Tech Handbook Null Yard

Debian - Practical Shell Handbook

This handbook covers day-to-day Debian administration from the shell. For shell choices see Shells in Debian, for automation see Shell Scripting, and for remote access see SSH and Remote Administration.

Debian - Administrator, System and Directories

Debian is a Linux distribution built around the Linux kernel, GNU/user-space tools, APT/dpkg package management and usually systemd.

sudo

Run one command with administrative privileges:

sudo COMMAND

Use sudo instead of staying logged in as root for routine administration.

Debian version

cat /etc/os-release
cat /etc/debian_version
uname -a

Important directories

/etc        system configuration
/var/log    logs
/home       user homes
/root       root home
/usr        programs/libraries
/usr/local  locally installed software
/srv        service data
/run        runtime state
/tmp        temporary files

1. Shell, terminal and commands - what are you actually doing?

The terminal displays input/output. The shell interprets commands. Bash is common on Debian interactive accounts, while /bin/sh is typically dash for system scripting.

2. pwd - where am I?

pwd

3. ls - what is in the directory?

ls

Important options

-l

Long listing with permissions, owner, size and timestamps.

-a

Include hidden files.

-h

Human-readable sizes with long listing.

-t

Sort by modification time.

-S

Sort by size.

Most common form

ls -lah

4. cd - change directory

cd /etc

Important variants

Parent directory

cd ..

Home directory

cd
cd ~

Previous directory

cd -

5. Relative and absolute paths

Absolute paths begin with /. Relative paths are interpreted from the current directory.

.

Current directory.

..

Parent directory.

6. touch - create empty file and update modification time

touch file.txt

7. mkdir - create directories

mkdir logs

-p

Create missing parents:

mkdir -p project/logs/archive

8. cp - copy

cp source target

-r

Recursive directory copy.

-i

Prompt before overwrite.

-v

Verbose output.

Typical case

Back up a configuration file before editing:

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

9. mv - move and rename

mv old.txt new.txt

-i

Prompt before overwrite.

-v

Show performed operations.

10. rm - delete

rm file.txt

-i

Prompt before removal.

-r

Recursive deletion.

-f

Force mode.

-rf

Recursive force. Dangerous when the path is wrong. Never type it mechanically.

11. cat - quickly display a file

cat /etc/hosts

12. less - read large files comfortably

less /var/log/syslog

Useful keys: / search, n next, G end, g start, q quit.

13. head - start of a file

head file.txt

-n

head -n 20 file.txt

14. tail - end of a file

tail file.txt

-n

tail -n 50 file.txt

-f

Follow a growing file:

tail -f /var/log/nginx/error.log

15. grep - search text

grep error file.log

-i

Case-insensitive.

-n

Show line numbers.

-R

Recursive search.

-v

Invert match.

-E

Extended regular expressions.

Very practical

grep -RniE 'error|warning' /etc /var/log 2>/dev/null

16. find - find files

-name

find /etc -name '*.conf'

-iname

Case-insensitive name matching.

-type

find /var -type f

-mtime

find /var/log -type f -mtime -7

-size

find /var -type f -size +100M

17. which, command -v, whereis - where is a program?

which nginx
command -v bash
whereis nginx

18. file - what kind of file is this?

file /bin/bash

19. stat - detailed file information

stat file.txt

20. man - command documentation

man systemctl
man 5 sshd_config

21. echo - print text and variables

echo hello
echo "$HOME"

22. Redirection > and >>

> overwrites, >> appends.

echo test > file.txt
echo second >> file.txt

23. Pipe |

ss -lntp | grep ':8080'

24. sort, uniq, wc

sort

sort names.txt

uniq

sort names.txt | uniq

wc

wc -l file.txt

25. whoami, id, groups

whoami
id
groups

26. who and w

who
w

28. uname - kernel and platform

-a

uname -a

-r

uname -r

30. hostname

hostname
hostnamectl

31. uptime

uptime

32. top - processes and load

top

Install htop if you prefer a more interactive display.

33. ps - process list

ps aux

34. pgrep - find PID by name

-f

Match full command line.

-l

Show process names.

pgrep -af nginx

35. kill and pkill

kill PID
pkill processname

36. df - free filesystem space

df -h

37. du - how much space does a directory use?

-s

Summary.

-h

Human-readable sizes.

du -sh /var/*

38. mount - what is mounted?

mount
findmnt

39. lsblk - disks on Debian

lsblk
lsblk -f

42. Debian - ip

ip addr
ip link
ip route

45. ping

ping 1.1.1.1
ping example.com

46. curl

curl -I https://example.com
curl -v https://example.com

47. DNS - host and dig

host example.com
dig example.com

Install dnsutils if needed.

48. Debian - ss

ss -lntup

Use it to see listening ports and sockets.

