Linux Shell Mastery: Variables, Redirection, Pipes & Aliases¶
This page explains how the Linux shell works, including different shell types, variables, and the powerful operators that allow you to chain commands together.
🏛️ Types of Shell¶
The shell is a command-line interpreter that acts as an interface between the user and the operating system kernel.
- sh: The original Bourne shell.
- bash (Bourne Again Shell): The default shell for most Linux distributions.
- zsh: A highly customizable shell popular for modern development.
- ksh (Korn Shell): A powerful shell often used in enterprise environments.
- csh: A C-like syntax shell.
Reference: https://www.shiksha.com/online-courses/articles/introduction-to-types-of-shell/
📊 Shell Variable Architecture¶
Understanding the difference between Shell Variables (local) and Environment Variables (global) is critical for managing processes and automation scripts.
graph TD
A[Linux Kernel] --> B[Parent Shell Session]
B --> C(Shell Variables - Local Only)
B --> D{export command}
D --> E[Environment Variables - Global]
E --> F[Child Process 1: Script]
E --> G[Child Process 2: Command]
C -.-> |X| F
C -.-> |X| G🛠️ Creating Shell Variables¶
Shell variables are only accessible within the current shell session.
[opc@new-k8s ~]$ clear
[opc@new-k8s ~]$ NAME="vignesh"
[opc@new-k8s ~]$ echo $NAME
vignesh
[opc@new-k8s ~]$ printenv NAME
printenv NAME -- doesn't show anything, since it's not an environment variable
Even in env, printenv command the NAME variable will not been shown
🌎 Managing Environment Variables using export¶
Environment variables are accessible to child processes as well.
1. Creating an Environment Variable¶
[opc@new-k8s ~]$ export NEW_NAME="Raghav"
[opc@new-k8s ~]$ echo $NEW_NAME
Raghav
[opc@new-k8s ~]$ printenv NEW_NAME
Raghav
2. Viewing Environment Variables¶
You can view a single variable with echo or list all environment variables using env or printenv.
[opc@new-k8s test]$ echo $PATH
/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/opc/.local/bin:/home/opc/bin
[opc@new-k8s test]$ env
XDG_SESSION_ID=172502
HOSTNAME=new-k8s
SELINUX_ROLE_REQUESTED=
TERM=xterm
SHELL=/bin/bash
HISTSIZE=1000
SELINUX_USE_CURRENT_RANGE=
SSH_TTY=/dev/pts/0
USER=opc
MAIL=/var/spool/mail/opc
PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/opc/.local/bin:/home/opc/bin
PWD=/home/opc
LANG=en_US.UTF-8
NEW_NAME=Raghav
SELINUX_LEVEL_REQUESTED=
HISTCONTROL=ignoredups
SHLVL=1
📍 The PATH Environment Variable¶
Most Linux commands can be executed from any directory because their paths are added to the PATH environment variable.
[opc@new-k8s ~]$ echo $PATH
/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/opc/.local/bin:/home/opc/bin
🔄 Shell Operators: Redirection¶
Redirection allows you to capture command output and save it to files.
> Overwrites the file content, if the file already exists
>> Appends the content to the existing content in the file
In both cases, if the file is not present, it will create the file and write the content to it. By default, the echo command prints the output to the screen. But if we use redirection arrows, it can store the output to files.
1. Overwrite (>)¶
[opc@new-k8s ~]$ mkdir redirection
[opc@new-k8s ~]$ cd redirection/
[opc@new-k8s redirection]$ pwd
/home/opc/redirection
[opc@new-k8s redirection]$ ll
total 0
[opc@new-k8s redirection]$ echo "hello devops" > hello.txt
[opc@new-k8s redirection]$ ll
total 4
-rw-rw-r--. 1 opc opc 13 Apr 17 14:11 hello.txt
[opc@new-k8s redirection]$ cat hello.txt
hello devops
[opc@new-k8s redirection]$ echo "I am learning devops" > hello.txt
[opc@new-k8s redirection]$ ll
total 4
-rw-rw-r--. 1 opc opc 21 Apr 17 14:11 hello.txt
[opc@new-k8s redirection]$ cat hello.txt
I am learning devops
2. Append (>>)¶
[opc@new-k8s redirection]$ pwd
/home/opc/redirection
[opc@new-k8s redirection]$ ll
total 0
[opc@new-k8s redirection]$ echo "I eat fruits daily" >> double-arrow.txt
[opc@new-k8s redirection]$ ll
total 4
-rw-rw-r--. 1 opc opc 19 Apr 17 14:13 double-arrow.txt
[opc@new-k8s redirection]$ cat double-arrow.txt
I eat fruits daily
[opc@new-k8s redirection]$ echo "I love banana" >> double-arrow.txt
[opc@new-k8s redirection]$ echo "I also like apples" >> double-arrow.txt
[opc@new-k8s redirection]$ ll
total 4
-rw-rw-r--. 1 opc opc 52 Apr 17 14:13 double-arrow.txt
[opc@new-k8s redirection]$ cat double-arrow.txt
I eat fruits daily
I love banana
I also like apples
🔗 Shell Operators: Pipe (|)¶
A pipe (|) is used to pass the output from one command/program to the input for another command.
