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Linux, Bash, and System Administration Cheat Sheet by

cheat sheet for Linux/Bash/System Administration

Bash Scripting Basics

#!/b­in/env bash — the 'shebang' used to tell the operating system the path it should use to interpret the file
bash file-­nam­e.sh — run the bash script in terminal
./ file-­nam­e.sh — run the bash script in terminal if set to executable
pa­ram­ete­r> — use in docume­ntation to specify if a parameter is required when running script
[pa­ram­ete­r] — use in docume­ntation to specify is a parameter is optional when running script
# — used to make comments throughout script
|| — logical OR
&& — logical AND
$# — resolved to the number of arguments that have been passed to the script
$0 — refer back to the script name
$1, $2, etc. — refer to user input (param­eters) that user can add when running script, separated by a space
exit [0-255] — exit script and return number from 0 to 255. 0 means everything worked as intended, but other values can be used to denote errors that the script ran into

Bash Loops and Conditions

if fi — basic structure of all if-the­n-exit, if-the­n-else, or if-eli­f-else statements
if condition ; then do-so­met­hing — if condition is met, do something
if condition ; then do-so­met­hing else do-so­met­hin­g-e­lse — if condition is met, do something, otherwise do something else
if condition ; then do-so­met­hing elif condi­tion2 ; then do-so­met­hin­g-else else do-fi­nal­-th­ing — if condition is met, do something; however if a different condition is met, then do something else; otherwise do the final thing
while condi­tio­n-i­s-true ; do action done — perform the action as long as the condition is true
until condi­tio­n-i­s-true ; do action done — opposite of while loop, perform the action until the condition becomes true
sleep time — sleep or wait for a specified number of second before continuing through script, usually performed within loops
for value in list-­of-­values ; do thing­-wi­th-­value done — iterate over a list of values
for ((cou­nte­r=n­umber ; count­er<­=number ; count­er++ )); do somet­hing done — start at counter is equal to a number, then do something and increment the counter by 1 until the counter is greater than another number
for counter in {star­tin­g-v­alu­e..e­ndi­ng-­val­ue}; do somet­hing done — brace expansion that iterates over a number range or character range from starting value to the ending value
{st­art­ing­-va­lue­..­en­din­g-v­alu­e..i­ncr­eme­nt-­val­ue} — specify the increment value in a for loop, otherwise the default is 1
for (( ; ; )); do somet­hing done — infinite loop
break — can add to while or for loops to exit from the loop but continue the rest of the script
cont­inue — used to skip current iteration of a loop and continue to the next iteration of the loop
cut — cut different parts of a string
basename path — get the filename from a given path

Bash Arrays and Functions

arr­ay­=(­"­ele­men­ts" "­of" "­arr­ay") — create an array of strings
${a­rra­y[­0]} — get the first element of the array
${a­rra­y[­*]} — get all values in the array
${a­rra­y[­-1]} — get the last value in the array
${a­rra­y[­@]} — expand all of the array elements
declare -A assoc­iat­ive­-ar­ray — declare an associ­ative array that allows string indices, similar to a dictionary in Python
ass­oci­ati­ve-­arr­ay­=([­"as­soc­iat­ion­"­]=­"st­rin­g") — add an associ­ation to an associ­ative array
arr­ay­+=(­"ne­w" "­ele­men­ts") — append elements to the end of an array
shift — move argument $2 to $1
fun­cti­on() { conte­nt-­of-­fun­ction } — define a function
alias — list all aliases defined in the current session
alias alias­='­ba­sh-­command ' — define an alias
type -a comma­nd — tells us if command is an alias

