63 Useful Linux Commands

Linux commands are instructions that can be issued to the linux operating system from a command line or script. The following are useful commands that are enough to competently use a linux box and perform basic administration functions.

Command	Common Usage
su	su someuser Switch user to someuser. This will prompt you for someuser's password. su Switch user to root.
sudo	sudo ls Execute the command ls with root permissions.
alias	alias ls='ls -la' Substitute one string for another when executing commands. Used to create custom commands and modify the behavior of existing commands. alias Shows a list of active aliases. If a command is behaving strangely, it may be aliased.
unalias	unalias ls Removes an alias associated with the string "ls."
clear	clear Clear the screen of a terminal.
ls	ls List files in the current working directory. ls /tmp List files in the directory /tmp. ls -l Show more details such as file permissions. ls -t Sort files by modification date in descending order with newest files first. ls -tr Sort files by modification date in ascending order with newest files last. Often more useful as you don't need to scroll up to see the newest files. ls -a Show hidden files. ls -altr A commonly used combination of command parameters for ls that lists a detailed view of all files in ascending order by modification date.
wc	ls -1 \| wc -l wc stands for word count. It is a tool for counting symbols in text. The command above lists the files in a directory in a single column and pipes this result to wc -l that counts the number of lines. This is a common way to see the number of files in a directory.
cd	cd /tmp Change the current working directory to /tmp. cd .. Move up one level in the directory path. cd ../../.. Move up three levels in the directory path. cd / Change the current working directory to the root directory. cd Change the current working directory to your home directory.
pwd	pwd Display the current working directory.
df	df Display file system disk space usage. df . Display file system disk space usage for the current directory. df -h Display in human readable format. df -T Display filesystem type.
du	du temp.txt Displays the size of the file temp.txt. du /tmp Recursively displays the disk usage of all directories and files under /tmp. du -h /tmp Displays disk usage in a human readable format. du -sh /tmp Displays a summary of disk usage as opposed to a long list of the disk usage of every directory and file. du -sh Displays the total disk usage of the current working directory
history	history Show a history of recently issued commands for this user
!	!! Repeat the last command issued !2030 history gives a numerical identifier for each command. The syntax above repeats the command with the identifier 2030. !ls Repeat the last command starting with the characters ls
echo	echo "hello world" Display the text "hello world". Used to create output for scripts. echo $PS1 Print the current value of the environment variable PS1. echo * Print the names of all the files and directories in the current working directory with spaces between them. Often used for scripting when you want to loop through the files in a directory. *echo .jpg Print the names of all the jpg files in the current directory. echo "script failed" > error.log Redirect the output of the echo command to a file. echo "script failed" >> error.log** Append to the output file as opposed to overwriting it.
cat	cat temp.txt Display the contents of the file temp.txt. cat temp.txt > temp2.txt Concatenate the contents of the file temp.txt to the file temp2.txt. cat temp.txt temp2.txt Display the contents of the files temp.txt and temp2.txt.
more	more temp.txt Display the contents of the file temp.txt one screen at a time. Used to interactively look through a file.
less	less temp.txt Display the contents of the file temp.txt one screen at a time. Has a more powerful interface than more that allows you to go forwards, go backwards and search through the file.
cp	cp temp.txt temp2.txt Copy the file temp.txt to temp2.txt. cp temp.txt /temp Copy the file temp.txt to the directory /temp. cp -rf /home/me /media/backups/ Recursively copy the directory /home/me to the directory /media/backups/. cp -rfp /home/me /media/backups/ Recursively copy the directory /home/me to the directory /media/backups/ and preserve the original file permissions. cp -i ftp.txt ftp.sh Copy the file ftp.txt to ftp.sh and confirm with the user before overwriting an existing file. Here i stands for interactive.
mv	mv tema.txt temp.txt Move the file tema.txt to temp.txt mv temp.txt /tmp Move the file temp.txt to the directory /tmp mv /home/me/data /tmp/data Move the directory /home/me/data to the directory /tmp/data mv -i temp.txt /tmp Interactive mode. Confirm with the user before overwriting. mv -f temp.txt /tmp Force overwrite without prompting the user. mv -u index.html /web/ Overwrite only if the source file is newer than the destination file
rm	rm file.txt Remove a file rm -f /tmp/* Remove the files in the /tmp directory and don't prompt the user for confirmations rm -rf /tmp Completely remove the directory /tmp and all subdirectories and files under /tmp without prompting the user. The parameter r stands for recursive and is a somewhat dangerous parameter to set. rm -i /tmp/* Remove all files in /tmp and prompt the user to confirm each deletion
