Ubuntu Shell Commands

The Ubuntu operating system is a popular operating system, however often accessing a gui is either not appropriate or simple not available. These command line operations help the administration and use of an Ubuntu host.

File systems

Ubuntu can use a range of different file systems, however here are the most typical

Extended File System EXT
- EXT4 is the standard for Ubuntu
- Windows cannot access these file systems without additional functionality
File Allocation Table FAT
- Useful to transfer files between Windows and *nix systems like Ubuntu as both operating systems support the standard implementation

Concepts and Glossary

Term	Meaning
Symbolic Link	- can cross the file system, - allows you to link between directories, - has different inode number and file permissions than original file, - permissions will not be updated, - has only the path of the original file, not the contents.
Hard link	-can’t cross the file system boundaries (i.e. A hardlink can only work on the same filesystem), - can’t link directories, - has the same inode number and permissions of original file, - permissions will be updated if we change the permissions of source file, - has the actual contents of original file, so that you still can view the contents, even if the original file moved or removed

Shells such as bash, csh, ksh and dash are implementations of the POSIX standard. POSIX Standard

The implementation used by default on a system is determined by either a symbolic link or hard link on the file /bin/sh which points to a POSIX implementation.

Command Line	Description
file -h /bin/sh	(Symbolic link) A portable way to determine which shell is being used by a system
find -L /bin -samefile /bin/sh	(Hard link) A portable way to determine which shell is being used by a system

Basic

Command Line	Description
ls	directory listing
ls -al	formatted listing with hidden files
cd directory	change directory to dir
cd or cd ~	change to home
pwd	show current directory
mkdir directory	create a directory dir
rm file	delete file
rm -r directory	delete directory dir
rm -f file	force remove file
rm -rf directory	force remove directory dir *
cp file1 file2	copy file1 to file2
cp -r dir1 dir2	copy dir1 to dir2; create dir2 if it doesn’t exist
mv file1 file2	rename or move file1 to file2 if file2 is an existing directory, moves file1 into directory file2
ln -s file link	create symbolic link link to file
touch file	create or update file
cat file	show the text characters within a file
cat > file	places standard input into file
more file	output the contents of file
head file	output the first 10 lines of file
tail file	output the last 10 lines of file
tail -f file	output the contents of f
man topic	man pages provide command line help documentation for most commands. e.g. man ls to learn more about the ls command
nano file	minimalist editor with cli gui
vi file	(vim) powerful minimalist editor with no gui

Search

Command Line	Description
grep pattern files	search for pattern in files
grep -r pattern dir	search recursively for pattern in dir
grep -iRl ‘pattern’ dir	searches recursively for a literal in files within dir, listing file names only
command	grep pattern	search for pattern in the output of command
locate file	find all instances of file
find . -name filename.txt	searches for files called filename.txt in current directory and subdirectories
find path -name *.txt	searches for any text files with txt extension in path and it’s subdirectories
find path -type f -empty	searches for empty files
find path -user username -mtime -7 -name *.txt	searches for txt files modified by user in the last 7 days. Note: -7 means today minus 7, not an argument flag to find itself.

Compare

Package	Tools
`sudo apt install diffutils`	cpm diff diff3 sdiff

Command Line	Description
diff [options] from_file to_file	Compare 2 files
diff3 [options] file_1 file_2 file_3	Compare 3 files
sdiff -o out_file [options] from_file to_file	Compare 2 files and merge interactively

System Details

Command Line	Description
date	show the current date and time
cal	show this month’s calendar
uptime	show current uptime
w	display who is online
whoami	who you are logged in as
finger user	display information about user
uname -a	show kernel information
cat /proc/cpuinfo	cpu information
cat /proc/meminfo	memory information
df	show disk usage
du	show directory space usage
free	show memory and swap usage
whereis app	show possible locations of app
which app	show which app will be run by default
lsusb	list usb devices
dmesg	show recent system log messages

