Linux Commands A-Z Part 1: alias, apt, awk, bc, cat, chattr and More with Examples

By Ravindra Bagale, trainer LinkedIn Last updated: 26 October 2026

This is Part 1 of the Linux Commands A-Z reference. It covers every command from alias to chown. Each command gets its own section: why you need it, what it is, a table of the options you will really use, and step-by-step examples with the command, the output you will see and what that output means. Commands already explained in depth in earlier chapters (cd, chmod, chown and cat with redirects) are linked, not repeated. awk gets a full deep dive with twelve worked examples.

What you'll learn in this chapter

Use alias, apropos and man -k to work faster and find commands
Install and patch software with apt and schedule one-time jobs with at
Process columns, sums and group-by reports with awk
Encode data and do decimal maths with base64 and bc
Manage jobs, disks and compression with bg, blkid, bzip2 and bunzip2
Control password ageing, file attributes and groups with chage, chattr and chgrp

1. An everyday example: the tool box at a Pune garage

Why do we need this?

Ganesh runs a two-wheeler garage near Swargate. His tool box has a drawer for every tool, labelled A to Z. When a new mechanic joins, Ganesh does not explain everything at once. He opens one drawer, shows why the tool is needed, what it is, the settings that matter, and then fixes a real bike with it while the new mechanic watches.

What is it?

Each Linux command is one drawer. Why is the job it solves. What is the tool itself. The options table is the settings. The step-by-step examples are the real bike being fixed, with the output you should see.

How to do it (step by step)

Six steps per command. About 10 to 20 minutes each.

2. How to use this A-Z part

Why do we need this?

A reference is useful only if you can find the command fast and trust the examples.

What is it?

Commands are in alphabetical order. Each has an anchor like #cmd-awk, so you can bookmark or share one command. The A-Z index page lists all 183 commands with links.

Section	What you get
Why	The real problem this command solves
What	How it works in plain words
Options	The options used in real jobs
Steps	Command, output and meaning

How to do it (step by step)

Open the help for any command.

awk --help | head -3

Output:

Usage: awk [POSIX or GNU style options] -f progfile [--] file ...

What it means: --help is quick; man is complete.

Search the manual inside man.

man awk   # then type /NF and press n

What it means: Inside man, /word searches and q quits.

3. man, --help and type: check before you trust

Why do we need this?

Versions differ between Ubuntu, RHEL and macOS. Checking the local manual avoids surprises.

What is it?

man shows the manual, --help prints a short summary, and type tells you whether a name is a builtin, an alias or a file.

How to do it (step by step)

Is it a builtin or a program?

type bg alias at

Output:

bg is a shell builtin
alias is a shell builtin
at is /usr/bin/at

What it means: Builtins are documented in help, programs in man.

Read help for a builtin.

help bg | head -2

Output:

bg: bg [job_spec ...]
    Move jobs to the background.

What it means: help is the man page for bash builtins.

4. alias: Make a short name for a long command

alias in depth: short names, listing and removing

Why do we need this?

You type the same long commands every day, like ls -lah or a long ssh line. An alias turns them into a short word, so you type less and make fewer typing mistakes.

What is it?

alias is a shell builtin. It maps a name to a piece of command text. The shell swaps the name for the text before running it. An alias lives only in the current shell unless you save it in ~/.bashrc.

Common options

Option	What it does
`alias`	List all aliases defined now.
`alias ll='ls -lah'`	Create an alias called ll.
`alias ll`	Show one alias.
`unalias ll`	Remove one alias.
`unalias -a`	Remove all aliases in this shell.
`\ls`	Run the real command, skipping the alias.

How to do it (step by step)

Create an alias for a long listing.

alias ll='ls -lah'

What it means: Nothing is printed. From now on, ll runs ls -lah.

Check what it points to.

alias ll

Output:

alias ll='ls -lah'

What it means: The shell shows the stored text.

Find out how the shell sees the name.

type ll

Output:

ll is aliased to `ls -lah'

What it means: type confirms ll is an alias, not a file on disk.

Make it permanent.

echo "alias ll='ls -lah'" >> ~/.bashrc && source ~/.bashrc

What it means: The alias is saved and loaded in every new shell.

Add a safety alias for rm.

alias rm='rm -i'

What it means: Now rm asks before deleting each file. Many RHEL root accounts ship with this.

Remove it again.

unalias rm

What it means: rm is back to normal in this shell.

Practise this in the tasks below: Practice tasks

Real use: A support engineer at a Hinjewadi IT park sets alias k=kubectl and alias prodssh to save hundreds of keystrokes a day.

5. apropos: Search man pages by keyword

apropos in depth: find a command when you forget its name

Why do we need this?

You know what you want to do, like "compress" or "partition", but not the command name. apropos searches the man page summaries for that word.

What is it?

apropos is the same as man -k. It reads the man page database (built by mandb) and prints each matching page with its section number and one-line summary.

Common options

Option	What it does
`apropos word`	Search names and summaries for word.
`apropos -e word`	Exact word match only.
`apropos -s 8 word`	Only section 8 (admin commands).
`apropos -a w1 w2`	Pages matching all the words.
`man -k word`	Same as apropos.
`mandb`	Rebuild the database (as root).

How to do it (step by step)

Search for compression tools.

apropos compress

Output:

bzip2 (1)  - a block-sorting file compressor
gzip (1)   - compress or expand files
xz (1)     - Compress or decompress .xz and .lzma files

What it means: Each line is a command, its man section, and what it does.

