Ubuntu-L3-Sergeant

Text Processing & Streams

echo — Print text or variables

Prints text or variable values to standard output. Commonly used in scripts and pipelines.

$ echo "Hello Ubuntu"

Hello Ubuntu

wc — Count lines, words, characters

Counts lines, words, and bytes from input. Useful for quick size metrics.

$ wc file.txt

 6  5 30 file.txt

sort — Sort lines in a file

Sorts input lines lexicographically or numerically. Commonly chained before uniq.

$ cat names.txt

CSP
Kathir
Aron
Kevin
Mohammed
Azhar
Meeran


$ sort names.txt

Aron
Azhar
CSP
Kathir
Kevin
Meeran
Mohammed

uniq — Remove duplicate lines

Removes consecutive duplicate lines. Requires sorted input for full deduplication.

$ cat cities.txt

Montreal
Toronto
Madurai
Toronto
Chennai
Montreal
Chennai

$ sort cities.txt | uniq

Chennai
Madurai
Montreal
Toronto

cut — Extract columns from text

Extracts selected fields from each line. Ideal for delimited data.

$ cat city.csv

index,city
1,Madurai
2,Toronto


$ cut -d',' -f1 city.csv

index
1
2


$ cut -d',' -f2 city.csv

city
Madurai
Toronto

tr — Translate or delete characters

Translates or removes characters from input. Works only on character streams.

$ echo "hello world" | tr 'a-z' 'A-Z'

HELLO WORLD

awk — Pattern scanning and processing

Pattern-action language for structured text. Powerful for column-based processing.

$ cat file.txt

John Smith 25
Alice Brown 30
Bob Lee 22 


$ awk '{print $1}' file.txt

John
Alice
Bob

sed — Stream editor for text replacement

Stream editor for search and replace. Processes text without opening files.

$ cat file.txt

John Smith 25
Alice Brown 30
Bob Lee 22


$ sed 's/John/Mike/g' file.txt

Mike Smith 25
Alice Brown 30
Bob Lee 22

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Environment & Shell Control

env — Show environment variables

Displays current environment variables. Useful for runtime inspection.

$ env

SHELL=/bin/bash
PYENV_SHELL=bash
PWD=/Users/csp/kact/lima-base
LOGNAME=csp
XDG_SESSION_TYPE=tty
HOME=/home/csp.linux
LANG=C.UTF-8
...

export — Set environment variables

Marks variables for child processes. Common in shell initialization.

$ export APP_ENV=production

$ echo $APP_ENV
production

source — Reload shell configuration

Executes a file in the current shell. Applies config changes immediately.

$ source ~/.bashrc

which — Locate a command binary

Shows the path of the command executed. Helps detect shadowed binaries.

$ which python

/home/csp.linux/.pyenv/shims/python

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Disk Usage & Filesystems

du -h — Directory size usage

Reports disk usage per directory. Human-readable sizes.

$ du -h --max-depth=1

0	./three
0	./one
0	./two
0	./two2
112K	./.git
164K	.

statx — Extended file statistics (newer Ubuntu)

Displays extended filesystem metadata. Provides low-level file attributes.

$ statx file.txt

file — Detect file type

Identifies file type by content. Does not rely on extensions.

$ file city.csv
city.csv: ASCII text


$ file file.txt
file.txt: ASCII text

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Scheduling & Automation

crontab — Schedule jobs

Manages recurring scheduled jobs. Runs commands automatically.

$ crontab -e

at — Run commands at a specific time

Schedules one-time future tasks. Requires the atd daemon.

$ echo "backup.sh" | at 22:00

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Services & Systemd

systemctl status — Check service status

systemctl status Shows service health and logs. Used for troubleshooting.

$ systemctl status ssh

● ssh.service - OpenBSD Secure Shell server
     Loaded: loaded (/usr/lib/systemd/system/ssh.service; disabled; preset: enabled)
     Active: active (running) since Wed 2026-02-04 06:44:55 EST; 14h ago
 Invocation: 7a98164a660b45b78f783e62fcef864c
TriggeredBy: ● ssh.socket
       Docs: man:sshd(8)
             man:sshd_config(5)
    Process: 118956 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS)
   Main PID: 118967 (sshd)
      Tasks: 1 (limit: 4535)
     Memory: 1.1M (peak: 1.8M)
        CPU: 24ms
     CGroup: /system.slice/ssh.service
             └─118967 "sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups"

systemctl start — Start a service

Starts a service immediately. Does not persist after reboot.

$ sudo systemctl start nginx

systemctl enable — Start service at boot

Enables service at system startup. Creates persistent activation.

$ sudo systemctl enable docker

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Networking & Sessions

ss — Inspect open ports and sockets

Displays active sockets and ports. Replaces legacy netstat.

$ ss -tuln

Netid                   State                    Recv-Q                   Send-Q                                         Local Address:Port                                       Peer Address:Port
udp                     UNCONN                   0                        0                                                    0.0.0.0:5353                                            0.0.0.0:*
udp                     UNCONN                   0                        0                                                 127.0.0.54:53                                              0.0.0.0:*

who — Logged-in users

Lists logged-in users. Shows session terminals.

$ who

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Power & Session Management

uptime — System running time

Shows system running time. Includes load averages.

$ uptime

 21:32:06 up 11 days, 14:05,  1 user,  load average: 0.00, 0.01, 0.00

shutdown — Power off or reboot

Safely powers off or reboots the system. Notifies active users.

$ sudo shutdown now

reboot — Restart system

Restarts the system. Requires root privileges.

$ sudo reboot

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