Linux command-line cheatsheet

Using help

apropos topic_name: Show list of manual pages related to topic
man topic_name: Show manual page for specified topic
info topic_name: Show info page for specified topic (sometimes easier to read than man)
whatis command_name: One-line summary for command
whereis command_name: Displays information where binary command is located; alternatively: "type command_name"

Working with filesystem

ls: List current directory content, options: -l (detailed), -l - h (detailed with size in kB/MB), -l -S (detailed sorted by size), -a (plus hidden files whose names starts with dot), -t (sort by access time); List current directory content, detailed
ls -l | more: List current directory content, detailed, piping output to more
du -hs /usr: Get /usr directory size
pwd: Get full path of current directory
cd: Change directory, i.e. cd /home/tomeko, cd .. to go one level up, cd without parameters moves to current user home directory, "cd -": switches between current and last directory; if path contains space use quotes, doublequotes or escape each space with backslash
mkdir: Make directory; mkdir -p path creates also all needed upper level directories needed
rmdir: Remove directory; rm -r for recursive, rm -rf for recursive with no confirmation
rm: Another way of removing files and directories, switch -r for recursive, -f for force, -i for interactive
cp source destination: Copy files; use * to match all; options: -a (archive, copy file properties as well), -p (preverse, copy file owners and permissions), -r (recursive), -u (update: copy only if destination is older or does not exist)
mv source destination: Move (or rename) files
touch filename: Update access and modification of specified file to current; if file does not exist new file is created (unless -c option is used)

File-matching metacharacters

*: matches any number of characters
?: matches single character
[...]: Matches any one of the characters between the brackets; may include dash-separated range of letters or numbers; use exclamation for negation
~: Replaces user home directory

File ownership and permissions

Every file and directory has an owner. To determine user permissions to access file system checks first if user is file owner. In this case user gets permissions specified for file owner. If not, it checks if user is a member of group owner. Otherwise permissions for "other" users are used. File/directory ownership can be listed with ls -l. Newly created file group ownership is the same as user primary group, type groups to get list of groups user belongs to.

chown username filename: Change file ownership
chown -R username directory: Change ownership recursively
chgrp groupname filename: Change group ownership
chmod 751 filename: Change permissions to read+write+execute for user (4+2+1), read+execute (4+1) for group and execute (1) for others
chmod -R 777 directory: Change permissions recursively
chmod +x filename: Add execute permission for all users
chmod g+w filename: Add write permission for group; multiple changes can be combined with colons, i.e. chmod g+w,o-r,u+x filename

Searching for files

find /var/log -name "*log*": Find files with names containing "log" in /var/log
find /usr/sbin -executable: Find executables
find /sbin -executable -size +100k: Find executables larger than 100 kB; all numeric arguments can be specified as +n (more than), -n (less than) or n (exact value)
find /var/log -mmin -10: Find files that were modified less than 10 minutes ago
find /var/log -amin -100: Find files that were accessed less than 100 minutes ago
locate file_pattern: Search tool working with indexed database - may be much faster than find

Working with archives

tar -czvf home.tar.gz /home: Create (c) zipped (z) file (f) with verbose output (v); more than one directory can be listed after space; option -C before source would cause storing paths as relative
tar -xzvf ./home.tar.gz: Extracts archive to current directory
tar -xvf /file.tar -C /dest_dir: Extracts archive to specified directory

Creating whole device (raw) backups

dd if=/dev/cdrom of=/image.iso: Create device image
mount -o loop /image.iso /mnt: Mount device image

Symbolic and hard links

ln -s file symlink: Create symbolic link; symbolic link will fail if original link would be removed
ln file hardlink: Create hard link; hard link behaves like a copy of the file that is synchronized continuously

