This is more of a Ubuntu cheatsheet than a Linux one. However, many shortcuts and commands are usable on other Linux systems.

Most of these are just example usages. You should check each command’s manual for more options (by using the man command).

Basics

Read the manual of any (supported) command
```
  man <command>
```
Change the current directory
```
  cd <path>
```
Go to home directory
```
  cd
```
Print the current working directory
```
  pwd
```

List files

  ls # List files in the current directory
  ls </path> # List files in a directory
  ls *.md # List files whose extention is `md`
  ls favicon* # List files that starts with `favicon`
  ls -l # List files with more information, like mod, size, owner
  ls -a # List all files, including hidden files

Bash shortcuts

Stop current command: <Ctrl> + C
Sleep program: <Ctrl> + Z
Go to start of line: <Ctrl> + A
Go to end of line: <Ctrl> + E
Cut from start of line: <Ctrl> + U
Cut to end of line: <Ctrl> + V
Select text: hold <Shift>, left click then drag your mouse
Copy selection: <Ctrl> + <Shift> + C
Paste selection: <Ctrl> + <Shift> + V
Search for a previously used command: <Ctrl> + R
Repeat last command: !!

Users and permissions

Check current user’s name
```
  whoami
```
Check the id of a user
```
  id <username>
```
Run a command as root
```
  sudo <command>
```
Run a command as another user
```
  sudo -u <username> <command>
```

Switch to another user

  su <username> # This requires the user's password
  sudo su <username> # Root can switch to any user without knowing the password

Add a new user
```
  sudo adduser <username>
```

Add a user into a group

  sudo usermod -aG <groupname> <username>
  sudo adduser <username> <groupname>

Delete a user
```
  sudo userdel <username>
```
Change the user’s password
```
  passwd
```
Change another user’s password
```
  sudo passwd <username>
```

Change mod of a file

  sudo chmod <mod> <filepath>

Variations:

  sudo chmod 600 ~/.ssh/authorized_keys
  sudo chmod +x <path> # make file at <path> executable
  sudo chmod a+x <path> # make file at <path> executable for all users
  sudo chmod -R <mod> <directory_path> # Change mod for all files under <directory>

Notes: In order to cd to a directory, the directory itself must be executable to the current user

Environment variables

Print all environment variables
```
  printenv
```

Set an environment variables

  export VARIABLE_NAME=<value> # There should be no spaces before and after `=`

Some useful environment variables:
- $USER: Current user’s name, same as output of whoami
- $UID: Current user’s id, same as output of id -u $USER
- $PWD: Path of the current working directory, same as output of pwd
- $HOME: Current user’s home directory
- $PATH: A set of directories where executable programs are located. These programs can be called directly without specifying the full path.

Basic programming

Create a variables

  VARIABLE_NAME=<value> # There should be no spaces before and after `=`

Checking a variable’s value
```
  echo $VARIABLE_NAME
```
Using variables in commands
```
  ls $HOME
  cd $HOME
```
Using the stdin output of a command as the argument of another command
```
  ls `pwd`
```
Piping the output of one command as the input of another command
```
  command1 .... | command2 ....
```

Redirecting a file or stream

  command .... >/output.log 2>error.log
  command < file.txt

Ignore error logs

  command arg1 arg2 arg3... 2>/dev/null

Ignore all logs

  command arg1 arg2 arg3... >/dev/null 2>&1

Run program in background (it’s still killed if the terminal is closed)
```
  command arg1 arg2 arg3... &
```

Files and folder

Create an empty file
```
  touch filename
```
Print content of a file
```
  cat </path/to/file>
```

Quickly create a file with a short content

  echo "<?php phpinfo(); ?>" > phpinfo.php

Appending content to file

  $ echo "1" > test
  $ echo "2" > test
  $ echo "3" >> test
  $ cat test
  2
  3
  $

Count number of words in a file
```
  wc </path/to/file>
```

Using grep to find a pattern in a file in directory

  grep <pattern> </path/to/file>

Find pattern in all files in a directory recursively

  grep -r <pattern> </path/to/directory>

Only show file names

  grep -rl <pattern> </path/to/directory>

Search incasesensitively

  grep -i <pattern> </path/to/file>

Using find to find a file

  find </path> -type f -name '<pattern>'