chown	chown root:rootg file1 Change the ownership of file1 to be root user and rootg group. This also works on directories. chown -R root:rootg /tmp Recursively change the ownership of all files and directories under /tmp to root user and rootg group.
diff	diff file1 file2 Display the differences between two files. diff -iW file1 file2 Ignore case and all white space.
find	*find . -name .txt** Find all text files. Performs a recursive search from the current directory. find . * Find all files under the current directory. find . -type f -empty Find all empty files under the current directory. find . -type d -empty Find all empty directories under the current directory.
grep	*find . \| grep "hello"** Find files that contain the text "hello". Grep searches within a file and is often used in combination with find as above. grep "hello there" * Find files that contain the text "hello there" in the current directory.
touch	touch hello.txt Change the access and modification times of hello.txt to the current time.
kill	kill 9090 Terminate the process with id 9090. kill -9 9090 Force the process with id 9090 to terminate. The id of linux processes can be found with the ps command (below).
ps	ps -ef Show all currently running processes with details. ps -ef \| grep apache Show all currently running processes with a name like "apache."
top	top Display a real time continuously updating view of running processes. top provides an extensive user interface that is a little tricky to learn. However, the default screen shows you what processes are using the most resources such as CPU and memory. This is a way to quickly check the health of your system to terminate processes that are running amok.
killall	killall -9 gimp-2.8 Force all processes with a name like "gimp-2.8" to terminate.
man	man ls Display the manual pages for the linux command ls. In theory, all valid commands on your linux instance have installed man pages.
whoami	whoami Print the currently logged in user.
crontab	crontab -e Edit the schedule of cron jobs for this user. Cron is a system process that runs commands according to a schedule.
nohup	nohup command Run a command in the background. nohup command &>/dev/null & Push a command into the background and discard its output. Nohup stands for "no hangup." This detaches a command from your terminal emulator so that the process will survive when your terminal session ends.
export	export Print all environment variables. export \| grep PWD Search for an environment variable called PWD export -p Print all environment variables for the current shell. export SOMETHING="great" Create an environment variable called SOMETHING and set its value to "great." This can now be accessed from any command or script in the same environment. For example, you can confirm the export with echo $SOMETHING. export -n SOMETHING Remove the environment variable SOMETHING.
ln	ln -s target link-name Create a symbolic link from link-name to target.
mkdir	mkdir directory-name Create a directory mkdir -m 777 directory-name Create a directory and initialize its permissions to 777. mkdir -p food/japanese/sushi Create the directory structure food/japanese/sushi
netstat	netstat -an Display all network connections. netstat -an \| grep 8080 Display any connections on port 8080. netstat -at Display all TCP connections. netstat -s Display networking statistics.
ping	ping hostname Send a ping request to a hostname such as the domain name of a website. This displays the IP of the host and the time it takes to received a response to the ping. This is a way to measure network latency.
traceroute	traceroute hostname Attempts to trace the route that a request will take to a host and display time estimates for each hop on the route. Used to debug networking issues.
passwd	passwd Change the password of the current user. You will be prompted for your current password and a new password that needs to be entered twice to detect typos. passwd apache Change the password for the user apache.
shutdown	shutdown -h now Power-off the box now. shutdown -h +5 Power-off the box in 5 minutes. shutdown -h 03:00 Power-off the box at 3 AM. shutdown -c Cancel a pending shutdown. shutdown -r Reboot the machine.
wall	wall message Displays a message to all terminal sessions across all users.
tar	tar -cvf home.tar /home/me Create an archive file with everything under the /home/me directory. tar -xvf home.tar ./ Extract the archive home.tar to the current directory. tar -cvzf home.tar.gz /home/me Create a tar file that is compressed with gzip.
gzip	gzip big.log Compress the file big.log. This replaces the file with the zip file big.log.gz. gzip -c big.log > big.log.gz Compress to a new file without overwriting the target.
gunzip	gunzip big.log.gz Deflate the compressed file big.log.gz to big.log.
tail	tail file.txt Show the last few lines of file.txt tail -f nohup.out Show the last few lines of the file nohup.out and continue to show any updates that are appended to the end of the file. Often used to monitor a log file.
vi	vi file.txt A non-graphical text editor that has a rich set of features. Considered powerful but is somewhat difficult to learn. For example, vi has an editing mode and a command mode. If you accidentally paste text when you are in command mode, vi will try to interpret the text as a series of commands. This could have interesting results.
vmstat	vmstat Display current memory statistics. vmstat 5 Continue to display current memory statistics with an update every 5 seconds. vmstat -s Another view with more statistics about processes, memory and cpu usage. This view doesn't auto-refresh. vmstat -D Display disk usage statistics.
