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Below are the most commonly used linux commands and its purpose:

&& — and
|| — or
apropos — returns all documentation containing the defined command
bzip/bunzip — bzip; compresses files
cat — outputs the contents of a file
cd — change directory
chmod — change file mode
chown — change ownership
clear — clears the screen
cp — copy
-f Force; will overwrite any existing files with the same name
-i Interactive; will ask to overwrite
-p Preserves file ownership
-r Recursive; will copy files under directories
-a Archive; preserve symlinks
-u Copy only if original file is newer
cut — manipulate data by columns
-c Range to cut [-c2-4]
-d Define a deliminator to use with fields
-f Return the numbered field
dig – look up IP addresses and DNS names
env - show environmental variables for current user
exit — terminates all current processes, including shell
find — locate file in system
. Search in current directory
finger – used to find out information regarding a user
free — reports on system memory
grep — search for string
-i Case insensitive
-n Include line number in output
groupadd — add group
-g Group ID
-p Password
-r Create system group
groupmod — modify group
-g GID
-p Password
gzip/gunzip — gzip; compresses files
head — output the first ten lines of a file
halt — shuts down operating system
id – outputs user and group IDs for a user
-u Show only UID
-g Show only GID
-G Show all groups to which a user belongs
-Gn Group name
if — conditional statements in bash scripting
-d See if directory exists
-e See if file exists
-f See if file exists and it is a regular file
-G See if file exists and it is owned by a defined group
-h See if file exists and it is symlinked
-L See if file exists and it is symlinked (same as -h)
-O See if file exists and it is owned by defined UID
-r See if file exists and has read permissions
-w See if file exists and had write permissions
-x See if file can be executed
ifconfig — shows NIC for network card; configures network settings
info — improved manual pages
init 0 — shuts down operating system
init 6 — reboots the operating system
ip addr — replaces ifconfig on newer systems
last – shows last command used by defined user
less — opens file in a vim-like environment for viewing
ln — link files
-s Create symbolic link
ls — list contents of directory
-a List all files, including hidden files
-l List files, including details such as owner and permissions
-p Adds a / to the end of directories
-R Enable recursion, including all sub-directories
man — manual pages
1 Executable programs and shell commands
2 System calls for the kernel
3 Library calls
4 Device files
5 File formats
6 Games
7 Misc.
8 Programs only run by the root user
9 Kernel routines
-k Returns all documentation containing search keyword (apropos)
mv — move; used to move or rename files
-m Set permissions mode
-p Force recursion
netstat — shows network status
-a All listening and non-listening sockets
-i Network interface statistics
-l Listening sockets
-s Summary of each protocol
passwd — change/set password
-S View password settings
-l Locks account
-u Unlocks account
-d Remove password
-n Set days before password change
-x Set maximum days before password change
-w Days to warn prior to password reset
-i Days to wait after expired password to disable account
ping — test connectivity of remote network device
ps — process status
-u Define user
pwck – verifies the integrity of the password files
pwconv – syncs /etc/passwd and /etc/shadow
pwd — print working directory
reboot — shuts down, then restarts the system
route — view routing tables
set — similar to env; works with environment variables
sort — sorts text in file
-r Sort in reverse order
-n Sort numerically
su – substitute user
sudo – run command as root
tail — outputs the last ten lines of a file
rm — remove
-r Enable recursion
rmdir — remove directory
shutdown — shuts down the system
-H Halts the machine
-P Power off
-r Reboot
-h Equivalent to powering off, overridden by --halt
-k Send warning, but perform no actions
-c Cancel a shutdown
su — substitute user
tar — archives files
-cf Create file
-xf Extract file
-v Verbose
-zcf Archive with gzip
-jcf Archive with bzip2
top — shows list of current applications/processes, and system usage
h Help
k Input process ID to kill
r Change process priorety
p Sort by CPU usage
m Sort by memory usage
touch — created empty file
-c Will not create file if one exists; will update timestamp
-d Manually set timestamp; can be used with directories
traceroute – traces the route a packet takes
uname — returns information related to the system
-s Displays kernel name
-n Displays hostname
-r Kernel release number
-v Kernel version number
-m Hardware architecture
-p Processor architecture
-i Hardware platform
-o Operating system
-a All information
useradd — adds new user
-D Outputs defaults
-c Comment
-e When to expire account
-f Number of days after password expiration to disable account
-g Default group
-G Additional groups
-M Do not create home directory
-m Set home directory
-p Define password
-r Create system user
-s Set default shell
-u Define UID
userdel – delete user
-r Delete home folder
usermod – modify user account
-c Comment
-e Expire date
-f Days after password expiration to disable account
-G Add groups
-l Change username
-L Lock account
-m Move home directory
-p Change password
-u Set user ID
-U Unlock account
w – displays user system information
wc — word count
-w Count words
-c Count characters
whatis — returns available man pages for command
which — shows location of application
who – displays who is logged in
-b Last boot time of machine
-m Hostname and associated user
-r Run level for current user
-q Number of logged-in users
-a All
whoami — outputs username of current user
zip/unzip — zip; compresses files



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