50. ssh

ssh user@server

Different port

ssh -p 2222 user@server

More diagnostics

ssh -vvv user@server

51. scp

scp file user@server:/tmp/

52. ssh-keygen

ssh-keygen -t ed25519

53. chmod

Add execute permission

chmod +x script.sh

Numeric mode

chmod 640 config.conf
chmod 755 script.sh

54. chown

sudo chown user:group file

55. Debian - apt

apt update

Refresh package metadata:

sudo apt update

apt upgrade

Upgrade installed packages without removing installed packages to satisfy dependency changes:

sudo apt upgrade

Typical update

sudo apt update && sudo apt upgrade

apt full-upgrade

Allows dependency changes/removals when required. Review the proposed transaction.

Install

sudo apt install nginx

Remove

sudo apt remove nginx

Purge

sudo apt purge nginx

Autoremove

sudo apt autoremove

apt search nginx

Information

apt show nginx
apt policy nginx

56. Debian - dpkg

dpkg -l
dpkg -L PACKAGE
dpkg -S /path/to/file

58. Debian - systemd and systemctl

systemctl status nginx
sudo systemctl start nginx
sudo systemctl restart nginx
sudo systemctl reload nginx
sudo systemctl enable --now nginx

59. Debian - journalctl

journalctl -u nginx
journalctl -u nginx -f
journalctl -b
journalctl -k

65. Debian - NetworkManager

nmcli device
nmcli connection show
nmcli connection show --active

66. dmesg

dmesg | tail -n 50

On restricted systems use sudo dmesg if required.

67. lscpu, lspci, lsusb - Debian

lscpu
lspci
lsusb

69. date

date
timedatectl

70. history

history

71. Ctrl+R

Reverse-search previous shell commands interactively.

72. Keyboard shortcuts

Common: Ctrl+C interrupt, Ctrl+D EOF/logout, Ctrl+L clear, Ctrl+Z suspend, Ctrl+A line start, Ctrl+E line end.

73. &&, ||, ;

&&

Run next command only on success:

sudo nginx -t && sudo systemctl reload nginx

||

Run next command only if the previous command failed.

;

Run sequentially regardless of exit status.

74. Background processes

long-command &
jobs
fg
bg

75. tmux

sudo apt install tmux
tmux new -s admin
tmux attach -t admin

76. tar archives

tar -czf backup.tar.gz directory/
tar -xzf backup.tar.gz

77. gzip, gunzip

gzip file
gunzip file.gz

78. crontab

crontab -e
crontab -l

81. Quick Debian diagnostics

cat /etc/os-release
uptime
df -h
free -h
ip addr
ip route
ss -lntup
systemctl --failed
journalctl -p err -b

Real-life examples - Debian

1. Website stopped responding

systemctl status nginx
nginx -t
ss -lntp | grep -E ':80|:443'
journalctl -u nginx -n 100
curl -v http://127.0.0.1/

2. Disk is almost full

df -h
df -i
sudo du -xhd1 /var | sort -h

3. You forgot where configuration is

dpkg -L PACKAGE | grep -E '/etc/|conf'
find /etc -iname '*name*'

4. Who uses port 8080?

sudo ss -lntp | grep ':8080'
sudo lsof -i :8080

5. Network does not work

ip addr
ip route
ping 1.1.1.1
dig example.com
resolvectl status

6. Safe SSH configuration change

sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
sudoedit /etc/ssh/sshd_config
sudo sshd -t
sudo systemctl reload ssh

Keep the current SSH session open and test a second login before disconnecting.

7. Application consumes CPU

top
ps aux --sort=-%cpu | head
pgrep -af appname

8. Watch logs while testing an application

journalctl -u myapp -f

9. Quickly recognize an unfamiliar server

cat /etc/os-release
hostname
uptime
df -h
ip addr
systemctl --failed
ss -lntup

10. Update Debian

sudo apt update
sudo apt upgrade

Review held packages and whether a reboot is required.

11. Long process over SSH

tmux new -s work
long-command

12. System configuration backup

sudo tar -czf /tmp/etc-backup.tar.gz /etc

For a real backup, copy it off the server and include application data/databases separately.

Debian - minimal set to remember

pwd / ls / cd
cp / mv / rm
less / tail / grep / find
ps / top / kill
df / du / lsblk
ip / ss / dig / curl
ssh / scp
apt / dpkg
systemctl / journalctl
tmux / tar / crontab

The practical Debian pattern is: inspect state, read logs, test one layer at a time, then make the smallest change you understand.

Official references

  • Debian Reference: https://www.debian.org/doc/manuals/debian-reference/
  • Debian manpages: https://manpages.debian.org/
  • Debian documentation: https://www.debian.org/doc/