Show Menu
Cheatography
  1. Home >
  2. Software >

Linux Commands & Options Cheat Sheet (DRAFT) by

NOS-120 Commands & Command Options (but for CPCC this time)

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Keyboard Shortcuts

Shortcut
What It Does
Notes
 
cmd
Tab Tab
Show all commands that start with "­cmd­"
Esc + .
Insert last argument entered
Repeat to go back to previous arguments
Up Arrow
Auto-fill last command entered
Repeat to go back to previous commands
Ctrl+Arrow (Right­/Left)
Move to beginn­ing/end of a word
Ctrl+Shift+T
Open new terminal tab
Ctrl+L
Clear terminal output
Ctrl+D
Logout / Exit

Important Direct­ories & Files

Directory/File
(Stands For) Purpose
/root
Root user's home directory
/home
Normal users' home direct­ories
/var
(Varia­bles) Database locations, spool files for mail, etc.
 
/usr
(User System Resources) System resources used by users
— /usr/bin
(Binaries) Execut­ables usable by normal users
— /usr/sbin
(System Binaries) Execut­ables only usable by root user
— /usr/lib
52-bit libraries
— /usr/lib64
64-bit libraries
 
/etc
(Extended Text Config­ura­tions) Config files
— /etc/passwd
User account config files
— /etc/group
Group membership info & config files
— /etc/shadow
User password info & config files
— /etc/sudoers
Main file for sudo config
— /etc/sudoe­rs.d/
Destin­ation for sudo config "­dro­pping files"
— /etc/login.defs
Defines default properties used for new user accounts

vim Editor Controls

--- Command Mode ---
o
Starts new line and switches to Insert mode
cw
"­Change word" - deletes word and switches to Insert mode
G
Go to end of file
dd
Delete or cut entire line
yy
Copy entire line
p
Paste copied text
x
Delete character cursor is on top of
dw
Delete word cursor is on top of
d$
Delete from cursor to end of line
ZZ
Save & quit (existing files)
--- Extended Command Mode ---
:w filename
Save (new files)
:wq
Save & quit (existing files)

Users & Groups

Command
Options
What It Does
 
id user1
Show UID, GID, & secondary groups
(current user if not specified)
 
getent passwd usr1
Find out if usr1 is known to the system
 
useradd user1
Add a new user
 
-u #
set a specific UID
  
-s /sbin/nologin
create a user without a login shell
 
usermod user1
Change properties of existing user
 
-c "text"
adds text to comment field
 
-g group1
changes primary group to group1
 
-G group1
replaces supple­mentary group with group1
 
-aG group1
appends group1 to supple­mentary groups
 
-L user1
locks user1's account (instead of deleting)
 
newgrp group1
Change current user's primary group
(temporary; current session only)
 
userdel user1
Deletes user1 but not their home directory
 
-r
deletes both user and home directory
 
groups user1
Displays simple list of groups user1 is a member of
 
passwd user1
Set password for user1
 
chage user1
Change password aging properties for user
 
-m days
minimum # of days between password changes
 
-M days
maximum # of days between password changes
 
-W days
warning period before password expires
 
-I days
inactivity period (password usable after expira­tion)
 
-d 0
require password change on next login
 
-E date
date when account expires
 
getent group grp1
Find out if grp1 is known to the system
 
groupadd group1
Create a new group
 
-g #
set a specific GID
 
-r
create a system group
 
groupmod group1
Change properties of existing group
 
-g #
change GID to specified number
To set the number of days from today when user's account expires:
chage -E $(date +days %Y-%m-%d)


To give full admin privileges to a user or group:
echo "[user1|%group1] ALL=(ALL) ALL" >> /etc/s­udo­ers.d/name

Managing Processes

w
Displays currently logged-in users, login method, time, & resource usage info
 
--from
shows users' IP addresses
(in addition to regular output)
 
-u username
limits output to specified username
 
command &
Starts a new job in the background
 
jobs
Displays jobs running in the background
("+" = default job)
 
fg %1
Brings job 1 to the foreground
 
bg %1
Sends job 1 to the background
 
pidof command
Returns the PID(s) of a currently running job
 
systemctl status cmd
Shows main PID of a process, among other things
(can use if 
pidof
returns multiple)
--- CPU Load Handling ---
 
uptime
Displays the load average for the last 1, 5, and 15 mins
 
lscpu
Displays number of CPUs in the system, among other things
 
-p
simplified output
(each row of numbers = 1 CPU)
--- Viewing & Managing Processes ---
 
ps
Shows info about processes; pipe to 
head
/
grep
/etc for less output
("[
output
]" = kernel thread)
 
aux
 
lax
 
-ef
 
--forest
shows output in visual "­tre­e" format
 
pstree
Shows processes in visual tree format
(may not always be installed)
 