Only admin commands about partitions.

apropos -s 8 partition

Output:

fdisk (8)  - manipulate disk partition table
parted (8) - a partition manipulation program

What it means: Section 8 holds system admin commands.

If you get "nothing appropriate", build the database.

sudo mandb

Output:

Processing manual pages under /usr/share/man...

What it means: On fresh cloud images the database is often empty.

Combine two words.

apropos -a copy file

Output:

cp (1) - copy files and directories

What it means: -a means all words must match.

Practise this in the tasks below: Practice tasks

Real use: A fresher in a Baner startup uses apropos user to discover useradd, usermod and userdel on day one.

6. apt: Install packages on Debian and Ubuntu

apt in depth: update, install, remove and search on Ubuntu

Why do we need this?

Ubuntu and Debian servers run a lot of the cloud. You need to install software, apply security fixes and remove old packages safely.

What is it?

apt is the friendly front end to the Debian package system (dpkg). It downloads .deb packages from repositories listed in /etc/apt/sources.list and /etc/apt/sources.list.d/, and also pulls in dependencies.

Common options

Option	What it does
`apt update`	Refresh the package lists (does not upgrade).
`apt upgrade`	Upgrade installed packages.
`apt install pkg`	Install a package.
`apt install -y pkg`	Install without the yes/no question.
`apt remove pkg`	Remove, keep config files.
`apt purge pkg`	Remove with config files.
`apt autoremove`	Remove unused dependencies.
`apt search word`	Search packages.
`apt show pkg`	Details of a package.
`apt list --installed`	List installed packages.

How to do it (step by step)

Refresh lists first.

sudo apt update

Output:

Hit:1 http://archive.ubuntu.com/ubuntu jammy InRelease
...
12 packages can be upgraded.

What it means: apt now knows the latest versions. Nothing is installed yet.

Install nginx.

sudo apt install -y nginx

Output:

Setting up nginx (1.18.0-6ubuntu14) ...

What it means: nginx and its dependencies are installed and the service starts.

Check what version you got.

apt show nginx | head -3

Output:

Package: nginx
Version: 1.18.0-6ubuntu14
Priority: optional

What it means: apt show reads package details.

Apply all security and bug fixes.

sudo apt upgrade -y

Output:

12 upgraded, 0 newly installed, 0 to remove

What it means: Installed packages move to the latest versions.

Clean unused packages.

sudo apt autoremove -y

Output:

Removing linux-image-5.15.0-91-generic ...

What it means: Old kernels and orphan libraries are removed to free disk.

Practise this in the tasks below: Practice tasks

Real use: A Kharadi fintech patches 40 Ubuntu servers every Tuesday with apt update and apt upgrade in a maintenance window.

7. at: Run a command once at a later time

at in depth: run a job once, later

Why do we need this?

Sometimes you need a task to run once at a set time, like a restart at 2 AM or removing temporary access tomorrow. cron is for repeating jobs; at is for one-time jobs.

What is it?

at reads commands from your keyboard or a file and queues them. The atd service runs them at the given time. Output is mailed to the user, so redirect it to a file if there is no mail.

Common options

Option	What it does
`at 02:00`	Run at 2 AM (today or tomorrow).
`at now + 30 minutes`	Run in 30 minutes.
`at -f job.sh 18:00`	Read commands from a file.
`atq`	List queued jobs.
`atrm 3`	Remove job number 3.
`at -c 3`	Show the full job script.

How to do it (step by step)

Make sure atd runs.

sudo systemctl enable --now atd

What it means: The at daemon is running and starts at boot.

Queue a one-time job.

echo 'systemctl restart nginx' | sudo at 02:00

Output:

job 4 at Tue Oct 27 02:00:00 2026

What it means: The job gets number 4 and the run time.

See the queue.

sudo atq

Output:

4	Tue Oct 27 02:00:00 2026 a root

What it means: Job 4 waits in queue a for user root.

Cancel it.

sudo atrm 4

What it means: Job 4 is gone. atq shows nothing.

Give temporary access that removes itself.

echo 'gpasswd -d vendor wheel' | sudo at now + 4 hours

Output:

job 5 at Mon Oct 26 05:10:00 2026

What it means: After four hours the vendor loses sudo rights automatically.

Practise this in the tasks below: Practice tasks

Real use: A hospital IT team in Deccan schedules a one-time database restart at 3 AM with at, so no doctor is disturbed.

8. awk: Work with columns, sums and reports in text

awk deep dive: fields, patterns, sums, groups and reports

Why do we need this?

Logs, CSV files and command outputs are all text in columns. awk lets you pick columns, filter rows, add up numbers and print reports in one line, without opening Excel or writing a Python script. It is one of the most asked commands in Linux interviews.

What is it?

awk is a small programming language for text. It reads input line by line. Each line is a record ($0). It splits the line into fields $1, $2 ... using spaces by default, or the separator you set with -F. A program is pattern { action }. If the pattern matches, the action runs. BEGIN runs before the first line and END after the last. NR is the line number, NF the number of fields.

