takes to the home directory
clear Clears the terminal
touch file1.txt Creates a file1.txt file in the current working directory
nano file1.txt Opens the file1.txt file in the nano editor
cat file1.txt Shows the contents of file1.txt
less file1.txt Shows less content of file1.txt (Hit q to exit)
mv file1.txt file2.txt Renames file1.txt to file2.txt
cp file1.txt file2.txt Copy the contents of file1.txt to file2.txt
cp file1.txt ~/dir2/file2.txt Copy a file to a different path
rm file1.txt Deletes a file
rmdir dir2 Deletes a directory
rm -R dir2 Deletes a directory with files

Search Commands

In this section, you will explore the commands that would help you to search for specific files, directories, or content within files

grep "five" file1.txt Search the text five in file1.txt
grep "five" blog/posts/file1.txt Search the text five in blog/posts/file1.txt
grep -r "five" blog Search in the blog directory and its child contents for text five
find -name file1.txt Search the file file1.txt in the current working directory
find blog/posts -name 'file1.txt' Finds the file file1.txt in the blog/posts directory
find blog/posts -name 'file*' Finds the files prefix with ‘file’ in the blog/posts directory
find -size +100M Finds the files which have more than 100 MB in size

System commands

Linux provides a wide range of system commands that allow users to interact with their operating system, gather information, and perform various administrative tasks. These commands play a crucial role in managing and maintaining a Linux system effectively.

uname -a Displays Linux system information
uname -r Displays kernel release information
uptime Displays how long the system has been running and the load average.
uptime -p Same as above but displays the output in a pretty format
uptime -s Displays how long the system has been up
hostname Displays the system hostname
hostname -I Display all local IP addresses of the host
last reboot Displays the system reboot history
date Displays the current date
cal Displays the calendar
w Displays the users who are online
whoami Displays the logged-in user
history Shows the audit of the user
which curl Shows the curl application path(installation location)

User commands

User management is a crucial aspect of Linux system administration. Linux provides a variety of commands to interact with user accounts, gather user-related information, and manage user privileges. These commands enable system administrators to effectively manage user accounts and ensure proper access control.

In this section, we will delve into some essential user commands that can assist you in user management tasks.

cat /etc/passwd Displays the list of users.
getent passwd Displays the list of users.
The cat /etc/passwd command and getent passwd command displays the list of users in the system. The output is typically presented in the following format

redis:x:112:119::/var/lib/redis:/usr/sbin/nologin

The above line consists of seven fields, which are as follows:

redis: User name.
x: Encrypted password (the letter "x" indicates that the actual password is stored in the /etc/shadow file).
112: User ID number (UID).
119: User's group ID number (GID).
/var/lib/redis: Full name of the user (GECOS).
/usr/sbin/nologin: User home directory.

These commands provide important information about users on the system, such as their usernames, user IDs, group IDs, and home directories.

Ok. As you saw the cat /etc/passwd and getent passwd commands will display the output with seven fields.

However, you can also extract the part of the output by giving specific commands.

awk -F: '{print $1, $2}' /etc/passwd Displays the first two fields out of 7 fields
cut -d: -f1,2,3 /etc/passwd Displays the first 3 fields out of 7 fields
sudo useradd test_user1 Creates a new user called test_user1
sudo passwd test_user1 Sets the new password for user test_user1
sudo useradd -m test_user2 Creates a new user called test_user2 and also creates a test_user2 directory under the home directory.
sudo useradd -m -d /opt/test_user3 test_user3 Creates a test_user3 user and also creates a test_user3 directory under opt directory.
sudo useradd -u 1600 test_user4 Creates a test_user4 user with 1600 id
id -u test_user4 Returns the id of the user
sudo useradd -g users login_user1 Creates a new user and sets the login group to users
id -gn login_user1 Displays the group name of the user
sudo useradd -g users -G user1,user2 login_user2 Creates a new user called login_user2 and sets the primary group as users and secondary groups as user1, and user2.
id login_user2 Displays the userid, primary, and secondary group ids
sudo groupadd managers Creates a new group called managers
sudo chgrp managers file1.txt Changes the group to managers on file1.txt
sudo usermod -aG managers login_user2 Adds login_user2 to the managers group

Permission commands

Understanding Permissions in Linux In Linux, permissions play a crucial role in controlling access to files and directories. They define what actions users can perform on specific resources, such as reading, writing, or executing files

For example, run the ls -l command in any of the directories which have some content.

drwxr-xr-x 1 777 shiva 4096 May 25 16:52 blog<br>-rw-r--r-- 1 shiva shiva 158 Jan 17 11:54 dump.rdb<br>drwxr-xr-x 1 shiva shiva 4096 May 25 12:52 shell-scripting

You will see similar output in your terminal. Let’s consider the first 10 characters (drwxr-xr-x) in the first line.