-p
includes PIDs in tree output
  
top
Linux's in-ter­minal equivalent of Task Manager
(Also shows load average)
Keyboard Controls:
k
send a signal to a process from within 
top
Shift+M
sort by memory usage
Shift+P
return to sorting by CPU usage
l / t / m
toggle "­loa­d", "­thr­ead­s", & "­mem­ory­" header lines
 
kill PID
Sends signal 15 to specified process
* Must use process ID *
 
-signal
sends specified signal
(can use signal name or number)
 
-l
displays list of signal options
 
killall process
Kills processes by name instead of PID
 
pkill
Look up & manage processes by name or other attributes
(default is signal 15)
 
-signal
specify signal to send
 
-u username
sends signal to user's account
(forces logout & shuts down their processes)
 
-t pts/22
kills specified terminal session (TTY)
  
nice -n x command
Starts a new process with an adjusted priority value (x)
 
renice -n x -p PID
Change the priority value (x) of an existing process
Process Priority Values: -20 to 19 (default = 0)
(negative = higher priority | positive = lower priority)

SSH

ssh user@ip.addr
Start ssh connection
 
-v
verbose; shows in detail what's happening while establ­ishing connection
 
-Y
enables graphical applic­ation support
 
-p port#
connect to ssh service not listening on default port 22
 
-i keyfile
reads private key from identity file for public key authentication
(not part of lecture; added from Google search when it came up in the quiz)
 
-o option dest_server


Example:
PreferredAuthenti
cations=password
configures options when connecting to specified destination
(Example: require a password; can use commas to specify multiple)
  
ssh-keygen
Generates a public­/pr­ivate key pair
 
-N '' -f filename
generates key pair without extra prompts
(
''
= no passph­rase; 
-f
specifies non-de­fault file location)
  
ssh-co­py-id user@s­erver
Installs public key on destin­ation server
 
-i filepath
specifies which public key to install
 
 

Wildcards & Shortcuts

Wildcard
What It Represents
Example
 
~
Current user's home directory
 
.
The directory you're currently in
 
..
Parent directory of your current directory
 
*
All files in current directory
 
ls i*
= all files starting with "­i"
 
?
"Any charac­ter­"
 
f??e
matches anything with 4 characters that starts with "­f" & ends with "­e"
 
[aou]
Match to any one of the enclosed characters
 
c[aou]t
matches to "­cat­", "­cot­", and "­cut­"
 
!!
"Last command run" (inserts into current command)
 
sudo !!
= rerun command as root
 
^x^y
Rerun last command but replace x with y
 
ls /usr
followed by 
^usr^tmp
will run 
ls /tmp
 
!3
Rerun 3rd command from command history
 
!ls
Runs most recent command from history that had "­ls" in it

Command Redire­ction

<
Gets input from somewhere other than stdin (keyboard)
 
>
Sends output somewhere other than stdout (monitor)
 
1>
Altern­ative to "­>" (not really used)
 
>>
Redirects stdout; appends it to existing content (instead of overwr­iting it)
 
2>
Redirects only error messages (stderr)
 
&>
Redirects both stdout and stderr
 
cmd1 | cmd2
Pipes cmd1 into cmd2
(uses the output of cmd1 as the input for cmd2)

Basic File/D­ire­ctory Commands

ls dir1
List contents of dir1
 
-l
additional info (owner, perms, etc.)
 
-a
lists all, including hidden files
 
-i
adds file inode to info displayed
 
-d
info for directory itself (rather than its contents)
 
mkdir dir1
Create a new directory
 
-p
creates full path (multiple direct­ories if needed)
 
-v
verbose (output shows action taken)
 
cp dir1 dir2
  
Copy dir1 into dir2
 
-r
recurs­ively ("and its conten­ts")
  
rm dir1
Removes (deletes) an empty directory
 
-r
recurs­ively ("and it's conten­ts"), including special files
  
mv file1 dir1
Move file1 into dir1
 
-v
verbose (output shows action taken)
  
ln target link
Creates a hard link from file "­lin­k" to file "­tar­get­"
 
-s
makes a symbolic link instead of a hard link

Ownership & Permis­sions

chown user:group
Change owning user and owning group
 
chown :group
Change only owning group
 
chgrp group file
Alternate way to change owning group
  
chmod [perms] file
Change file/d­ire­ctory permis­sions
 
-R
recursive; applies to directory and all contents
 
g+r
grants owning group the read permission
 
a-x
removes execute perms for all parties (without changing other perms)
 
-R a=rwX
recurs­ively set perms for all parties to rw, with x perms on directory only
 