Common options

Option	What it does
`awk '{print $1}'`	Print the first field of each line.
`awk -F: '{print $1}'`	Use : as the field separator.
`awk -F, -v OFS=, '...'`	Set input and output separators for CSV.
`awk '$3 > 100'`	Print lines where field 3 is above 100.
`awk '/ERROR/'`	Print lines matching a regex.
`awk 'NR==5'`	Print line 5 only.
`awk '{print NF}'`	Print the number of fields.
`awk '{print $NF}'`	Print the last field.
`awk '{s+=$2} END{print s}'`	Sum a column.
`awk '{c[$1]++} END{for(k in c) print k,c[k]}'`	Count by key (group by).
`awk -v t=80 '$5+0>t'`	Pass a shell value into awk.
`awk 'BEGIN{printf "%.2f\n", 10/3}'`	Decimal maths and formatted print.
`awk '!seen[$0]++'`	Remove duplicate lines, keep order.
`awk -f report.awk file`	Run a program from a file.

How to do it (step by step)

Print user names from /etc/passwd.

awk -F: '{print $1}' /etc/passwd | head -3

Output:

root
bin
daemon

What it means: -F: splits on colons. $1 is the user name.

Print name and shell, but only for real login users.

awk -F: '$3>=1000 {print $1, $7}' /etc/passwd

Output:

arjun /bin/bash
priya /bin/bash

What it means: $3 is the UID. Normal users start at 1000 on most distros.

Find filesystems above 80 percent.

df -h | awk 'NR>1 && $5+0>80 {print $6, $5}'

Output:

/var 91%

What it means: NR>1 skips the header. $5+0 turns 91% into the number 91.

Count requests per IP in a web log.

awk '{c[$1]++} END{for(ip in c) print c[ip], ip}' access.log | sort -rn | head -3

Output:

812 10.0.4.17
310 10.0.4.22
95 10.0.9.3

What it means: c is an array keyed by IP. END prints the totals; sort ranks them.

Total and average of a CSV column.

awk -F, 'NR>1 {s+=$3; n++} END{printf "total=%d avg=%.2f\n", s, s/n}' sales.csv

Output:

total=48200 avg=1606.67

What it means: Skip the header, add column 3, count rows, then print with printf.

Sum sales per city (group by).

awk -F, 'NR>1 {t[$2]+=$3} END{for(k in t) print k, t[k]}' sales.csv

Output:

Pune 21000
Nashik 9800
Nagpur 17400

What it means: t[city] keeps a running sum for each city.

Print the last field of each line.

echo 'a b c d' | awk '{print $NF, NF}'

Output:

d 4

What it means: $NF is the last field; NF alone is the count.

Lines between two markers.

awk '/BEGIN CERT/,/END CERT/' bundle.pem | wc -l

Output:

What it means: A range pattern prints from the first match to the second.

Pass a shell variable safely.

limit=500; awk -v l="$limit" '$2>l' orders.txt

Output:

ORD-9 750
ORD-14 1200

What it means: -v sets an awk variable. Do not glue shell variables inside the quotes.

Remove duplicate lines but keep the first one in order.

awk '!seen[$0]++' emails.txt

Output:

a@x.com
b@y.com

What it means: seen counts each line. Only the first time (0, which is false, then !) prints.

Change a CSV column and keep commas.

awk -F, -v OFS=, 'NR>1 {$3=$3*1.18} 1' sales.csv | head -2

Output:

date,city,amount
2026-10-01,Pune,1180

What it means: OFS keeps commas in output. The lone 1 means print every line.

Slow requests from a log with response time in the last field.

awk '$NF>2.0 {print $7, $NF}' access.log | head -2

Output:

/api/report 3.41
/api/export 2.77

What it means: Numbers compare as numbers, so 3.41 > 2.0 works.

Practise this in the tasks below: Practice tasks

Real use: A Pune e-commerce ops team runs one awk line every morning to count 5xx errors per API path from nginx logs, and a CA firm in Sadashiv Peth totals GST columns from CSV exports with awk.

9. base64: Encode or decode data as base64 text

base64 in depth: encode, decode and Kubernetes secrets

Why do we need this?

Some systems only accept plain text, like JSON, YAML, email or HTTP headers. base64 turns any bytes into safe text and back. You see it in Kubernetes secrets, basic auth and certificates.

What is it?

base64 is encoding, not encryption. Anyone can decode it. It uses 64 safe characters (A-Z, a-z, 0-9, + and /) and = for padding. Output grows by about one third.

Common options

Option	What it does
`base64 file`	Encode a file.
`base64 -d`	Decode.
`base64 -w 0`	No line wrapping (one long line).
`echo -n text \| base64`	Encode text without a newline.
`base64 -i`	Ignore garbage when decoding.

How to do it (step by step)

Encode a password for a Kubernetes secret.

echo -n 'Pune@2026' | base64

Output:

UHVuZUAyMDI2

What it means: -n stops echo adding a newline, which would change the result.

Decode it back.

echo 'UHVuZUAyMDI2' | base64 -d

Output:

Pune@2026

What it means: -d decodes. This is why base64 is not security.

Encode a small file on one line.

base64 -w 0 logo.png | head -c 40

Output:

iVBORw0KGgoAAAANSUhEUgAAAEAAAABACAYAAAC

What it means: -w 0 gives one line, easy to paste into JSON.

Read a secret from a cluster.

kubectl get secret db -o jsonpath='{.data.password}' | base64 -d

Output:

S3cret!

What it means: Kubernetes stores secret values in base64.

