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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Linux Cheatsheet</title>
<!-- Bootstrap CDN -->
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/font/bootstrap-icons.min.css">
<!-- CSS -->
<link rel="stylesheet" href="assets/style.css">
</head>
<body data-theme="light">
<div class="container">
<!-- Theme Toggle -->
<div class="button float-end"><i class="bi bi-circle-half"></i></div>
<h3 class="title">LINUX & BASH</h3>
<div class="section-list">
<ul>
<li><a href="#linux-basics">Linux Basics</a></li>
<li><a href="#bash-scripts">Bash Scripts</a></li>
<li><a href="#variables">Variables</a></li>
<li><a href="#strings">Strings</a></li>
<li><a href="#arrays">Arrays</a></li>
<li><a href="#input">Input</a></li>
<li><a href="#output">Output</a></li>
<li><a href="#operators">Operators</a></li>
<li><a href="#escape-sequence">Escape Sequence</a></li>
<li><a href="#variable-substitution">Variable Substitution</a></li>
<li><a href="#command-substitution">Command Substitution</a></li>
<li><a href="#decision">Decision Making</a></li>
<li><a href="#loops">Loops</a></li>
<li><a href="#functions">Functions</a></li>
<li><a href="#command-line-arguments">Command Line Arguments</a></li>
<li><a href="#output-redirection">Output Redirection</a></li>
<li><a href="#piping">Piping</a></li>
<li><a href="#bash-aliases">Bash Aliases</a></li>
<li><a href="#brace-expansion">Brace Expansion</a></li>
<li><a href="#regex">RegEx</a></li>
<li><a href="#grep">Grep</a></li>
<li><a href="#tr">Tr</a></li>
<li><a href="#find">Find</a></li>
<li><a href="#eval">Eval</a></li>
<li><a href="#expr">Expr</a></li>
<li><a href="#file-permissions">File Permissions</a></li>
<li><a href="#linux-filesystem-hierarchy">Linux Filesystem Hierarchy</a></li>
</ul>
</div>
<div class="sections" id="linux-basics">
<h4 class="heading"># Linux Basics</h4>
<div>
<h5>• Shell :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Commands</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>!!</code> or <code>!$</code></td>
<td>execute previous command</td>
</tr>
<tr>
<td><code>![x]</code></td>
<td>run xth command</td>
</tr>
<tr>
<td><code>!-[x]</code></td>
<td>run last xth command</td>
</tr>
<tr>
<td><code>which [command]</code></td>
<td>print full path of (shell) commands</td>
</tr>
<tr>
<td><code>whereis [command]</code></td>
<td>locate binary, source, and manual-page files for command</td>
</tr>
<tr>
<td><code>clear</code></td>
<td>clear terminal screen</td>
</tr>
<tr>
<td><code>exit</code></td>
<td>exit shell</td>
</tr>
<tr>
<td><code>exit [x]</code></td>
<td>exit shell with exit status x</td>
</tr>
<tr>
<td><code>history</code></td>
<td>shell history</td>
</tr>
<tr>
<td><code>history [x]</code></td>
<td>last x shell history</td>
</tr>
<tr>
<td><code>sleep [x]</code></td>
<td>sleep (pause) terminal for x sec</td>
</tr>
<tr>
<td><code>reset</code></td>
<td>initialize the terminal (resets terminal mode)</td>
</tr>
<tr>
<td><code>tty</code></td>
<td>print file name of terminal connected to standard input</td>
</tr>
</tbody>
</table>
</div>
</div>
<div>
<h5>• System and User :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Commands</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>su</code></td>
<td>switch user - allow switch to another user</td>
</tr>
<tr>
<td><code>sudo</code></td>
<td>superuser do - allow executing commands with elevated/root privileges</td>
</tr>
<tr>
<td><code>df</code></td>
<td>filesystem disk space usage</td>
</tr>
<tr>
<td><code>du</code></td>
<td>estimate file space usage of set of files</td>
</tr>
<tr>
<td><code>free</code></td>
<td>print free and used memory in system</td>
</tr>
<tr>
<td><code>arch</code></td>
<td>print CPU architecture</td>
</tr>
<tr>
<td><code>dmesg</code></td>
<td>print or control kernel ring buffer (logs)</td>
</tr>
<tr>
<td><code>lsblk</code></td>
<td>list block devices information</td>
</tr>