[opc@new-k8s test]$ pwd
/home/opc/test
[opc@new-k8s test]$ ll
total 8
drwxrwxr-x. 2 opc opc 27 Mar 17 14:03 client
-rw-rw-r--. 1 opc opc 77 Apr 12 12:26 Dockerfile
-rw-rw-r--. 1 opc opc 0 Apr 12 12:54 hello.txt
-rw-rw-r--. 1 opc opc 23 Apr 12 12:56 mani.txt
-rw-rw-r--. 1 opc opc 0 Mar 17 14:03 server
drwxrwxr-x. 3 opc opc 18 Apr 13 12:46 vignesh
[opc@new-k8s test]$ ll | wc -l
7
📜 Creating a Shell Script and Exporting to PATH¶
We will create a shell script to print the username, hostname, date, present working directory, and the files in that directory.
Filename: myinfo
#!/bin/bash
echo $USER
hostname
date
pwd
ls -l
Making it Executable and Testing PATH¶
[opc@new-k8s ~]$ ll
total 3072008
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rw-rw-r--. 1 opc opc 48 Apr 15 10:56 myinfo
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test
[opc@new-k8s ~]$ cat myinfo
#!/bin/bash
echo $USER
hostname
date
pwd
ls -l
[opc@new-k8s ~]$ chmod +x myinfo
[opc@new-k8s ~]$ ll
total 3072008
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rwxrwxr-x. 1 opc opc 48 Apr 15 10:56 myinfo
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test
[opc@new-k8s ~]$ myinfo
bash: myinfo: command not found
[opc@new-k8s ~]$ ./myinfo
opc
new-k8s
Sat Apr 15 10:56:49 GMT 2023
/home/opc
total 3072008
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rwxrwxr-x. 1 opc opc 48 Apr 15 10:56 myinfo
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test
[opc@new-k8s ~]$ pwd
/home/opc
[opc@new-k8s ~]$ echo $PATH
/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/opc/.local/bin:/home/opc/bin
[opc@new-k8s ~]$ export PATH=$PATH:/home/opc
[opc@new-k8s ~]$
[opc@new-k8s ~]$
[opc@new-k8s ~]$ myinfo
opc
new-k8s
Sat Apr 15 10:57:54 GMT 2023
/home/opc
total 3072008
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rwxrwxr-x. 1 opc opc 48 Apr 15 10:56 myinfo
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test
[opc@new-k8s ~]$
[opc@new-k8s ~]$
[opc@new-k8s ~]$ cd /tmp
[opc@new-k8s tmp]$ pwd
/tmp
[opc@new-k8s tmp]$ ll
total 4
-rw-------. 1 root root 1097 Apr 15 10:02 dhclient-exit-hooksRZi.log
drwx------. 3 root root 17 Jul 4 2021 systemd-private-c60b800098604975be26dbbb3215bd47-chronyd.service-ZzaKpF
drwx------. 3 root root 17 Mar 27 20:01 systemd-private-c60b800098604975be26dbbb3215bd47-unified-monitoring-agent.service-561YDH
-rw-rw-r--. 1 opc opc 0 Apr 12 12:30 vignesh.txt
[opc@new-k8s tmp]$ myinfo
opc
new-k8s
Sat Apr 15 10:58:09 GMT 2023
/tmp
total 4
-rw-------. 1 root root 1097 Apr 15 10:02 dhclient-exit-hooksRZi.log
drwx------. 3 root root 17 Jul 4 2021 systemd-private-c60b800098604975be26dbbb3215bd47-chronyd.service-ZzaKpF
drwx------. 3 root root 17 Mar 27 20:01 systemd-private-c60b800098604975be26dbbb3215bd47-unified-monitoring-agent.service-561YDH
-rw-rw-r--. 1 opc opc 0 Apr 12 12:30 vignesh.txt
⚡ Alias Command¶
Aliases allow you to create shortcut commands. We will create a command named myls which will print the current date and list files.