Practise this in the tasks below: Practice tasks

Real use: A Magarpatta DevOps engineer decodes Kubernetes secrets with base64 -d while debugging a login failure.

10. basename: Get the file name from a path

basename in depth: file name from a path

Why do we need this?

Scripts often get a full path but need only the file name, for example to name a backup or a log line.

What is it?

basename removes everything up to the last slash. It can also remove a suffix like .log. It does not check if the file exists; it works on the text only.

Common options

Option	What it does
`basename PATH`	Print the last part.
`basename PATH .ext`	Also remove the suffix.
`basename -s .ext a b`	Remove suffix from many paths.
`basename -a p1 p2`	Handle many paths.
`${p##*/}`	Pure bash way, no new process.

How to do it (step by step)

Get the file name.

basename /var/log/nginx/access.log

Output:

access.log

What it means: Everything before the last slash is gone.

Drop the extension too.

basename /var/log/nginx/access.log .log

Output:

access

What it means: The second argument is the suffix to cut.

Many files at once.

basename -s .csv /data/jan.csv /data/feb.csv

Output:

jan
feb

What it means: -s works on every path given.

Use it in a backup script.

f=/srv/app/config.yml; cp "$f" "/backup/$(basename "$f").$(date +%F)"

What it means: The copy is named config.yml.2026-10-26.

Practise this in the tasks below: Practice tasks

Real use: A media company in Aundh renames thousands of uploaded videos using basename in a loop.

11. bc: Calculator with decimals

bc in depth: decimal maths in the shell

Why do we need this?

Bash maths $(( )) works only with whole numbers. For GST, percentages, averages or disk sizes you need decimals. bc gives exact decimal maths.

What is it?

bc is an arbitrary-precision calculator language. It reads expressions from input. scale sets the number of decimal places. -l loads the maths library and sets scale to 20.

Common options

Option	What it does
`echo '5/2' \| bc`	Whole number result (scale 0).
`echo 'scale=2; 5/2' \| bc`	Two decimal places.
`bc -l`	Maths library, sqrt, s(), l().
`echo '2^10' \| bc`	Power.
`echo 'obase=2; 10' \| bc`	Convert to binary.
`echo '1.5 > 1.2' \| bc`	Compare: prints 1 for true.

How to do it (step by step)

See why bash is not enough.

echo $((10/3))

Output:

What it means: The decimals are lost.

Do it with bc.

echo 'scale=3; 10/3' | bc

Output:

3.333

What it means: scale=3 keeps three decimals.

Add 18 percent GST.

echo 'scale=2; 1499*1.18' | bc

Output:

1768.82

What it means: Exact money maths.

Compare decimals in a script.

load=2.75; if (( $(echo "$load > 2.0" | bc) )); then echo high; fi

Output:

high

What it means: bc prints 1 or 0, which bash (( )) can test.

Convert a number to binary.

echo 'obase=2; 192' | bc

Output:

11000000

What it means: Handy for subnet masks.

Practise this in the tasks below: Practice tasks

Real use: A billing script at a Chakan auto-parts supplier uses bc to add GST to every invoice line.

12. bg: Resume a stopped job in the background

bg in depth: continue a stopped job in the background

Why do we need this?

You started a long copy and forgot the &. Instead of killing it and starting again, you can pause it and push it to the background.

What is it?

bg is a shell builtin for job control. Ctrl+Z sends SIGTSTP and stops the foreground job. bg sends SIGCONT and lets it run in the background. The job still belongs to your shell.

Common options

Option	What it does
`Ctrl+Z`	Stop the foreground job.
`bg`	Resume the last stopped job in background.
`bg %2`	Resume job 2.
`jobs -l`	List jobs with PIDs.
`disown %1`	Detach job 1 from the shell.

How to do it (step by step)

Start a long job without &.

rsync -a /data/ /backup/data/

What it means: The terminal is busy.

Pause it.

Ctrl+Z

Output:

[1]+  Stopped                 rsync -a /data/ /backup/data/

What it means: Job 1 is stopped, not killed.

Send it to the background.

bg

Output:

[1]+ rsync -a /data/ /backup/data/ &

What it means: It runs again, and you get the prompt back.

Check it.

jobs -l

Output:

[1]+ 41822 Running                 rsync -a /data/ /backup/data/ &

What it means: Running in background with PID 41822.

Keep it alive after logout.

disown %1

What it means: The shell will not send it SIGHUP when you log out.

Practise this in the tasks below: Practice tasks

Real use: An admin at a Pimpri plant pauses a long log compression with Ctrl+Z, runs bg, and checks a live alert in the same SSH session.

13. blkid: Show UUID and type of disks and partitions

blkid in depth: UUIDs and filesystem types

Why do we need this?

Device names like /dev/sdb can change after a reboot or when you add disks. /etc/fstab should use UUIDs. blkid tells you the UUID and type of each partition.

What is it?

blkid reads the metadata of block devices and prints attributes like UUID, TYPE (xfs, ext4, swap, LVM2_member), LABEL and PARTUUID. Run it as root for full, fresh results.

Common options

Option	What it does
`blkid`	All devices.
`blkid /dev/sdb1`	One device.
`blkid -s UUID -o value DEV`	Only the UUID value.
`blkid -t TYPE=xfs`	Devices with a given type.
`blkid -o list`	Table output (older versions).
`lsblk -f`	Similar view as a tree.