<tr>
<td><code>lspci</code></td>
<td>list PCI devices (buses) information</td>
</tr>
<tr>
<td><code>lsusb</code></td>
<td>list USB devices (buses) information</td>
</tr>
<tr>
<td><code>hostid</code></td>
<td>print numeric identifier for current host</td>
</tr>
<tr>
<td><code>hostname</code></td>
<td>print hostname of current system</td>
</tr>
<tr>
<td><code>uname</code></td>
<td>print system information</td>
</tr>
<tr>
<td><code>uptime</code></td>
<td>show uptime (how long the system has been running)</td>
</tr>
<tr>
<td><code>halt</code></td>
<td>halts system by stopping all processes but does not power off</td>
</tr>
<tr>
<td><code>poweroff</code></td>
<td>poweroff system</td>
</tr>
<tr>
<td><code>shutdown</code></td>
<td>shutdown system</td>
</tr>
<tr>
<td><code>reboot</code></td>
<td>reboot system</td>
</tr>
<tr>
<td><code>last reboot</code></td>
<td>print system reboot history</td>
</tr>
<tr>
<td><code>id</code></td>
<td>print user and group id</td>
</tr>
<tr>
<td><code>users</code></td>
<td>print all logged in users on system</td>
</tr>
<tr>
<td><code>w</code></td>
<td>print who is logged-in and what running</td>
</tr>
<tr>
<td><code>who</code></td>
<td>print logged in user</td>
</tr>
<tr>
<td><code>whoami</code></td>
<td>print current user</td>
</tr>
<tr>
<td><code>passwd</code></td>
<td>change user password</td>
</tr>
<tr>
<td><code>ps</code></td>
<td>snapshot of current processes</td>
</tr>
<tr>
<td><code>top</code></td>
<td>view running system in real-time interactively</td>
</tr>
<tr>
<td><code>kill [PID]</code></td>
<td>kill/terminate process</td>
</tr>
</tbody>
</table>
</div>
</div>
<div>
<h5>• Networking :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Commands</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>ifconfig</code></td>
<td>Interface Configuration (ifconfig) used to configure network interfaces <b>(deprecated)</b></td>
</tr>
<tr>
<td><code>nslookup [URL]</code></td>
<td>Name Server Lookup (Nslookup) used to query DNS server interactively <b>(deprecated)</b></td>
</tr>
<tr>
<td><code>netstat</code></td>
<td>print network connections, routing tables, interface statistics, masquerade connections, multicast memberships etc <b>(deprecated)</b></td>
</tr>
<tr>
<td><code>route</code></td>
<td>display IP/kernel routing table <b>(deprecated)</b></td>
</tr>
<tr>
<td><code>traceroute [URL]</code></td>
<td>print route packets trace to network host</td>
</tr>
<tr>
<td><code>dig</code></td>
<td>Domain Information Groper (dig) used for retrieving information about DNS name servers</td>
</tr>
<tr>
<td><code>ip</code></td>
<td>show or manipulate routing, network devices, and tunnels</td>
</tr>
<tr>
<td><code>ping [URL|IP]</code></td>
<td>Packet Internet Groper (ping) sends ICMP ECHO_REQUEST to network hosts</td>
</tr>
<tr>
<td><code>arp</code></td>
<td>display and manipulate Address Resolution Protocol (ARP) cache. ARP is protocol used to map IP addresses to MAC addresses on local network.</td>
</tr>
<tr>
<td><code>nmap</code></td>
<td>Network Mapper is network scanning tool that identifies devices, open ports, and services of target hosts on network. It can detect OS and service versions and automate network scanning.</td>
</tr>
<tr>
<td><code>tcpdump</code></td>
<td>TCP dump is packet analyser tool used to capture and analyse packets and network traffic in real-time over network.</td>
</tr>
</tbody>
</table>
</div>
</div>
<div>
<h5>• File Management :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Commands</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>ls</code></td>
<td>list current directory contents (except hidden)</td>
</tr>
<tr>
<td><code>ls [dir]</code></td>
<td>list directory contents</td>
</tr>
<tr>
<td><code>ls -a</code></td>
<td>list directory contents include hidden</td>
</tr>
<tr>
<td><code>ls -l</code></td>
<td>list directory contents in detailed format</td>
</tr>
<tr>
<td><code>pwd</code></td>