[opc@new-k8s tmp]$ alias myls="date && ls -l"
[opc@new-k8s tmp]$ myls
Sat Apr 15 11:01:37 GMT 2023
dhclient-exit-hooksRZi.log systemd-private-c60b800098604975be26dbbb3215bd47-unified-monitoring-agent.service-561YDH
systemd-private-c60b800098604975be26dbbb3215bd47-chronyd.service-ZzaKpF vignesh.txt
💾 Persisting Environment Variables and Aliases (.bashrc)¶
Manually exported environment variables and aliases will be lost once the terminal session is closed. The .bashrc file in the user's home directory is executed every time a new terminal session is started. By placing commands in the .bashrc file, they will be automatically executed for every new session.
.bashrc file¶
[opc@new-k8s ~]$ clear
[opc@new-k8s ~]$ pwd
/home/opc
[opc@new-k8s ~]$ ls -lart
total 3072064
-rw-r--r--. 1 opc opc 193 Nov 22 2019 .bash_profile
-rw-r--r--. 1 opc opc 18 Nov 22 2019 .bash_logout
-rw-r--r--. 1 opc opc 172 Apr 2 2020 .kshrc
drwxr-xr-x. 3 opc docker 33 Jul 4 2021 .kube
drwxrw----. 3 opc opc 19 Jul 4 2021 .pki
drwxrwxr-x. 4 opc opc 30 Nov 26 2021 .config
drwxrwxr-x. 4 opc opc 30 Nov 26 2021 .cache
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc opc 82 Jan 11 2022 .docker
drwx------. 2 opc opc 80 May 26 2022 .ssh
drwxr-xr-x. 4 root root 32 Apr 13 12:25 ..
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt
-rwxrwxr-x. 1 opc opc 48 Apr 15 10:56 myinfo
-rw-r--r--. 1 opc opc 285 Apr 15 11:19 .bashrc
-rw-------. 1 opc opc 6596 Apr 15 11:19 .viminfo
drwxr-x---. 10 opc opc 4096 Apr 15 11:19 .
-rw-------. 1 opc opc 26674 Apr 15 11:19 .bash_history
[opc@new-k8s ~]$ cat .bashrc
# .bashrc
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
# Uncomment the following line if you don't like systemctl's auto-paging feature:
# export SYSTEMD_PAGER=
# User specific aliases and functions
export NEW_NAME="Raghav"
alias myls="date && ls -l"
[opc@new-k8s ~]$ myls
Sat Apr 15 11:21:19 GMT 2023
total 3072008
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rwxrwxr-x. 1 opc opc 48 Apr 15 10:56 myinfo
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test
[opc@new-k8s ~]$ echo $NEW_NAME
Raghav
🚦 Exit Codes ($?)¶
$? is a special variable which holds the status code of the last executed command. In Linux, 0 means success, any other value indicates failure.
[opc@new-k8s ~]$ ll
total 3072008
-rw-rw-r--. 1 opc opc 852 Apr 15 03:15 fruits.txt
-rwxrwxr-x. 1 opc opc 81 Apr 15 13:27 newtest
drwxrwxr-x. 2 opc opc 25 Nov 26 2021 prometheus
-rw-r--r--. 1 root root 3145728000 Jan 11 2022 swapfile
drwxrwxr-x. 4 opc opc 100 Apr 15 13:04 test
[opc@new-k8s ~]$ echo $?
0
[opc@new-k8s ~]$ ddhghg
-bash: ddhghg: command not found
[opc@new-k8s ~]$ echo $?
127
🧠 Quick Quiz — Shell Basics & Environment¶
Which environment variable determines where Linux looks for executable commands?
📝 Want More Practice?¶
To strengthen your understanding and prepare for interviews, try the full 20-question practice quiz based on this chapter:
👉 Start Shell Basics & Environment Quiz (20 Questions)
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