How to do it (step by step)

List all.

sudo blkid

Output:

/dev/nvme0n1p1: UUID="9c1e..." TYPE="xfs"
/dev/nvme1n1: UUID="4a7d..." TYPE="ext4"

What it means: Each device with its UUID and filesystem type.

Get only the UUID for fstab.

sudo blkid -s UUID -o value /dev/nvme1n1

Output:

4a7d6f2b-1c3e-4d8a-9b0f-2e5c7a1d3f60

What it means: Clean value, ready to paste.

Write the fstab line.

echo "UUID=$(sudo blkid -s UUID -o value /dev/nvme1n1) /data ext4 defaults,nofail 0 2" | sudo tee -a /etc/fstab

Output:

UUID=4a7d... /data ext4 defaults,nofail 0 2

What it means: nofail stops a boot failure if the disk is missing.

Find all swap areas.

sudo blkid -t TYPE=swap

Output:

/dev/vdb2: UUID="e2f..." TYPE="swap"

What it means: Filters by type.

Practise this in the tasks below: Practice tasks

Real use: A cloud team in Viman Nagar adds an EBS volume and uses blkid to put its UUID in fstab so the mount survives reboots.

14. bunzip2: Uncompress .bz2 files

bunzip2 in depth: unpack .bz2 files

Why do we need this?

Old log archives, source code and data dumps often come as .bz2. You need to open them safely, sometimes keeping the original.

What is it?

bunzip2 is the same as bzip2 -d. It replaces file.bz2 with the uncompressed file unless you ask it to keep the original.

Common options

Option	What it does
`bunzip2 f.bz2`	Uncompress and remove the .bz2.
`bunzip2 -k f.bz2`	Keep the .bz2.
`bunzip2 -c f.bz2`	Write to standard output.
`bunzip2 -t f.bz2`	Test the file only.
`bzcat f.bz2`	Read without unpacking.
`tar xjf a.tar.bz2`	Extract a .tar.bz2.

How to do it (step by step)

Test before opening.

bunzip2 -tv dump.sql.bz2

Output:

  dump.sql.bz2: ok

What it means: The archive is not corrupt.

Unpack and keep the original.

bunzip2 -k dump.sql.bz2 && ls dump.sql*

Output:

dump.sql  dump.sql.bz2

What it means: Both files now exist.

Read the first lines without unpacking.

bzcat app-2025.log.bz2 | head -2

Output:

2025-01-01 00:00:01 INFO start
2025-01-01 00:00:02 INFO ready

What it means: Handy for big archives.

Extract a tar.bz2.

tar xjf source.tar.bz2 && ls

Output:

source  source.tar.bz2

What it means: j tells tar to use bzip2.

Practise this in the tasks below: Practice tasks

Real use: A research lab at a Pashan institute unpacks .bz2 genome data with bunzip2 -k so the original stays for checksums.

15. bzip2: Compress files with bzip2

bzip2 in depth: compress with better ratio than gzip

Why do we need this?

Disk and bandwidth cost money. bzip2 makes text files like logs and SQL dumps much smaller, usually smaller than gzip, at the cost of more CPU time.

What is it?

bzip2 compresses one file at a time and adds .bz2. It removes the original by default. Levels -1 to -9 control block size; -9 is the default. For folders, use tar with j.

Common options

Option	What it does
`bzip2 f`	Compress f to f.bz2.
`bzip2 -k f`	Keep the original.
`bzip2 -d f.bz2`	Decompress.
`bzip2 -9 f`	Best compression.
`bzip2 -v f`	Show ratio.
`tar cjf a.tar.bz2 dir`	Archive a folder with bzip2.

How to do it (step by step)

Compress a log and keep it.

bzip2 -kv app.log

Output:

  app.log:  9.812:1,  0.815 bits/byte, 89.81% saved

What it means: The file is about ten times smaller.

Compare sizes.

ls -lh app.log app.log.bz2

Output:

-rw-r--r-- 1 arjun arjun 120M app.log
-rw-r--r-- 1 arjun arjun  13M app.log.bz2

What it means: 120 MB became 13 MB.

Archive a folder.

tar cjf reports.tar.bz2 reports/ && ls -lh reports.tar.bz2

Output:

-rw-r--r-- 1 arjun arjun 2.1M reports.tar.bz2

What it means: c create, j bzip2, f file name.

Decompress it later.

bzip2 -d app.log.bz2

What it means: app.log is back (an error appears if app.log still exists).

Practise this in the tasks below: Practice tasks

Real use: A telecom billing team in Yerawada keeps a year of CDR files as .bz2 to save storage.

16. cal: Show a calendar

cal in depth: quick calendars in the terminal

Why do we need this?

When planning maintenance windows or cron schedules, you need to know which date is a Sunday or the last day of a month, without leaving the server.

What is it?

cal prints a calendar of the current month by default. You can ask for a month, a year or three months around today. On some systems it is ncal or comes from util-linux.

Common options

Option	What it does
`cal`	This month.
`cal 2027`	Whole year.
`cal 2 2027`	February 2027.
`cal -3`	Previous, current and next month.
`cal -m`	Weeks start on Monday.
`cal -j`	Day of year numbers.

How to do it (step by step)

This month.

cal

Output:

    October 2026
Su Mo Tu We Th Fr Sa
             1  2  3
...
25 26 27 28 29 30 31

What it means: Today is highlighted on most terminals.