<td>print current/working directory (full path)</td>
</tr>
<tr>
<td><code>cd [dir]</code></td>
<td>change directory</td>
</tr>
<tr>
<td><code>cd -</code></td>
<td>change to last directory</td>
</tr>
<tr>
<td><code>cd ..</code></td>
<td>change to one level above current directory</td>
</tr>
<tr>
<td><code>cd</code> or <code>cd ~</code></td>
<td>change to home directory</td>
</tr>
<tr>
<td><code>mkdir [dir]</code></td>
<td>create directory</td>
</tr>
<tr>
<td><code>mkdir -p [dir1]/[dir2]</code></td>
<td>create directory inside non-existing parent directory</td>
</tr>
<tr>
<td><code>rmdir [dir]</code></td>
<td>remove empty directory</td>
</tr>
<tr>
<td><code>cp [source] [destin]</code></td>
<td>copy files and directories from source to destination</td>
</tr>
<tr>
<td><code>cp -r [dir1] [dir2]</code></td>
<td>copy directories & its content recursively</td>
</tr>
<tr>
<td><code>mv [source/old] [destin/new]</code></td>
<td>rename or move file and directory</td>
</tr>
<tr>
<td><code>rm [file]</code></td>
<td>remove files</td>
</tr>
<tr>
<td><code>rm -r [file]</code></td>
<td>remove files/directories & its content recursively</td>
</tr>
<tr>
<td><code>cat [file]</code></td>
<td>concatenate and print files on standard output</td>
</tr>
<tr>
<td><code>tac [file]</code></td>
<td>concatenate and print files in reverse</td>
</tr>
<tr>
<td><code>head [file]</code></td>
<td>print first 10 lines of file</td>
</tr>
<tr>
<td><code>head -[x] [file]</code></td>
<td>print first x lines of file</td>
</tr>
<tr>
<td><code>tail [file]</code></td>
<td>show last 10 lines of file</td>
</tr>
<tr>
<td><code>tail -[x] [file]</code></td>
<td>show last x lines of file</td>
</tr>
<tr>
<td><code>more [file]</code></td>
<td>paging through content one screenful at time <b>(it is primitive, use less instead)</b></td>
</tr>
<tr>
<td><code>less [file]</code></td>
<td>paging through content one screenful at time with enhancements</td>
</tr>
<tr>
<td><code>nl [file]</code></td>
<td>number lines of files</td>
</tr>
<tr>
<td><code>touch [file]</code></td>
<td>update modification and access time of file (create if not exist)</td>
</tr>
<tr>
<td><code>touch -c [file]</code></td>
<td>update modification and access time of file but do not create new file</td>
</tr>
<tr>
<td><code>sort [file]</code></td>
<td>sort lines of text files (default alphabetical order)</td>
</tr>
<tr>
<td><code>uniq [file]</code></td>
<td>filter adjacent matching (duplicate) lines from files</td>
</tr>
<tr>
<td><code>wc [file]</code></td>
<td>print line, word and byte counts of files</td>
</tr>
<tr>
<td><code>wc -l [file]</code></td>
<td>print lines count (<code>l-line, m-char, c-byte, w-word</code>)</td>
</tr>
<tr>
<td><code>tree [dir]</code></td>
<td>list contents of directories in trees format</td>
</tr>
<tr>
<td><code>file [file]</code></td>
<td>print file type</td>
</tr>
<tr>
<td><code>stat [file]</code></td>
<td>display file or file-system status</td>
</tr>
<tr>
<td><code>locate [file]</code></td>
<td>search for entries (files) in locate database</td>
</tr>
<tr>
<td><code>ln [file] [link]</code></td>
<td>create hard link to file</td>
</tr>
<tr>
<td><code>ln -s [file] [link]</code></td>
<td>create symbolic/soft link to file</td>
</tr>
<tr>
<td><code>wget [URL]</code></td>
<td>non-interactive network retriever (downloader)</td>
</tr>
<tr>
<td><code>curl [URL]</code></td>
<td>transfers data from or to server</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div class="sections" id="bash-scripts">
<h4 class="heading"># Bash Scripts</h4>
<p>A Bash script is a plain text file that contains a series of commands that are executed in sequence. Scripts have <code>.sh</code> extension (for example: <code>script.sh</code>). Shell scripts begin with the shebang (<code>#!/bin/sh</code>), which tells the system which shell interpreter to use to run the script.</p>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code>#!/bin/bash
