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I spend part of my time as a system admin for our servers. Since I am fairly new to Linux, I assembled a guide of the most common CLI commands as a reference for myself. I decided to share my command reference guide in case anyone else finds it useful. Since we are mostly focused on Ubuntu 8.04 LTS, this guide is aimed at that operating system, but these common commands should work with other Linux distributions as well. Since Ubuntu does not recommend using the system as root user, most commands require the sudo command to elevate to root privileges. User input is highlighted in blue. Optional input is surrounded by curly brackets like this {optional parameter}.

Users & Groups
Devices
File system
General
Network configuration
Server application configuration

Users & Groups

Edit adduser configuration

sudo nano /etc/adduser.conf

Change default directory mode:

DIR_MODE=0750

This make user directories private. The digits work as follows:

Mode digits: 2 = write only, 4 = read only, 5 = read and execute, 6 = read & write, 7 = read, write & execute

(execute means show listing for directories)

In order, the numbers apply to the owning user first, his group second and everyone else third

Add/create user

sudo adduser -m username

-m adds a user directory under home with all required files

Delete user (and his home directory)

sudo userdel -r username

Change own password

passwd

Change user’s password

sudo passwd username

Edit password security settings

sudo nano /etc/pam.d/common-password

See Ubuntu Guide on setting stricter password control

Add User group

sudo addgroup groupname

Add user to group

sudo adduser username groupname

Remove user from group

sudo deluser username groupname

Show all groups and users in them

cat /etc/group | less

Show list of all users with names

cat /etc/passwd | awk -F: '{print $1,",",$5}' | more

Show groups you are in

groups

Edit sudoers file / add users to sudo list

sudo visudo

under root, add:

username ALL=(ALL) ALL

Devices

Get processor info

cat /proc/cpuinfo

Get detailed system information

sudo lshw

Device messages (peripherals)

dmesg

Combine with | grep message to search for particular messages

Find new external disk after connection (shows recent messages)

dmesg | tail

Display volume information

sudo vgdisplay

List of mounted partitions

mount

Mount a disk

mount /mnt/mountdirectory/

Unmount a disk

umount /mnt/mountdirectory/

sync

See current mount points and used/free space

df -H


See physical volumes

pvdisplay


Show RAID partitioning

cat /proc/mdstat


List of disks and their partitions

sudo fdisk -l

File system

Directory listing with useful info

ls -la

Path to current directory

pwd

Switch to previous directory used

cd -

Create symbolic link (file reference)

ln -s targetpath linkname

Show text file contents

less filename

Number of files in a directory and its subdirectories

ls -1R | wc -l

Edit file in text editor nano (simple)

nano filename

Edit file in text editor vim (powerful)

vim filename

Remove empty directory

rmdir directoryname

Remove directory and all contained files and directories

rm -r directoryname

Find files by name

sudo find / -name *filename* -print

Find files by file owner

sudo find / -user username or userid -print

Securely erase files, partitions or hard drives

sudo shred -vfz -n [times] [your hard drive]
— Read on for more information on shred on Ubuntu

General

Display date, time and time zone

date

Change time zone

sudo dpkg-reconfigure tzdata

Logout

logout

Change file owner

chown owner-username{:groupname} filename

Change permissions

chmod mode-such-as-777 filename

Update time via network time server (one time)

sudo ntpdate timeserver {additional-timeserver...}

Setup daily cron job to update time

sudo nano /etc/cron.daily/ntpdate
enter ntpdate timeserver {additional-timeserver...}
(Don’t forget to save the file)
sudo chmod 755 /etc/cron.daily/ntpdate
(Makes the file executable)

Get APT repository list

sudo nano /etc/apt/sources.list
(For example, uncomment Universe for wider selection)

Clear screen

clear

Show all command aliases

alias

Change command alias

alias name='some command w/ options'
(for example alias ls = 'ls -lah'
(put in .bashrc to save permanently)

Find where is the command located

which command

Find out if a process is running

ps -fa | grep process name
(e.g. apache)

List active processes

sudo ps aux

Reboot

sudo reboot

Shutdown (immediately)

sudo poweroff
See also ‘Shutdown’ versus ‘poweroff’ versus ‘halt’ on Ubuntu server

Network configuration

Server application configuration