The first letter denotes the content type such as directory or file. d refers to the directory - refers to the file.
The second three letters refer to the user permissions
The third three letters refer to the group permissions
The last three letters refer to the other's permissions

r read, w write, x execute, - no permission

u user, g group, o other people from the outside world, anall

+ adds permissions, - removes permissions, = adds new permissions and overrides existing

chmod o+w file1.txt gives write access to the outside people on file1.txt
chmod o-w file1.txt removes write access for the outside people on file1.txt
chmod g+wr file1.txt gives read-and-write access to the groups
chmod u=rwx file1.txt gives read, write, and execute access to the user and overrides the existing permissions

Absolute Numeric Mode

Number	Permission Type	Symbol
0	No permission	`---`
1	Execute	`--x`
2	Write	`-w-`
3	Execute + Write	`-wx`
4	Read	`r--`
5	Read + Execute	`r-x`
6	Read + Write	`rw-`
7	Read + Write + Execute	`rwx`

chmod 111 file1.txt gives execute access to the user, group, and others
chmod 750 file1.txt gives read+write+execute access to users and read+execute access to groups and no permissions to outside people.
chmod -R 777 posts give read+write+execute access to users, groups, and others on the directory posts and their contents.

Process commands

Processes are an integral part of any operating system, including Linux. They represent running programs or tasks that consume system resources. Linux provides a range of commands to manage processes efficiently, allowing users to monitor, control, and terminate them as needed.

ps displays currently running user processes
ps -ef displays currently running system processes
ps -ef grep 12693 displays the process information of 12693
top shows the processes running(hit q to exit)
htop alternative for the top (interactive process viewer)
kill 24567 kills the process 24567

Network and storage commands

Linux provides a variety of commands that enable users to interact with the network and storage components of their system. These commands allow you to gather network information, perform network diagnostics, manage storage devices, and retrieve files from the web. Understanding these commands can greatly enhance your ability to work with network and storage resources effectively.

ifconfig shows the network information
iwconfig shows wireless information
ping google.com returns the continuous response from the host
blkid shows the storage information
df shows the available and unavailable disk space
lsusb shows the connected devices list
lspci shows PCI information
wget https://www.sample-videos.com/img/Sample-jpg-image-50kb.jpg downloads image file

Compression commands

Compression commands are indispensable when it comes to managing large sets of files or reducing file sizes for storage or transfer. Linux provides a wide range of compression commands that allow you to package files into archives, extract their contents, and compress them using various algorithms. Following are some of the example compression commands.

tar cf testDir.tar testDir compress the testDir and creates testDir.tar file
tar xf testDir.tar extract the contents of testDir.tar
tar czf testDir.tar.gz testDir creates gzip compressed tar file
tar xzf testDir.tar.gz extract the compressed gzip file
tar -tvf testDir.tar displays the contents of the tar file
tar -tvf testDir.tar.gz displays the contents of gzip tar file
tar -xf nodejs.tar.xs Extracts the contents of nodejs.tar.xs

SSH and File transfer commands

Secure Shell (SSH) and file transfer commands are essential tools for remote access and transferring files between local and remote systems in Linux. SSH provides a secure encrypted connection, while file transfer commands enable efficient and secure file transfer over SSH. Following are some of the example SSH commands.

ssh root@domain.com connect to the domain.com server with the root user (prompts for password)
scp file1.txt user1@domain.com /tmp copies the file securely to the tmp folder in the domain.com server
sudo scp -r user1@domain.com:/var/www /temp copies files recursively from the server to the local machine

Ok. I know the list is too big. But, it is worth going through it. If you have these commands in your arsenal, you can definitely win the command line war.

Mastering Shell Scripting: Your Ultimate Cheat Sheet for Efficient Automation

Shiva Prasad Gajula — Wed, 31 May 2023 11:02:15 +0000

Bash is a powerful scripting language used in the Linux terminal to automate tasks and perform various operations. Here are some essential commands and concepts to help you get started with Bash scripting.

Creating a script:

Create a file name whatever you want. But, make sure it ends with .sh. Open that file in your favorite text editor and write the first line to begin your shell script journey.

my_script.sh

#! /bin/bash

The #! /bin/bash line is called the shebang or hashbang, and it is used to specify the interpreter to be used for executing the script. In this case, #!/bin/bash indicates that the script should be interpreted and executed using the Bash shell.