Disk

Command Line	Description
df	List all file systems and display the free disk space, and used disk space for each
df /path	Find free disk space on the device a path
`echo 'Hello, World!' > /dev/device_path`	Used while device is unmounted, will permit the device to be used to persist data even without a file system. ! This is a destructive operation!
`head -n 1 /dev/sdx`	Read from an unmounted device, even when the device has no filesystem.
sudo mount /dev/device_path /path/mount_point	Mounts a device on a mount point
sudo umount /dev/device_path	Unmounts a device using the device path. Typically /dev/device_name e.g. /dev/sd`[a-z]` for disks and /dev/sd`[a-z][0-9]` for partitions
sudo umount /path/mount_point	Unmounts a device using the mount point.
dd if=/dev/device_path of=output_image_path.img	Backup a device, creating an image of disk device. !! This is a destructive method, overwriting the output image path !!
dd if=input_image_page.img of=/dev/device_path	Restore a device, using an image to overwrite the current device content. !! This is a destructive method, erasing the device !!
1. `losetup -f` then 2. `losetup /dev/loop[0-9] /image/path.img` then 3. `mount /dev/loop[0-9] /mnt/path`	Mount an image file (in 3 steps). 1.Find the next available loop device 2. Associate loop device with image file 3. mount loop device at mount point
1. `losetup -f` then 2. `losetup -P /dev/loop[0-9] /image/path.img` then 3. `mount /dev/loop[0-9]p[0-9] /mnt/path`	Mount a partition from an image file (in 3 steps). 1.Find the next available loop device 2. Associate loop device with image file, where each partition will be given a loop device name like /dev/loop`[0-9]`p`[0-9]` 3. Mount loop device (representing a single partition) at mount point
1. `umount /mnt/path` 2. `losetup -d /dev/loop[0-9]`	Unmount a device (in 2 steps). 1. Unmount the mount path 2. Disassociate the loop device from any current image file.
lsblk	List block devices, also showing mount points and volume sizes
sudo fdisk -l	Lists 1. disks, getting the device name, disk label, size, sectors, units, sector sizes and i/o sizes 2. Partitions and their device name, Boot flag, Start Offset, End Offset, Sectors, Size, Id, Type
sudo fdisk -l \| grep ‘^Disk /’	List all disks, including loop disks
fsck /dev/device_path	!Use while unmounted only! Checks a disk for consistency errors, and automatically attempts to resolve the errors found
fdisk /dev/path	Partition a device, interactively. m - print help p - print partition table n - create a new partition table d - delete a partition q - quit without saving changes w - write new partition table to the device and exit
mkfs.ext3 /dev/device_path[0-9]	Format the device with a EXT3 file system type
mkfs.ext4 -L myExt4Drive /dev/sdx1	Format the device with a EXT4 file system type. Flag -L adds a label for the file system. May be inappropriate for portable devices, as it maintains file permissions
mkfs.exfat -n myExFatDrive /dev/sdx1	Formats the device /dev/sdx1 and makes a exFAT file system on the device
sudo e2label /dev/sda1 new_label	Updates the current label on the disk partition
`find /path/to/folder -size 0 -type f`	List all files with zero (0) file length and delete them. Note: `-print` flag may be required on some OS.
`sudo find /path/to/folder -size 0 -type f -delete`	Remove all files with zero (0) file length and delete them. The sudo elevation is required for deletions on protected file systems.

Compression

Command Line	Description
tar cf file.tar files	create a tar named file.tar containing files
tar xf file.tar	extract the files from file.tar
tar czf file.tar.gz files	create a tar with Gzip compression
tar xzf file.tar.gz	extract a tar using Gzip
tar cjf file.tar.bz2	create a tar with Bzip2 compression
tar xjf file.tar.bz2	extract a tar using Bzip2
gzip file	compresses file and renames it to file.gz
gzip -d file.gz	decompresses file.gz back to file

Processes

May require elevation to modify processes not owned by the current user

Command Line	Description
ps	display your currently active processes
ps -ax	display all processes currently active on a system
top	display all running processes
kill pid	kill process id pid
killall proc	kill all processes named proc *
bg	lists stopped or background jobs; resume a stopped job in the background
fg	brings the most recent job to foreground
fg n	brings job n to the foreground

Command Chaining Syntax

Command Line	Description
op1 && op2	execute op1 and then op2 immediately after op1 completes, if and only if op1 returns zero (0)
op1 \|\| op2	execute op1, and then op2 immediately after op1 completes, if and only if op1 returned non-zero

Privileges

Command Line	Description
sudo command	run a command as a root user
sudo su	switch to a shell which is elevated to root user
sudo -s	open a root user shell (same as above)
sudo -s -u username	open a user shell as username
sudo -k	forget sudo passwords (prevents sudo privileges persisting in the current shell. e.g. remote assistance)
gksudo command	opens a gnome sudo dialog
kdesudo command	opens a kde sudo dialog
passwd	change your password
chmod permissions file	change to rights on a specific file or directory. See ‘man chmod’ for permissions
chown user:group file	change ownership of a specific file or directory, to a provided user and/or group. See ‘man chown’

Display

Command Line	Description
sudo /etc/init.d/gdm restart	restart X window manager and display login
sudo /etc/init.d/kdm restart	restart X window manager and display login
(file) /etc/X11/xorg.conf	X11 display configuration

System Services

sudo start service sudo stop service sudo status service /etc/init.d/service start | start service /etc/init.d/service stop | stop service /etc/init.d/service status | display service /etc/init.d/service restart | restart service runlevel | gets the current runlevel