Plan a Sunday window.

cal 11 2026

Output:

   November 2026
Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7

What it means: Sundays are the first column.

Monday-first view.

cal -m 11 2026

Output:

   November 2026
Mo Tu We Th Fr Sa Su
                   1

What it means: Matches the Indian office week.

Three months together.

cal -3

What it means: Good for release planning.

Practise this in the tasks below: Practice tasks

Real use: A school ERP admin in Kothrud checks cal before scheduling exam-week server freezes.

17. cat: Print and join files

cat in depth: print, number, join and spot hidden characters

Why do we need this?

cat is the fastest way to see a small file, join parts into one file, or check for hidden Windows line endings that break scripts.

What is it?

cat means concatenate. It reads files in order and writes them to standard output. With no file it reads the keyboard. For big files use less. Writing with > and >> is covered in the redirect section linked below.

Common options

Option	What it does
`cat f`	Print a file.
`cat a b`	Print a then b.
`cat -n f`	Number all lines.
`cat -b f`	Number non-empty lines.
`cat -A f`	Show tabs (^I), line ends ($) and ^M.
`cat -s f`	Squeeze repeated blank lines.
`tac f`	Print lines in reverse.

How to do it (step by step)

Show a config with line numbers.

cat -n /etc/hostname

Output:

     1	web01

What it means: Line numbers help when someone says "check line 12".

Find Windows line endings.

cat -A deploy.sh | head -2

Output:

#!/bin/bash^M$
echo start^M$

What it means: ^M before $ means CRLF. Bash will fail with $'\r': command not found.

Join split parts.

cat backup.part* > backup.tar && ls -lh backup.tar

Output:

-rw-r--r-- 1 arjun arjun 3.0G backup.tar

What it means: Parts join in name order.

Reverse a log to see newest first.

tac app.log | head -1

Output:

2026-10-26 00:50:12 INFO done

What it means: tac is cat backwards.

Practise this in the tasks below: Practice tasks

Real use: A bank support desk in Camp uses cat -A to find a hidden tab that broke a payment config file.

18. chage: Set password ageing rules

chage in depth: password expiry and account ageing

Why do we need this?

Audits (ISO 27001, PCI, RBI rules for banks) ask that passwords expire and that contractor accounts end on a date. chage sets these rules per user.

What is it?

chage changes the ageing fields in /etc/shadow: last change, minimum and maximum days, warning days, inactive days and account expiry. Defaults for new users come from /etc/login.defs.

Common options

Option	What it does
`chage -l user`	List the rules.
`chage -M 90 user`	Max 90 days between changes.
`chage -m 1 user`	Min 1 day between changes.
`chage -W 7 user`	Warn 7 days before.
`chage -E 2026-12-31 user`	Account expires on a date.
`chage -d 0 user`	Force change at next login.
`chage -I 10 user`	Lock 10 days after expiry.

How to do it (step by step)

See the current rules.

sudo chage -l priya

Output:

Last password change			: Oct 20, 2026
Password expires			: never
Account expires				: never

What it means: No expiry is set yet.

Set a 90-day policy with a 7-day warning.

sudo chage -M 90 -W 7 priya

What it means: Priya must change her password every 90 days.

End a contractor account.

sudo chage -E 2026-12-31 vendor1

What it means: After that date, vendor1 cannot log in.

Force a new password at first login.

sudo chage -d 0 newjoin

What it means: The user must set their own password when they log in.

Check again.

sudo chage -l vendor1 | grep Account

Output:

Account expires				: Dec 31, 2026

What it means: The rule is saved in /etc/shadow.

Practise this in the tasks below: Practice tasks

Real use: A cooperative bank in Shivajinagar sets chage -E on every vendor account to pass its RBI IT audit.

19. chattr: Set special file attributes like immutable

chattr in depth: immutable and append-only files

Why do we need this?

Even root can delete files by mistake, and attackers love to edit logs or /etc/resolv.conf. chattr can lock a file so nobody, not even root, can change or delete it until the lock is removed.

What is it?

chattr sets filesystem attributes on ext4 and xfs. +i makes a file immutable. +a allows only appending, good for logs. lsattr shows the attributes. Only root can set i and a.

Common options

Option	What it does
`chattr +i f`	Make immutable.
`chattr -i f`	Remove immutable.
`chattr +a f`	Append only.
`chattr -R +i dir`	Apply recursively.
`lsattr f`	Show attributes.
`lsattr -d dir`	Attributes of the folder itself.

How to do it (step by step)

Lock DNS settings.

sudo chattr +i /etc/resolv.conf

What it means: Now nothing can overwrite this file.

Check.

lsattr /etc/resolv.conf

Output:

----i---------e------- /etc/resolv.conf

What it means: The i flag is set. e is normal on ext4 (extents).

Try to delete as root.

sudo rm /etc/resolv.conf

Output:

rm: cannot remove '/etc/resolv.conf': Operation not permitted

What it means: Even root is blocked.

Unlock when you really need to edit.

sudo chattr -i /etc/resolv.conf

What it means: Normal again.

Make an audit log append-only.

sudo chattr +a /var/log/app-audit.log && echo 'x' | sudo tee /var/log/app-audit.log

Output:

tee: /var/log/app-audit.log: Operation not permitted

What it means: Overwrite fails; appending with tee -a still works.