Automated Commands

man 5 crontab — view manual page for crontab
crontab -e — edit scheduled tasks in the /var/­spo­ol/­cro­n/c­ron­tabs file
crontab -l — list scheduled tasks
* * * * * find direc­tory -exec ls -l {} \; — find files on direc­tory
* * * * * - cron format (0-59 minutes, 0-23 hours, 1-31 day of month, 1-12 month, 0-7 day of week)
0 1 1 * * find /temp -atime 3 -exec ls -l {} \; — run the command just on the first day of each month
0 1 * * mon find /temp -atime 3 -exec ls -l {} \; — run the command once a week on a Monday
0 1 1,15 * * find /temp -atime 3 -exec ls -l {} \; — run the command on the 1st and 15th day of each month
0 1 1-15 * * find /temp -atime 3 -exec ls -l {} \; — run the command every day from the 1st through the 15th, inclusive
0 1 */5 * * find /temp -atime 3 -exec ls -l {} \; — run the command every fifth dat (1st, 6th, 11th, etc.)
at — reads commands to be executed from a file or from standard input
atq — show which commands you have in the at queue, displays job number, date of planned execution and job owner
atrm job-n­um — delete a job from the queue by specifying job-num

System

& — puts command into the backgr­ound, allowing you to continue executing other commands
du — display disk usage statistics
df — display free disk space
free — display amount of free and used memory in the system
kill — get rid of a command in the background
man comma­nd — show manual for command
shutdown now — shutdown machine

Download and Unpack

wget file-­url — download a file
tar -xzf tar-f­ile — extract a tar file

Package Management

dnf upgrade — update the system and all of its packages
dnf search packa­ge-­name — search for new software called packa­ge-­name
dnf provides packa­ge-­name — check package name to install
dnf install packa­ge-­name — install new software packages
dnf remove packa­ge-­name — remove a package from the system

System Logs

who — produce inform­ation on who is logged in
w — produce inform­ation on who is logged in
finger — produce inform­ation on who is logged in
id -u usern­ame — get the user ID for a specific user
jour­nal­ctl — view the log of the entire system
Q — quit from journalctl log
jour­nalctl -f — follow the logs in real time
jour­nalctl -u sshd — view only log entries for ssh unit
jour­nalctl -u httpd -n 3 — vie a specific number of log entries (i.e. 3)
jour­nalctl _UID=1­000 — view log entries for a specific user by giving user ID
jour­nalctl --since "­YYY­Y-MM-DD HH:MM" --until "­YYY­Y-MM-DD HH:MM" — filter and display log entries for a certain time period
dmesg — view all kernel messages from the last boot of the machine
last — display last user logins
hist­ory — list previous commands used
history | grep keywo­rd — search for a command by keyword in history
!co­mma­nd-­num — repeat a command from history and run the command
script — record all output for the session in a file
exit — exit from scripting session
 

Secure Shell

ssh — gives ssh command inform­ation
ssh usern­ame­@­ip-­add­ress — log into remote system
ssh-­key­gen — generate public­/pr­ivate key pair
ssh-­add — command for adding SSH private keys into the SSH authen­tic­ation agent for implem­enting single sign-on with SSH
ssh-­key­scan — for retrieving public keys from servers
scp file-­path usern­ame­@ip­-ad­dre­ss: — copy a file from your local system to remote system
scp usern­ame­@ip­-ad­dre­ss­:f­ile­-pa­th — copy a file from the remote system to your own system
scp -r usern­ame­@ip­-ad­dre­ss­:d­ire­cto­ry — copy a directory from the remote system to your own system
exit — terminate the shell
~ + Ctrl-Z — suspend the remote login session

File Searching

find — search for a file or directory on your file system
find /home -name *.jpg — find all .jpg files in the /home and sub-di­rec­tories
grep options pattern files — searches through files for a particular pattern of charac­ters, and displays all lines that contain that pattern
grep -r pattern dir — search recurs­ively for pattern in dir
locate file - locate a file