770
octally set rwx perms for owning user & group, 0 for other
 
chmod [perms] file
(cont'd) Change special permis­sions
 
o+t dir
set Sticky bit (in other)
* For direct­ories only *
 
1xxx
set Sticky bit – octal form
(xxx = regular perms)
 
g+s dir/file
set SetGID bit (in group)
* For direct­ories and files *
 
2xxx
set SetGID bit – octal form
 
3xxx
set both Sticky & SetGID bits
(chmod "­1"+"2­")
 
u+s file
set SetUID bit (in user)
* For files only *
 
4xxx
set SetUID bit – octal form
  
umask
Displays currently set umask
(when used by itself)
 
077
removes default perms
(0 from user, 7 from group/­other)
 
0077
preserves any special perms when removing defaults
Octal Expression Values: r=4 | w=2 | x=1

Default permis­sions (on vanilla Linux):
direct­ories=777 | files=666

Process States & Signals

Running
 
R
currently using CPU
Runnable
 
R
queued & waiting for CPU time
Sleeping
 
S
defined (has PID) but not requesting CPU time
 
I
(idle)
not counted when calcul­ating load
 
D
(not yet addressed)
 
K
(not yet addressed)
 
Stopped
 
T
defined (has PID) but prevented from getting CPU cycles
--- Signals ---
 
1) SIGHUP
Hang Up
toss current config file in memory & reload it from the file system
 
9) SIGKILL
Kill it dead!
immedi­ately end process that isn't responding to "­ter­min­ate­" signal
 
15) SIGTERM
Terminate
shut down process
(default used by 
kill
)
 
18) SIGCONT
Continue
("unstop")
resume allowing process to get CPU time
 
19) SIGSTOP
Stop
suspend process & prevent from getting CPU time

Managing Services & Daemons

--- System Commands ---
 
systemctl reboot
reboots the system
 
systemctl daemon­-reload
tells systemd you've made changes to a unit file & it needs to reinit­ialize those unit files
 
systemctl get-de­fault
shows which target is the default target that the system summons whenever it starts up
 
systemctl set-de­fault graphi­cal.target
sets default target to "­gra­phi­cal.ta­rge­t"
--- Unit Commands ---
 
systemctl list-units
shows all the active & loaded units on the system
 
-t type
filters output by specified type of unit
 
-a
shows all units known to systemd
(including "­ina­cti­ve" & "­not­-fo­und­")
 
systemctl status unit
shows status & details of specified unit (including location of unit files)
 
systemctl is-enabled unit.s­ervice
shows whether the service is enabled
(i.e. will start automa­tically on system boot)
 
systemctl is-active unit.s­ervice
shows whether the service is active
 
systemctl enable --now unit.s­ervice
enables the service & start it immedi­ately (sets "­ena­ble­" persis­tently)
 
systemctl disable --now unit
stops service immedi­ately & sets to "­dis­abl­ed" persis­tently
 
systemctl reload unit
sends the "hang up" signal; drops config file loaded in memory & reload it from file system
 
systemctl restart unit
kills the process & starts it up again fresh
(main PID will change)
 
systemctl reload­-or­-re­start unit
reloads if possible, otherwise restart
(for when you don't know if unit supports reload)
 
systemctl list-dependencies unit.service
shows list of units required by unit.s­ervice for it to work
 
--reverse
shows units that call unit.s­ervice
 
systemctl mask unit
prevents unit from being started automa­tically or manually
(not even via 
enable --now
command)
 
systemctl unmask unit
undoes masking on a service

Network Info Commands

ip -br addr
shows current IP & netmask for all interfaces
 
ip link show
link-level properties of all interfaces (MAC addresses, etc.)
 
ip address show
equivalent of 
ipconfig

(same info as 
link show
, plus IP addresses)
 
ip a s
abbrev­iated form of 
ip address show
 
ip route show
shows default route
 
ip r s
abbrev­iated form of 
ip route show
 
ping
| 
ping6
IPv4 ping | IPv6 ping
 
tracepath
| 
tracepath6
equivalent of 
tracert
/
traceroute
| IPv6 version
 
mtr address
"My tracer­out­e" program; gives extra info & continuous running stats
 
nmap -sS host
scans ports on host; shows port number­/type, open state, associated service, etc.
 
ss
sockets state inform­ation
 
ss -p
processes respon­sible for opening ports
 
ss -l
listening sockets
 
ss -u
UDP sockets
 
ss -n
translates names to numbers
(e.g. both process name & PID)
 
ss -t
TCP sockets

Misc. Commands

whatis command
Displays a short, one-line summary of what the command does
 
echo
Creates output (equiv­alent of 
print
in Python)
 
date
Displaying, setting, calculating...
basically all things date-r­elated
 
+%F
display current date in intern­ational format (YYYY-­mm-dd)
 
+%R
display current time (24-hr clock)
 
+%s
display # of seconds since epoch
  
tmux
Splits terminal window into multiple separate panes
(I think)
 
gnome-­cha­racters
Opens window of gnome icons/­emotes
 
gnome-­cal­culator
Opens built-in calculator (like Windows)
 
sleep seconds
Waits & does nothing for specified time