mkdir Docs
cp doc1.txt Docs/
rm doc1.txt
clear</code></pre>
<h5>Executing Scripts :</h5>
<p><b>Note :</b> The script must have executable permission.</p>
<pre class="code-box"><code>./[script]
sh [script]
bash [script]
# Output each command execution :
bash -x [script]</code></pre>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code>./script.sh
bash script.sh</code></pre>
</div>
<div class="sections" id="variables">
<h4 class="heading"># Variables</h4>
<h6>Declaring :</h6>
<pre class="code-box"><code>greet="Hello World"
num=17</code></pre>
<h6>Access :</h6>
<pre class="code-box"><code>echo $greet
echo "Number is ${num}"</code></pre>
<h6>Read-Only Variable :</h6>
<div class="code-box"><code>readonly COUNT=1</code></div>
<h6>Declare Integer :</h6>
<div class="code-box"><code>declare -i num=10</code></div>
<h6>Remove Variable :</h6>
<div class="code-box"><code>unset num</code></div>
</div>
<div class="sections" id="strings">
<h4 class="heading"># Strings</h4>
<h6>Declaring :</h6>
<div class="code-box"><code>greet="Hello World"</code></div>
<h6>Access :</h6>
<div class="code-box"><code>echo $greet</code></div>
<h6>Length of string :</h6>
<div class="code-box"><code>echo ${#greet}</code></div>
<h6>String Replace :</h6>
<pre class="code-box"><code>greet="Hello World"
echo "Before: $greet"
greet=${greet/World/Universe}
echo "After: $greet"</code></pre>
</div>
<div class="sections" id="arrays">
<h4 class="heading"># Arrays</h4>
<h6>Declaring Indexed Array :</h6>
<div class="code-box"><code>declare -a fruits=("apple" "mango" "banana" "orange")</code></div>
<h6>Declaring Associative Array :</h6>
<pre class="code-box"><code>declare -A india["capital"]="New Delhi"
declare -A capital=(["India"]="New Delhi", ["Nepal"]="Kathmandu", ["Bhutan"]="Thimphu")</code></pre>
<h6>Indexing :</h6>
<div class="code-box"><code>fruits[0]="grapes"</code></div>
<h6>Access Element :</h6>
<div class="code-box"><code>echo ${fruits[0]}</code></div>
<h6>Print All Elements :</h6>
<div class="code-box"><code>echo ${fruits[@]}</code></div>
<h6>Print Key of Associative Array :</h6>
<div class="code-box"><code></code>echo ${!india[@]}</div>
<h6>Length of Array :</h6>
<div class="code-box"><code>echo ${#fruits[@]}</code></div>
<h6>Input :</h6>
<div class="code-box"><code>read -a fruits</code></div>
</div>
<div class="sections" id="input">
<h4 class="heading"># Input</h4>
<h6>Input (STDIN) :</h6>
<div class="code-box"><code>read [var]</code></div>
<h6>Prompt Input :</h6>
<div class="code-box"><code>read -p "Username : " [var]</code></div>
<h6>Secure Prompt Input :</h6>
<div class="code-box"><code>read -sp "Password : " [var]</code></div>
<h6>Input NChar :</h6>
<div class="code-box"><code>read -n [x] [var]</code></div>
<h6>Array Input :</h6>
<div class="code-box"><code>read -a [array]</code></div>
</div>
<div class="sections" id="output">
<h4 class="heading"># Output</h4>
<h6>Print (STDOUT) :</h6>
<p>The <code>echo</code> prints output followed by a new line character.</p>
<pre class="code-box"><code>echo "Hello World"
msg="Hello World"
echo $msg
echo "Hii, ${msg}"</code></pre>
<h6>Enable Backslash Escapes :</h6>
<div class="code-box"><code>echo -e "\n Hello World"</code></div>
<h6>Special Char in Output :</h6>
<div class="code-box"><code>echo '$Hello World'</code></div>
<h6>Print (STDOUT) :</h6>
<p>The <code>printf</code> prints output in the specified format; it does not append an implied newline.</p>
<div class="code-box"><code>printf "\n%s\n\n" "Hello World"</code></div>
</div>
<div class="sections" id="operators">
<h4 class="heading"># Operators</h4>
<div>
<h5>• Arithmetic Operator :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Operator</th>
<th>Operation</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>+</code></td>
<td>addition</td>
</tr>
<tr>
<td><code>-</code></td>
<td>subtraction</td>
</tr>
<tr>
<td><code>*</code></td>
<td>multiplication</td>
</tr>
<tr>