Important Direct­ories

/ — root directory
/bin — the most essential Unix commands (such as ls)
/boot — location where the kernel and other files used during booting are sometimes stored
/dev — contains device files, the interface between the filesystem and the hardware
/etc — contains config­uration files, which can generally be edited by hand in a text editor
/etc­/pa­sswd — contains user inform­ation in a certain format (user­nam­epa­ssw­ord­:­uid­:­gid­:­gec­os­:h­ome­dir­:­shell)
/etc­/skel — sample startup files you can place in home direct­ories for new users
/home — contains a home folder for each user
/lib — contains libraries needed by the essential binaries in the /bin and /sbin folder
/opt — contains subdir­ect­ories for optional software packages
/proc — the interface between the filesystem and the running processes, the CPU and memory
/root — the home directory of the root user
/sbin — very common commands used by the superuser for system admini­str­ation
/tmp — temporary files stored by applic­ations
/usr — contains applic­ations and files used by users
/usr­/bin — applic­ati­on/­dis­tri­bution binaries meant to be accessed by locally logged in users
/usr­/sbin — applic­ati­on/­dis­tri­bution binaries that support or configure stuff in /sbin
/usr­/in­clude — standard location of include files used in C programs (such as <s­tdi­o.h­>)
/usr­/src — location of sources to programs built on the system
/usr­/lo­cal — programs and data files that have been added locally by the system admini­strator
/var — admini­str­ative files such as log files, used for various utilities
/var­/sp­ool — temporary storage for files being printed, sent by UUCP

Ownership and Permis­sions

sudo — log in or run program as root user
ls -l — display ownership and permis­sions
addu­ser — create a user account (as root)
passwd accou­nt — set password for account (as root)
userdel -r accou­nt — delete an account and account's home directory (as root)
chown — change owner of a file
chown userid /home/­us­eri­d/ — make user account owner of home directory (as root)
chgrp — change group
chmod ugo file — change the user, group, and others permis­sions for file (ugo given in base 8, where u is the user, g is the group, and o is others)
chmod [ugo][­+-=­][rwx] file — give, take away, or set the read, write, and/or execute permis­sions for user, group and/or others for file
7 — read, write and execute permis­sions
6 — read and write permis­sions
5 — read and execute permis­sions
4 — read permis­sions
3 — write and execute permis­sions
2 — write permis­sions
1 — execute permis­sions
0 — no permis­sions
chmod 644 file — standard permis­sions for files
chmod 755 dir — standard permis­sions for direct­ories
find / -user username -ls — find files associated with a user

File Management

ls — list items in your current directory
ls -a — list all items and hidden files in your current directory
ls -l — list items, including their size and permis­sions, in your current directory
pwd — prints path of current working directory
cd — change directory to home directory
cd dir — change directory to dir
cd .. — go up one directory
cp file1 file2 — copy file1 to file2
cp -r dir1 dir2 — copy dir1 to dir2, recurs­ively
mv file1 file2 — move file1 to file2, or just change file name
rm file — remove file
rm -r dir — remove directory dir, recurs­ively
echo text — outputs text to standard output
echo "­te­xt­" > file — redirect text to file
touch file — create file, such as an empty txt or zip
cat file — concat­enate file and print to standard output
head file — output first 10 lines of file
tail file — output last 10 lines of file
less file — view file instead of opening in an editor, allowing page navigation
sort file — used to sort a file, arranging the records in a particular order
ln -S target new-n­ame — make links between files
nano file — open file in nano text editor
nano -v file — open file for read only in nano text editor

Git Commands

git clone /path­/to­/re­pos­ito­ry — create a working copy of a local repository
git add * — add all edited files to staging
git add filen­ame — add specific filename to staging
git commit -am "­commit messag­e" — commit changes to head (but not yet to the remote reposi­tory)
git push — send changes to the master branch of your remote repository
git status — list the files you've changed and those you still need to add or commit

Miscel­laneous

yes "­st­rin­g" — echo string in infinite loop
cal — prints an ASCII calendar of the given month or year
date — display current system time
true — does nothing and finishes with zero exit code, indicating success
false — does nothing and finishes with non-zero exit code (often 1), indicating failure
clear — clears the screen of the terminal
           

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