Process management

ps aux: Process list
ps aux | grep keyword: Process list with filter
pstree: Process tree
top: Process monitor; use top -u username to limit to only specified user processes; Shift+M to sort by memory usage, Shift+P to sort by CPU usage, k to kill process with specified signal (15 = SIGTERM, 9 = SIGKILL), q to quit
kill [-s signal] process_pid: Kill process with specified PID; by default uses signal 15, SIGKILL
program_name &: Start program in background
jobs: Get the list of programs running in background
fg job_number: Bring job to foreground
bg job_number: Resume (stopped with Ctrl+Z) job in background
kill %job_number: Kill process related to job number

Network

netstat -platune: Information about current network connections, ports listening and programs involved
nmap ip_address: Port scan

Kernel

lsmod: List currently loaded kernel modules
modinfo module_name: Detailed module informations
modprobe module_name: Load module (-v for verbose, -f to force ignoring kernel version)
modprobe -r module_name: Unload module
dmesg: Display kernel message log

Other useful commands

env: List environment variables
history: Recently used commands (not necessary this session only); add number to limit to N recent entries
!command_number: Execute command from history from specified position
!cd: Execute last "cd" command instance from history
!?string?: Run again last command containing specified string
passwd: Change password for the current user (also applies to root)
su [username]: Temporary change your identity; type exit to go back to your account
shutdown -r now: Reboot computer
shutdown -h now: Shut down (halt)
w: Get list of currently logged users and their activity
clear: Clear screen
ldd executable_name: Show shared library dependencies for specified executable (required system libraries)
lsof: List open files; lsof -u username to limit to specified user
uname -a: Shows basic informations about kernel, processor, current user, helps to identify distro
whoami: Display current user account name
last: Recent logins, reboots and shutdowns
date: Shows current date and time
cal: Display calendar (whole year if without arguments)
man ascii: Displays ASCII table
ssh username@ip_address: Initiate ssh session
scp filename username@ip_address:/home/my_path: Transfer file to remote computer (-r for recursive directory copying)

Some shortcuts

Ctrl+Alt+F1 ... Ctrl+Alt+F6: Switching to tty1 ... tty6
Ctrl+Alt+F7: Switching back to graphical environment
Tab, double Tab: While typing command: file/directory name autocompletion

Vi(m): text editor

Pretty non-intuitive command line editor. Use nano if available.

vi filename: Create new or open existing file for editing
Esc: switching from input mode to command mode
i: switching from command mode to input mode
:w newfile (command mode): Save file with new name
:q!: Quit without saving
:w: Save file
:wq!: Save current file and exit
u: Undo last changes
G: Go to the bottom of the file

Displaying text file content

cat filename: Plain lister that just dumps content on the screen
cat filename: Plain lister; tac displays lines in reverse order
cat filename | grep keyword: Show file lines containing specified keyword
tail -f /var/log/syslog: Shows last lines (10 by default) of specified file and waits for file updates
head /var/log/syslog: Shows first lines (10 by default) of specified file
less /var/log/syslog: Lister with browsing capabilities. Type :q (similar to vim) to exit.
diff --side-by-side file1 file2: Shows diff in human-readable form

Common directories

/bin: Essential binaries available to all users
/sbin: Binaries for the system administrator (i.e. fdisk)
/boot: Kernel image (vmlinuz) and other vital components
/dev: Files corresponding to system hardware
/etc: Configuration files
/home: Users personal files. Current user home directory can be referred to with shortcut ~
/mnt and/or /media: Mounted data storage
/usr: Most all-user accessible files; most programs are stored in /usr/bin
/opt: Used as directory for complex software packages (i.e. office suite)
/proc: Interface to system memory and some system features, current state of the kernel, i.e. /proc/cpuinfo (print processor specification: cat /proc/cpuinfo), /proc/meminfo, /proc/net/sockstat, /proc/sys/fs/file-max, /proc/process_PID_number
/root: Used as root home directory (instead of /home)
/srv: Important services like HTTP or FTP and their files
/sys: Directory similar to /proc, but stored directly on disk
/tmp: Directory where every user can write to
/var: i.e. print spool