Practise this in the tasks below: Practice tasks

Real use: A payment gateway team in Baner sets chattr +a on audit logs so nobody can quietly rewrite history.

20. chgrp: Change the group of a file

chgrp in depth: change only the group

Why do we need this?

Shared folders for a team need the right group, so every member can work with the files. chgrp changes the group without touching the owner.

What is it?

chgrp sets the group owner of files and folders. A normal user can change the group only to a group they belong to. -R works through a whole tree. chown :group does the same job.

Common options

Option	What it does
`chgrp grp f`	Set group of f.
`chgrp -R grp dir`	Recursive.
`chgrp -v grp f`	Verbose.
`chgrp -c grp f`	Report only changes.
`chgrp --reference=a b`	Copy the group of a to b.
`chgrp -h grp link`	Change a symlink itself.

How to do it (step by step)

Make a team folder.

sudo mkdir /srv/finance && sudo groupadd finance

What it means: A folder and a group exist.

Give it to the group.

sudo chgrp -R finance /srv/finance

What it means: Every file inside now has group finance.

Check.

ls -ld /srv/finance

Output:

drwxr-xr-x. 2 root finance 6 Oct 26 01:10 /srv/finance

What it means: Owner stays root, group is finance.

Allow group writing and keep the group on new files.

sudo chmod 2775 /srv/finance

What it means: The 2 (setgid) makes new files inherit the finance group.

See only what changed.

sudo chgrp -c finance /srv/finance/*

Output:

changed group of '/srv/finance/q3.xlsx' from root to finance

What it means: -c prints only real changes.

Practise this in the tasks below: Practice tasks

Real use: An accounts team at a Hadapsar manufacturing firm shares month-end files through a folder set with chgrp and setgid.

21. Commands in this range already covered in depth

Why do we need this?

These commands already have a full why, what and how section in an earlier chapter. Repeating them here would only make the page longer.

What is it?

Each link opens the exact section with options, examples and output.

How to do it (step by step)

cd: Change the current folder: cd ~, cd - and cd ..
chmod: Change permissions: numeric vs symbolic
chown: Change owner and group: user:group and -R
cat-redirect: cat with > and >>: overwrite vs append

22. Real examples from many industries

Why do we need this?

The same Linux skill shows up in every kind of company. Seeing it in many places helps you remember it.

What is it?

Each row is a made-up team. It shows who uses this and how.

How to do it (step by step)

Team and city	How they use it	Command
Sneha, SaaS support, Hinjewadi	Uses alias k=kubectl and awk to count errors per tenant	`awk '{c[$4]++} END{for(t in c) print t,c[t]}' app.log`
Rahul, bank IT, Shivajinagar	Sets chage expiry on vendor accounts for audit	`chage -E 2026-12-31 vendor1`
Meera, hospital IT, Erandwane	Schedules one-time DB restarts with at	`echo 'systemctl restart postgresql' \| at 03:00`
Imran, cloud ops, Viman Nagar	Puts EBS UUIDs into fstab with blkid	`blkid -s UUID -o value /dev/nvme1n1`
Pooja, CA firm, Sadashiv Peth	Adds GST with bc in billing scripts	`echo 'scale=2; 2500*1.18' \| bc`
Kunal, payments, Baner	Protects audit logs with chattr +a	`chattr +a /var/log/pay-audit.log`

23. Common errors and how to fix them

Why do we need this?

Everyone hits these errors. Knowing the cause saves hours.

What is it?

Read the exact message first. Then match it to a row below.

How to do it (step by step)

Error you see	Why it happens	How to fix
`alias: ll: not found`	Alias was set in another shell or not saved	Add it to ~/.bashrc and source it. `source ~/.bashrc`
`nothing appropriate.`	The man database is empty	Build it once as root. `mandb`
`E: Unable to locate package nginx`	Package lists are old	Refresh the lists first. `apt update`
`Can't open /var/run/atd.pid to signal atd. No atd running?`	atd service is stopped	Start and enable it. `systemctl enable --now atd`
`awk: cmd. line:1: unexpected newline or end of string`	Quotes or braces not closed	Wrap the program in single quotes and close every brace. `awk '{print $1}' f`
`base64: invalid input`	Text has extra characters or a missing padding	Strip spaces or use -i. `base64 -di`
`(standard_in) 1: syntax error`	bc got shell text like 5% or a comma	Pass plain numbers only. `echo '5*1.05' \| bc`
`bash: bg: no current job`	Nothing is stopped in this shell	Stop a job with Ctrl+Z first. `jobs -l`
`rm: cannot remove 'x': Operation not permitted (as root)`	The file has the immutable attribute	Check and remove the attribute. `lsattr x; chattr -i x`
`chgrp: changing group of 'f': Operation not permitted`	You are not in that group and not root	Use sudo or join the group. `sudo chgrp finance f`

24. Troubleshooting step by step

Why do we need this?

When a command from this part does not behave as shown, check these in order.

What is it?

Go top to bottom. Stop at the first step that shows the problem.

How to do it (step by step)

Is the command installed and which one runs?

type -a awk

Which version is it?

awk --version | head -1

Is a needed service running (atd)?

systemctl is-active atd

Do you have permission?

id; sudo -l | head -3

Are there hidden characters in the input?

cat -A input.txt | head -3

Read the manual for the exact option.

man chage

25. Lab 1: Build a sales report with awk, bc and column

Why do we need this?