Network

Package	Utilities
`sudo apt install wireless-tools`	iwconfig iwlist iwspy iwpriv ifrename
`sudo apt install net-tools`	ifconfig hostname domainname nmap arp rarp netstat iptunnel ipmaddr route dnsdomainname nisdomainname ypdomainname slattach

Command Line	Description
ifconfig	show network details
iwconfig	show wireless information
sudo iwlist scan	scan and list wireless networks
sudo /etc/init.d/networking restart	restart network, used for manual configuration
(file) /etc/network/interfaces	manual configuration
ifup interface	bring interface up
ifdown interface	bring interface down
ping host	ping host and output results
whois domain	get whois information for domain
dig domain	get DNS information for domain
dig -x host	reverse lookup host
wget file	download file
wget -c file	continue a stopped download
sudo /etc/init.d/dns-clean start	flush dns cache
sudo systemd-resolve –flush-caches	flush dns caches
sudo systemd-resolve –statistics	validate dns cache size
traceroute host	traces the route to a host machine
sudo netstat -plnt	list active listening sockets with PID
sudo netstat -plnt \| grep ‘:80’	list active listening sockets with PID on port 80
nmap -sP 192.168.2.0/24	List all devices on the network
nmcli dev wifi	View a list of the available WiFi networks, transfer rate, signal strength and security
wpa_passphrase ssid passkey » wpa_supplicant.conf	Note: Suitable for a Raspberry Pi, not Ubuntu appends a new SSID and passkey combination to the wpa_supplicant.conf, using encryption to store the passkey in an form which avoids plain text

Firewall

Command Line	Description
ufw enable	enable firewall
ufw disable	disable firewall
ufw default allow	allow all connections by default
ufw default deny	deny all connections by default
ufw status	display current firewall status and rules
uwf allow port	allow port
ufw deny port	deny port
ufw deny from ip	deny specified ip address

Secure Shell - SSH

Command Line	Description
ssh user@host	connect to host as user
ssh -p port user@host	connect to host on port port as user
ssh -R listening_port:localhost:22 remote_user@remote_host -i key.pem	Enable SSH Tunnelling on a machine, establishing communication through which a reverse connection can be made.
ssh listening_user@localhost -p listening_port	Connect to listening SSH Reverse Tunnel
ssh-copy-id user@host	add your key to host for user to enable a keyed or passwordless login
sshfs user@host:/remote/path /local/path	mount a remote path to a local path using ssh

Package Management

Command Line	Description
apt-get update	retrieve list of available packages
apt-get upgrade	upgrade all packages
apt-get dist-upgrade	upgrade Ubuntu version
apt-get install pkg	install package pkg
apt-get purge pkg	uninstall package pkg
apt-get autoremove	remove all obsolete packages
apt-get -f install	fix broken packages
dpkg –configure -a	fix broken packages
(file) /etc/apt/sources.list	all apt packages configuration
snap find package	searches for a snap package named package
sudo snap install package	installs a snap package named package
snap list	display list of installed snaps
sudo snap refresh package	attempt to get latest version of snap package
sudo snap refresh	update all snap packages
sudo snap revert package	rollback to the previous version of a snap package

Snippets

Reverse SSH Tunnel

To enable a persistent SSH Reverse Tunnel, use autossh rather than ssh as it will attempt to reestablish the tunnel open, if the connection is lost.

Prerequisite: sudo apt install autossh

Create and add the following to /etc/systemd/system/autossh.service Note: the argument flags -t -t ensures the SSH pseudo terminal is not rejected

 [Unit]
 Description=Auto Reverse SSH
 Requires=systemd-networkd-wait-online.service
 After=systemd-networkd-wait-online.service
 [Service]
 ExecStart=/path/to/autossh -i /path/to/key.pem -R <port>:localhost:22 ubuntu@<remote.fqdn.host> -t -t
 [Install]
 WantedBy=multi-user.target

Enable using sudo systemctl enable autossh.service to ensure it starts on the next boot
Start the service immediately, rather than wait for the next boot using sudo systemctl start autossh.service Note: the .service is not mandatory here and can be dropped but rather used to demonstrate what is really being done by systemctl
Check the status using systemctl status autossh to ensure that the service has correctly established the connection. Look for a line beginning with Active: active (running) to indicate succes. If a failure has occurred, the easiest way to find it is using the command journalctl | grep autossh.

Shell Alias shortcuts in .profile

If there are operations which are regularly typed into the shell or which are complex, specific and/or verbose, you can add an alias into your .profile file.

Add a record to your ~/.profile file as follows

  alias pstree="ps axjf"

Log out of the account, exit and log back in again
In command shell type pstree to see the system process in a hierarchical tree
To see all aliases which are defined just type alias