Linux Commands¶

A reference guide for Linux commands: file navigation, process management, user administration, and networking.

Getting Help¶

Command	Description
`man <command>`	Open the manual page for a command
`whatis <command>`	Display a one line description of a command
`apropos <keyword>`	Search manual pages for a keyword
`<command> --help`	Quick help for a command

# Examples
man usermod           # full manual for usermod
whatis nano           # quick description of nano
apropos -a change password  # search for commands related to changing passwords

Listing and Navigating¶

Command	Description
`ls`	List directory contents
`ls -l`	Long format, shows permissions, owner, size, date
`ls -a`	Show all files including hidden
`ls -la`	Long format + hidden files
`pwd`	Print working directory
`cd <dir>`	Change directory
`cd ..`	Go to parent directory
`cd ~`	Go to home directory

Tip

Type cd ../D and press ++tab++ to autocomplete directories starting with "D".

File and Directory Management¶

Command	Description
`mkdir <name>`	Create a directory
`mkdir 'my cool file'`	Create a directory with spaces in the name
`rmdir <dir>`	Remove an empty directory
`touch <file>`	Create an empty file
`rm <file>`	Remove a file
`cp <src> <dest>`	Copy a file
`cp -r <src> <dest>`	Copy a directory recursively
`mv <src> <dest>`	Move or rename a file

cp *.png ~/Desktop    # copy all PNGs to the Desktop

Viewing File Contents¶

Command	Description
`cat <file>`	Display entire file contents (concatenate)
`less <file>`	Browse a file interactively
`head <file>`	Show the first 10 lines
`tail <file>`	Show the last 10 lines

Navigating in `less`¶

Key	Action
Arrow keys	Scroll up/down
`g`	Go to start of file
`G`	Go to end of file
`/<word>`	Search for a word
`q`	Quit

Searching¶

Command	Description
`grep <pattern> <file>`	Search for a pattern in a file
`find <path> -name <pattern>`	Search for files by name

# Search for files containing "log" in the projects directory
find /home/analyst/projects -name "*log*"
# * is a wildcard representing unknown characters

Input, Output, and Piping¶

Operator	Description
`echo <text>`	Print text to stdout
`>`	Redirect output to a file (overwrites)
`>>`	Redirect output to a file (appends)
`2>`	Redirect error messages (stderr)
`/dev/null`	Discard output (the "dark void")
`\\|`	Pipe, send output of one command as input to another

echo "hello" > output.txt       # write to file
echo "more" >> output.txt       # append to file
ps -ef | grep Chrome            # pipe process list into grep

Piping

The | symbol sends the output of the command on the left as input to the command on the right.

Users and Groups¶

User Management¶

Command	Description
`sudo`	Run a command as superuser
`su`	Switch user (`sudo su -` for root)
`exit`	Logout from current user
`passwd`	Change password (stored in `/etc/shadow`)
`sudo passwd -e <user>`	Expire a user's password (force reset)
`sudo useradd <user>`	Create a new user
`userdel <user>`	Delete a user (keeps their files)
`userdel -r <user>`	Delete a user and all their home directory files

Group Management with `usermod`¶

Command	Description
`usermod -g <group> <user>`	Change a user's primary group
`usermod -a -G <group> <user>`	Add user to a group without replacing existing groups
`usermod -d <dir> <user>`	Change user's home directory
`usermod -l <newname> <user>`	Change user's login name
`usermod -L <user>`	Lock the account

useradd -G finance,admin david     # create user in multiple groups
usermod -a -G finance david        # add david to finance without removing other groups

Permissions and Ownership¶

Command	Description
`ls -l <path>`	View permissions and ownership
`chmod <perms> <file>`	Change file permissions
`chown <user> <file>`	Change file owner
`chown :<group> <file>`	Change file group (note the `:` prefix)

Permission Values¶

Value	Permission
`r` = 4	Read
`w` = 2	Write
`x` = 1	Execute
`rwx` = 7	Full permissions

chmod u+x my_cool_file               # give owner execute permission
sudo chown david access.txt           # david is now the owner
sudo chown :security access.txt       # the "security" group is now the group owner

Package Management¶

APT (Debian/Ubuntu)dpkgOther

Command	Description
`sudo apt install <pkg>`	Install a package
`sudo apt remove <pkg>`	Remove a package
`apt update`	Update package lists
`apt upgrade`	Upgrade installed packages
`sudo apt full-upgrade`	Upgrade including kernel if possible

Command	Description
`sudo dpkg -i <file>.deb`	Install a `.deb` package
`sudo dpkg -r <pkg>`	Remove a package
`dpkg -l`	List installed packages
`dpkg -l \\| grep <name>`	Search for a specific package

uname -r                  # check kernel version
7zip e my_file.tar        # extract an archive

Processes¶

Command	Description
`ps`	List running processes
`ps -x`	List all processes for the current user
`ps -ef`	All processes with full details
`top`	Live view of most CPU-heavy processes
`uptime`	System uptime, number of users
`lsof`	List open files and which processes use them

Managing Processes¶

Command	Description
++ctrl+c++	Interrupt a running process
`kill <PID>`	Terminate a process gracefully
`kill -KILL <PID>`	Force kill without waiting for cleanup
`kill -TSTP <PID>`	Suspend a process
`kill -CONT <PID>`	Resume a suspended process

ps -ef | grep Chrome     # find Chrome's PID
kill 12345               # then kill it

Disk Management¶

Command	Description
`parted -l`	List disk partitions
`mkpart`	Create a partition
`mkfs -t ext4`	Format a partition as ext4
`mount`	Mount a filesystem
`umount`	Unmount a filesystem
`mkswap`	Create swap space

mkpart primary ext4 1MiB 5GiB    # create a 5GB ext4 partition

Networking and Remote Access¶

Command	Description
`ifconfig`	Configure / display network interfaces
`scp <src> <dest>`	Copy files between computers securely
`dig <domain>`	Query DNS name servers

# Serve files from current directory over HTTP
python -m http.server 8080

Services¶

Command	Description
`service <name> status`	Check a service's status
`service <name> reload`	Reload config without stopping the service
`systemctl enable <name>`	Enable a service on boot
`systemctl disable <name>`	Disable a service on boot

sudo vim /etc/vsftpd.conf    # edit a service config file

Imaging¶

Command	Description
`dd`	Copy input to output (used for disk imaging)

Warning

dd is powerful but dangerous — double-check your if= (input) and of= (output) parameters before running.