Create a small CSV and produce a city-wise report with totals and GST, nicely formatted.

What is it?

A full hands-on lab. Run every step on a test Linux machine (an EC2 instance, a VM, or WSL).

How to do it (step by step)

Create the CSV.

printf 'date,city,amount\n2026-10-01,Pune,1000\n2026-10-01,Nashik,450\n2026-10-02,Pune,700\n' > s.csv

Total per city.

awk -F, 'NR>1{t[$2]+=$3} END{for(c in t) print c, t[c]}' s.csv

Output:

Pune 1700
Nashik 450

Grand total.

awk -F, 'NR>1{s+=$3} END{print s}' s.csv

Output:

Add 18% GST with bc.

echo 'scale=2; 2150*1.18' | bc

Output:

2537.00

Format as a table.

awk -F, 'NR>1{t[$2]+=$3} END{for(c in t) print c, t[c]}' s.csv | column -t

Output:

Pune    1700
Nashik  450

Save a dated copy with bzip2.

cp s.csv "s-$(date +%F).csv" && bzip2 -k s-*.csv && ls s-*

Output:

s-2026-10-26.csv  s-2026-10-26.csv.bz2

26. Lab 2: Secure a contractor account and a config file

Why do we need this?

Create a contractor user with an end date, a team group folder, a locked config and a one-time cleanup job.

What is it?

A full hands-on lab. Run every step on a test Linux machine (an EC2 instance, a VM, or WSL).

How to do it (step by step)

Create the user and group.

sudo useradd vendor9 && sudo groupadd ops9

Set the account end date.

sudo chage -E 2026-12-31 vendor9 && sudo chage -l vendor9 | grep Account

Output:

Account expires				: Dec 31, 2026

Make a group folder.

sudo mkdir /srv/ops9 && sudo chgrp ops9 /srv/ops9 && sudo chmod 2770 /srv/ops9 && ls -ld /srv/ops9

Output:

drwxrws---. 2 root ops9 6 Oct 26 /srv/ops9

Lock a config.

sudo cp /etc/hosts /srv/ops9/hosts.ref && sudo chattr +i /srv/ops9/hosts.ref && lsattr /srv/ops9/hosts.ref

Output:

----i---------e------- /srv/ops9/hosts.ref

Queue a cleanup in 2 hours.

echo 'chattr -i /srv/ops9/hosts.ref' | sudo at now + 2 hours

Output:

job 7 at ...

Check the queue.

sudo atq

Output:

7	Mon Oct 26 03:40:00 2026 a root

Ravindra Bagale's Tip

Do not try to memorise all 183 commands. Learn one drawer at a time: run every example in this part on a test machine, then close the page and do the practice tasks from memory. awk alone can replace many small Python scripts in daily ops work, so spend extra time on it.

27. Frequently asked questions

Q. Why are cd, chmod and chown not explained here?

They already have full sections with options and examples in earlier chapters. The links take you straight there.

Q. Is awk still worth learning when Python exists?

Yes. For one-line column work on servers, awk is faster to write and is installed everywhere.

Q. apt or dnf: which should I learn?

Both. apt for Ubuntu and Debian, dnf for RHEL, Rocky and Amazon Linux 2023. dnf is in Part 2.

Q. Is base64 safe for passwords?

No. It is encoding, not encryption. Anyone can decode it.

Q. at or cron?

at for one time, cron or systemd timers for repeating jobs.

Q. Can chattr +i stop root?

It stops changes until root removes the flag with chattr -i. It is a safety lock, not full protection.

Q. Why does bc print no decimals?

The default scale is 0. Set scale=2 or use bc -l.

Q. Where is the full list of commands?

On the Linux Commands A-Z Index page, linked under Related chapters.

28. Interview questions and answers

Beginner level

Q. What does alias do?

It gives a short name to a longer command in the shell.

Q. How do you find a command if you forget its name?

apropos keyword or man -k keyword.

Q. apt update vs apt upgrade?

update refreshes lists; upgrade installs newer versions.

Q. How do you print the first column with awk?

awk '{print $1}' file.

Mid level level

Q. How do you sum a column in awk?

awk '{s+=$3} END{print s}' file.

Q. Why use blkid for fstab?

UUIDs do not change when device names change.

Q. How do you force a password change at next login?

chage -d 0 user.

Q. What does chattr +a do?

Allows only appending, good for logs.

Senior level

Q. Group by and count in one pass without sort?

awk '{c[$1]++} END{for(k in c) print k,c[k]}' uses an associative array.

Q. How would you give a vendor time-limited sudo?

Add to the group, then at now + N hours removes them; also chage -E as a backstop.

Q. Why is awk -v better than gluing shell vars into the program?

It avoids quoting bugs and code injection from data.

Try at home

Make three aliases you will really use and save them in ~/.bashrc.
Download any public CSV and use awk to get the total and the top 3 rows by one column.
On a test VM, set chattr +i on a copy of /etc/hosts and try to edit it as root.

Practice Tasks

About this scenario

Vidarbha Cotton Mills is a textiles business in Nagpur. Ganesh Wankhede looks after its IT and cloud work. They count money in INR and run their AWS resources in the ap-south-1 Region. In the tasks below you do real jobs for them using what this lesson teaches. All names, account IDs, IP addresses and numbers are made up for practice.

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