<td><code>/</code></td>
<td>division</td>
</tr>
<tr>
<td><code>%</code></td>
<td>modulus</td>
</tr>
<tr>
<td><code>++</code></td>
<td>increment</td>
</tr>
<tr>
<td><code>--</code></td>
<td>decrement</td>
</tr>
</tbody>
</table>
</div>
<p><b>Note :</b> Arithmetic operations should be in double braces: <code>count=$((count+1))</code></p>
</div>
<div>
<h5>• Logical Operator :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Operator</th>
<th>Operation</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>![expr]</code></td>
<td>not</td>
</tr>
<tr>
<td><code>[expr] || [expr]</code> or <code>[expr] -o [expr]</code></td>
<td>or</td>
</tr>
<tr>
<td><code>[expr] && [expr]</code> or <code>[expr] -a [expr]</code></td>
<td>and</td>
</tr>
</tbody>
</table>
</div>
</div>
<div>
<h5>• Relational Operator :</h5>
<p>(Numerical Comparison)</p>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Operator</th>
<th>Operation</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>[expr] -eq [expr]</code></td>
<td>equal</td>
</tr>
<tr>
<td><code>[expr] -ne [expr]</code></td>
<td>not equal</td>
</tr>
<tr>
<td><code>[expr] -gt [expr]</code></td>
<td>greater than</td>
</tr>
<tr>
<td><code>[expr] -lt [expr]</code></td>
<td>less than</td>
</tr>
<tr>
<td><code>[expr] -ge [expr]</code></td>
<td>greater than or equal</td>
</tr>
<tr>
<td><code>[expr] -le [expr]</code></td>
<td>less than or equal</td>
</tr>
</tbody>
</table>
</div
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code>if [ 80 -gt 40 ]
then
echo "Greater"
fi</code></pre>
</div>
<div>
<h5>• Relational Operator :</h5>
<p>(String Comparison)</p>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Operator</th>
<th>Operation</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>[string] = [string]</code></td>
<td>equal</td>
</tr>
<tr>
<td><code>[string] != [string]</code></td>
<td>not equal</td>
</tr>
<tr>
<td><code>[string] == [string]</code></td>
<td>equal for pattern matching</td>
</tr>
<tr>
<td><code>-n [string]</code></td>
<td>true if length of string is nonzero</td>
</tr>
<tr>
<td><code>-z [string]</code></td>
<td>true if length of string is zero</td>
</tr>
</tbody>
</table>
</div>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code>if [ "Hello" = "Hi" ]
then
echo "Matched"
else
echo "Not Matched"
fi</code></pre>
</div>
<div>
<h5>• Lexicographic Comparison :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Operator</th>
<th>Operation</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>[string] > [string]</code></td>
<td>true if left string is greater than right in alphabetical order</td>
</tr>
<tr>
<td><code>[string] < [string]</code></td>
<td>true if right string is greater than left string in alphabetical order</td>
</tr>
</tbody>
</table>
</div>
</div>
<div>
<h5>• File Testing Operator :</h5>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Operator</th>
<th>Operation</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>-e [file]</code></td>
<td>true if file exists</td>
</tr>
<tr>
<td><code>-f [file]</code></td>
<td>true if file exists and is regular file</td>
</tr>
<tr>
<td><code>-d [file]</code></td>
<td>true if file exists and is directory</td>
</tr>
<tr>
<td><code>-c [file]</code></td>
<td>true if file exists and is character file</td>
</tr>
<tr>
<td><code>-b [file]</code></td>
<td>true if file exists and is block file</td>
</tr>
<tr>
<td><code>-r [file]</code></td>
<td>true if file exists and read permission is granted</td>
</tr>
<tr>
<td><code>-w [file]</code></td>
<td>true if file exists and write permission is granted</td>
</tr>
<tr>
<td><code>-x [file]</code></td>
<td>true if file exists and execute permission is granted</td>
</tr>
<tr>
<td><code>-s [file]</code></td>
<td>true if size of file is greater than zero</td>
</tr>
</tbody>
</table>
</div>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code>if [ -f text.txt ]
then
echo "text.txt exists and it is regular file."