Or finding a directory

  find </path> -type d -name '<pattern>'

Check available diskspace
```
  df -ah
  du -sh
```

System

Show system and kernel
```
  uname -a
```
Check system disk space usage
```
  df -h
```
Show system date
```
  date
```
Show uptime
```
  uptime
```
Show current free and used memory in the system
```
  free -m
```

Set the CPU affinity of a process

  taskset 0x01 command # Set the process's CPU's afinity to CPU 1
  taskset 0x03 command # Set the process's CPU's afinity to CPU 1 and 2
  taskset 0x07 command # Set the process's CPU's afinity to CPU 1, 2 and 3
  taskset 0x15 command # Set the process's CPU's afinity to CPU 1, 2, 3 and 4
  ...

01, 03, 07, and 15 are just decimal values of 0001, 0011, 0111, 1111

List all block devices (except RAM disks)

  lsblk

This is useful to find the device path of your usb, sdcard, or hard drive. For example:

  NAME                  MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
  sda                     8:0    0 465,8G  0 disk
  ├─sda1                  8:1    0   487M  0 part /boot
  ├─sda2                  8:2    0     1K  0 part
  └─sda5                  8:5    0 465,3G  0 part
  ├─ubuntu--vg-root   252:0    0 457,4G  0 lvm  /
  └─ubuntu--vg-swap_1 252:1    0   7,9G  0 lvm  [SWAP]
  sdb                     8:16   1  14,6G  0 disk
  └─sdb1                  8:17   1  14,6G  0 part /media/usb
  sr0                    11:0    1  1024M  0 rom

The device path of my usb’s first partition is /dev/sdb1

Mount a filesystem (i.e a usb drive, a hard drive…)

  sudo mount </device/path> </mount/point>

Example:

  sudo mkdir /media/usb
  sudo mount /dev/sdb1 /media/usb

Unmount a file system from a mount point
```
  umount </mount/point>
```
Note: This only unmount the file system at the particular mount point, not all mount points that the file system is mounted to
Eject a device
```
  sudo eject </device/path>
```

Shutdown and restart

  sudo shutdown -P now # shutdown the machine
  sudo shutdown -r now # restart the machine

Tips and tricks

Use tree to visualize folder’s structure

  tree <path>

For example:

  $ tree ./
  ./
  ├── docker-compose.yml
  ├── logs
  ├── nginx
  │   └── site.conf
  └── www
      └── index.php

However, tree must be installed first

  sudo apt-get install tree -y

Use watch to execute a program periodically, then show the out put on the screen.
```
  watch -n <seconds> <command>
```
For exampple: watch your memory usage every 5 seconds
```
  watch -n 5 free -m
```
Use htop to monitor your system resource usage
```
  htop
```
htop must first be installed
```
  sudo apt-get install htop
```
Use tmux to multiplex your terminal
```
  sudo apt-get install tmux
```
Shorthand command to resume your tmux session when available, else start a new session
```
  TMUX_ALIAS="tfox"
  TMUX_SESSION_NAME="fox"
  alias "$TMUX_ALIAS"="(tmux has -t $TMUX_SESSION_NAME && tmux attach -t $TMUX_SESSION_NAME) || tmux new -s $TMUX_SESSION_NAME"
```
Put the above script to your ~/.bashrc, then everytime you want to use tmux, you can just type tfox instead.

Connect to your tmux session when doing ssh. Start a new session if not available

  TMUX_SESSION_NAME="fox"

  # Automatically start attach tmux session when under ssh
  if [ -n "$SSH_CLIENT" ] || [ -n "$SSH_TTY" ]; then
  if [[ "$TERM" != "screen" ]]; then
      (tmux has -t $TMUX_SESSION_NAME && tmux attach -t $TMUX_SESSION_NAME) || tmux new -s $TMUX_SESSION_NAME;
  fi
  fi

Add the following command as keyboard shortcut (Fox example, ScrollLock) to toggle fullscreen mode of the currently focused window
```
  wmctrl -r ":ACTIVE:" -b toggle,fullscreen
```

The IT Fox

Inside a developer's mind

Linux cheatsheet