wget	wget url Download a URL to the current directory. wget url -O filename Download a URL and save it to filename. wget url --no-check-certificate Do not validate the URL's SSL certificate. Often used for local testing where certificates may not work.
xargs	*find . -name .log \| xargs grep "user id 7799989" The command above searches for text in all .log files in a directory tree. The xargs** command is not used alone but is used to pass arguments to other commands.
apt-get	apt-get is a package management system for Debian, Ubuntu and related Linux distributions that is commonly used to add and remove software packages. It doesn't do a great job of managing package dependencies but is a convenient administrative tool nonetheless. apt-get update Update lists of available packages from their sources. This should be done regularly to ensure you are installing the latest software from repositories listed in the file /etc/apt/sources.list. apt-get upgrade Install the newest versions of all packages currently installed. This is often done in an attempt to resolve dependency problems on a box. Always perform update before upgrade. apt-get install package-names Remove packages. apt-get purge package-names Remove packages and try to clean up directory structures and configuration files.
apt-cache	A companion to apt-get that doesn't change your system but provides information about installed and available packages. Often used to look up package names. apt-cache policy package-name Display information for a package including whether it is currently installed and if it is available from repositories. apt-cache search java Search all package names and descriptions for the text "java."
date	date Display the current date. date --date="1 day ago" Display past dates. This can be used to pass date arguments to commands. date -d "+10 days" The future date ten days from now to the second. date +"%Y/%m/%d" Output the date in the format 2020/04/30.
ssh	ssh is a tool for logging on to a remote machine. It provides secure two way communication between hosts. As such, a variety of other linux commands are built on top of ssh. ssh user@host Connect to the ssh server as a client on the default port. Prompts for a password. ssh -p 8080 user@host Specify that the server is running on a nonstandard port.
sftp	sftp user@host A file transfer tool that uses ssh for encryption. Provides an interactive environment with commands for exploring remove file systems, downloading and uploading files.
scp	scp source-file user@host:destination-folder Copies files remotely using ssh for security. Prompts for passwords. As with cp, wildcards can be used to specify source files such as *.jpg.
gpg	gpg is an encryption and signing services tool. gpg -c --no-use-agent --passphrase $password backup.tar.gz Encrypt the file backup.tar.gz using a password. This form is useful in a script where you are encrypting a backup for archiving but isn't particularly secure. gpg --encrypt --recipient 'Somebody' target.tar This will prompt you for a password before encrypting the file target.tar. gpg --output target.tar --decrypt target.tar.gpg Decrypt an encrypted file. Prompts for password.
last	last -n 5 Show the last five logons. Includes details such as user and remote host.
nice	nice is a command to alter the priority of a process. This is often used to run a heavy process with low priority so that it doesn't impede other uses of a box. Niceness ranges from -20 to 19 with bigger numbers slowing a process down and small numbers speeding it up, in theory. nice Print the current default niceness. nice --adjustment=-10 ./heavy.sh Run the script heavy.sh with a low priority. nice --adjustment=10 ./heavy.sh Run the script heavy.sh with a high priority. nohup nice -n -10 ./heavy.sh / > out.txt & Run the script ./heavy.sh as somewhat high priority in the background and redirects its output to out.txt.
renice	renice changes the priority of a running command. renice -n 19 -p 8004 Change the priority of the process with id 8004 to the lowest possible priority. Useful when a process is slowing down a box.
stat	stat file.txt Display detailed information about a file. stat -f /tmp Display detailed information about a file system.
tee	tee writes standard output to a file without interrupting standard output. For example, it allows you to write standard output to a file while displaying it at the same time. command \| tee command.log Execute command and write the output to command.log without disrupting the output to the screen. command \| tee -a command.log Append to the output file as opposed to overwriting it.
uname	uname -a Print a variety of information about this linux instance including machine, kernel, node name and operating system version. uname -p Print the processor type.
users	users Print the users that are currently logged into the system.

63 Useful Linux Commands

su

sudo

alias

unalias

clear

ls

wc

cd

pwd

df

du

history

!

echo

cat

more

less

cp

mv

rm

chown

diff

find

grep

touch

kill

ps

top

killall

whoami

crontab

nohup

export

ln

mkdir

netstat

ping

traceroute

passwd

shutdown

wall

tar

gzip

gunzip

tail

vi

vmstat

wget

xargs

apt-get

apt-cache

date

ssh

sftp

scp

gpg

last

nice

renice

stat

tee

uname

users

Linux Commands

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