fi</code></pre>
<p>
<b>Note :</b>
<ul>
<li>Expressions can be grouped with <code>( [expression] )</code></li>
<li>Conditional expressions must be in square braces <code>[]</code></li>
<li>Space must be given between the operands and the operator.</li>
</ul>
</p>
</div>
</div>
<div class="sections" id="escape-sequence">
<h4 class="heading"># Escape Sequence</h4>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Escape</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>\\</code></td>
<td>backslash</td>
</tr>
<tr>
<td><code>\a</code></td>
<td>alert (BEL)</td>
</tr>
<tr>
<td><code>\b</code></td>
<td>backspace</td>
</tr>
<tr>
<td><code>\c</code></td>
<td>suppress trailing newline</td>
</tr>
<tr>
<td><code>\f</code></td>
<td>form feed</td>
</tr>
<tr>
<td><code>\n</code></td>
<td>new line</td>
</tr>
<tr>
<td><code>\r</code></td>
<td>carriage return</td>
</tr>
<tr>
<td><code>\t</code></td>
<td>horizontal tab</td>
</tr>
<tr>
<td><code>\v</code></td>
<td>vertical tab</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="sections" id="variable-substitution">
<h4 class="heading"># Variable Substitution</h4>
<div class="table-responsive">
<table class="table table-sm table-bordered table-striped">
<thead>
<tr>
<th>Var</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>${x}</code></td>
<td>substitute value of variable x</td>
</tr>
<tr>
<td><code>${x:+y}</code></td>
<td>if x is set, y is substituted for x. (value of x does not change)</td>
</tr>
<tr>
<td><code>${x:-y}</code></td>
<td>if x is null or unset, y is substituted for x. (value of x does not change)</td>
</tr>
<tr>
<td><code>${x:=y}</code></td>
<td>if x is null or unset, x is set to value of y.</td>
</tr>
<tr>
<td><code>${x:?y}</code></td>
<td>if x is null or unset, y is printed to standard error.</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="sections" id="command-substitution">
<h4 class="heading"># Command Substitution</h4>
<p>Command substitution allows the output of a command to replace the command itself. Using a backquote is old-style; instead, the second form is recommended.</p>
<div>
<h5>• Back-Quote :</h5>
<h6>Syntax :</h6>
<div class="code-box"><code>`[command]`</code></div>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code># Example 1:
DATE=`date`
echo "Date is $DATE"
# Example 2:
USERS=`who | wc -l`
echo "Logged in user are $USERS"</code></pre>
</div>
<div>
<h5>• Dollor Sign & Parenthesis :</h5>
<h6>Syntax :</h6>
<div class="code-box"><code>$([command])</code></div>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code># Example 1:
echo "Today is $(date)"
# Example 2:
for i in $(cat file.txt)
do
echo "$i"
done</code></pre>
</div>
</div>
<div class="sections" id="decision">
<h4 class="heading"># Decision Making</h4>
<div>
<h5>• If Statement :</h5>
<h6>Syntax :</h6>
<pre class="code-box"><code>if [ condition ]
then
#Statement
fi
# with OR
if [ condition ] || [ condition ]
then
#Statement
fi
# with AND
if [ condition ] && [ condition ]
then
#Statement
fi</code></pre>
<h6>Ex :</h6>
<pre class="code-box code-box-color"><code>x="Y"
if [ "$x" = "y" ] || [ "$x" = "Y" ]
then
echo "YES"