Running the script:

Open your terminal and go to the directory where you created the script file. Then run the below command to execute the script

./my_script.sh

At this stage, you won't see any output since we have not written anything other than the shebang line. Let's add some fun stuff.

Display statements:

You can use the echo command to print statements or messages in the terminal. For example:

echo Hello World

You will see Hello World in your terminal

Variables:

You can assign values to variables in Bash using the assignment operator (=). Variable names are usually uppercase by convention and can contain letters, numbers, and underscores. Here's an example:

NAME="Shiva"
echo "My name is $NAME"

User input:

To read user input in Bash, you can use the read command. It prompts you to enter a value and stores it in a variable. For example:

read -p "Enter your name: " NAME
echo "Hello $NAME, nice to meet you!s"

Gives you the following output.

Enter your name: Shiva
Hello Shiva, nice to meet you!s

Conditional Statements:

Bash supports various conditional statements to control the flow of your script. Here are a few examples:

Simple If Statement:

if [ "$NAME" == "Shiva" ]
then
  echo "Your name is Shiva"
fi

If Statement with Else:

if [ "$NAME" == "Shiva" ]
then
  echo "Your name is Shiva"
else
  echo "Your name is not Shiva"
fi

Else If Statement:

if [ "$NAME" == "Shiva" ]
then
  echo "Your name is Shiva"
elif [ "$NAME" == "Jack" ]
then
  echo "Your name is Jack"
else
  echo "Your name is NOT Shiva or Jack"
fi

Below is an example of comparing two numbers using -gt operator in the shell script

NUM1=3 
NUM2=5
if [ "$NUM1" -gt "$NUM2" ] 
then 
    echo "$NUM1 is greater than $NUM2"
else 
    echo "$NUM1 is less than $NUM2"
fi

You can use any of the following operators to do comparisons in a shell script.

Equal to: == or =
- Example: [ "$var1" == "$var2" ]
- Checks if $var1 is equal to $var2.
Not equal to: !=
- Example: [ "$var1" != "$var2" ]
- Checks if $var1 is not equal to $var2.
Greater than: >
- Example: [ "$var1" -gt "$var2" ]
- Checks if $var1 is greater than $var2.
Greater than or equal to: -ge
- Example: [ "$var1" -ge "$var2" ]
- Checks if $var1 is greater than or equal to $var2.
Less than: <
- Example: [ "$var1" -lt "$var2" ]
- Checks if $var1 is less than $var2.
Less than or equal to: -le
- Example: [ "$var1" -le "$var2" ]
- Checks if $var1 is less than or equal to $var2.

Following is the example to check whether a file exists with the given or not

FILE="test.txt"
if [ -e "$FILE" ]
then
  echo "$FILE exists"
else
  echo "$FILE does not exist"
fi

Case Statement:

The case statement allows you to evaluate the value of a variable against multiple patterns and execute different blocks of code based on the matching pattern.

Following is the example code snippet.

read -p "Are you 21 or over? Y/N " ANSWER
case "$ANSWER" in
    [yY] | [yY][eE][sS])
        echo "You can have a beer :)"
        ;;
    [nN] | [nN][oO])
        echo "Sorry, no drinking"
        ;;
    *)
        echo "Please enter y/yes or n/no"
        ;;
esac

The above code produces the following output.

# Example 1
Are you 21 or over? Y/N y
You can have a beer :)
# Example 2
Are you 21 or over? Y/N N
Sorry, no drinking

Simple for loop

The for loop allows you to iterate over a list of values and perform a set of actions for each value.

NAMES="Brad Kevin Alice Mark"
for NAME in $NAMES
do
    echo "Hello $NAME"
done

Following is the output of the above code.

Hello Brad
Hello Kevin
Hello Alice
Hello MarkWhile loop

A while loop in a shell script allows you to repeatedly execute a block of code as long as a certain condition is true. Here is an example of for loop.

LINE=1
while read -r CURRENT_LINE
do
    echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "./contents.txt"

Functions:

In shell scripting, you can define and use functions to encapsulate a block of code that can be executed multiple times throughout your script.

sayHello() {
    echo "Hello World"
}

# Call the function
sayHello

Function with parameters:

greet() {
    echo "Hello, I am $1 and I am $2 years old"
}

# Call the function and pass arguments
greet "Shiva" "26"
FUNCTION WITH PARAMS

Create a folder and write to a file

mkdir hello
touch "hello/world.txt"
echo "Hello World" >> "hello/world.txt"
echo "Created hello/world.txt"

Ok. In conclusion, this article covers basic shell scripting concepts anyone can begin with.