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Notes on Linux commands, Bash, SSH

practice Linux -, Git for Windows, Visual Studio Code, Powershell

  • ~ home dir
  • $ standard user
  • # root user
  • period - current directory
  • switches: -l = long list / -la = full details (perms, date, user) / -a = all - ls -a
  • Linux is case sensitive
  • spaces in file name - use backslash \


cmd info examples
dir lists directories and files in current dir dir / dir yourDirName
ls list storage / lists files or folders in current dir ls / ls new-dir or ls /bin / ls -a (all hidden files) / ls -d lists dir / ls -s lists file size / ls -S sorts by file size / ls -t sorts by time and date / ls -x sorts by extension size / `ls $HOME
pwd print working directory - full path/absolute path
cd change directory cd (goes to home folder) cd /dir cd .. (up one lvl) cd ~ (goes to home dir) cd - (goes back to last folder used)
mkdir create dir mkdir newdir / mkdir newdir1 newdir2 newdir3 (creates 3 dirs) mkdir newDir && cd newDir (creates dir and cd to it) mkdir -p newDir/{subDir1,subDir2} (creates a dir and sub dirs inside dir)
touch create file touch newfile.txt
rm removes/deletes dir or file rm dir / rm file.txt / rm -r newdir (dels dir) / rm file?.txt (dels file1, file2, file3, etc)
rmdir removes empty dir rmdir dir1 / rmdir * (all empty dir del)
cp copy file/dir from 1 loc to another cp oldfile.txt /dir2/newfile.txt (copy, rename, move to another dir) / cp ~/.bashrc bashrc (cp from home to current dir)
mv renames file mv newcopy.txt newcopy2.txt / mv *.txt dir (moves all txt files to dir) mv dir/* . (moves files in dir to current folder)
echo prints message echo Hello World
cat prints content in file cat newcopy2.txt / cat > outputs to new file created or replaces old text if file already exists - cat > file2.txt / ls -al / > file.txt (full filesystem and hidden files) / cat >> - appends to bottom - cat >> newfile.txt // ctrl + d - done - after typing text (cat > newfile.txt)
2> redirects error msgs ls -l video.mpg 2> errors.txt / see 'STDERR' / echo > file.txt 2>&1 - sends standard output and errors to file
head lists 1st 10 lines in file head file.txt / head -20 file.txt - view 20 lines
tail lists last 10 lines in file tail file.txt

| - piping - output one cmd/prog and inputs to another

  • ls -al | cat > lsout.txt (list files in current dir inside lsout.txt)
  • echo Hello World | cat >> newfile.txt - adds message to bottom of newfile.txt

global regular expression print. grep searches files for keywords | grep keyword file.txt

  • grep options:
    • grep keyword filename - prints all lines containing the keyword
    • grep '\!$' filename - using regular expression to match lines ending in a !
    • --color=always - highlight matched text in color
    • -c - how many lines matched
    • -n - matches and line number
    • -i - case insensitive grep -ni "keyword" file.txt
    • -v - invert match, find all lines that don't match reg exp
    • -l - only show the filenames of the files that matched
    • -L - shows names of files that don't contain matching lines
    • -r - recursive - search all files in dirs and subdirs
    • -R - ls -R folder-name lists files and folders inside folder-name
    • -o - only print matching part of the line, not whole line
    • -a - search binaries - treat binary data like its text instead of ignoring it
    • -F - don't treat the match string as a regex
    • -E - extended support of meta characs, escapes these special characters by default (use if you want regexps like '.+' to work, otherwise you need to use '.+'
    • grep -A 3 keyword shows 3 lines of context after a match
    • cat auth.log | grep "keyword" | awk '{print $4}' | sort -u > file.txt prints sorted list from column 4 from the auth.log file to file.txt,
    • (avoid egrep and fgrep)
    • grep alternatives - ack, ag, ripgrep - better for searching code


cmd info examples
-h or --help short desc / long desc with options of a cmd pushd -h / mkdir --help
man manual/man pages - about the command (desc, name, options) man appname
more prints content in file / spacebar to go page to page more newfile.txt
less use arrows to read content page by page, 'q' to exit less file.txt
sed stream editor - find and replace keyword sed 's/mysql/MySQL/g file.txt > newfile.txt' / s - substitute, g - everywhere/replace globally, '>' - to save results to newfile / replace 'g' with a number and will replace that occurence of the word (sed s/mysql/MySQL/2 = 2nd occurence replaced)
pushd jump from current dir to another pushd /dir
popd back to last dir used after pushd cmd
wc counts lines (-l), words (-w), characters (-c) in file wc - lw newcopy2.txt
uniq removes dupes in file uniq file.txt
history lists all cmds typed
locate locate file in dir with this keyword or locate app locate keyword
sudo updatedb use to quickly update db if file not found with 'locate'
apt-get install/update/upgrade software packages apt-get install pkgname / apt-get remove pkgname / apt-get purge pkgname - removes pkg and config files / apt-get update - update out-of-date pkgs / apt-get upgrade - upgrade out-of-date pkgs
git clone clone software git clone (github URL)
which find out if app/cmd is installed which appname
whois info about domain name's owner whois less
whatis short info about a cmd whatis pushd
whereis locate binary file, get source, man page, location of file
whoami displays username
wget d/l file from a website to current dir wget
ps lists processes running on terminal ps aux
top processes running the most resources
kill used to end process kill -15 (PID#) / kill (PID#) - PID - process ID #, listed under ps or top
jobs displays list of current jobs running in bg
fg moves a background process into the foreground fg (job #)
ctrl + z pauses process and moves to bg
passwd changes password
df total storage in sys df -h
ln link files together, content in 1st file is updated in 2nd file ln -s file.txt newfile.txt
info simple info page info cmd
apropos searches and lists cmds related to that cmd apropos cmd
uname -a info about kernel version and name, hostname, OS, date and time
uptime time, # of users, load, up for # of hours
awk prints selected line awk '{print}' file.txt / awk '{print $1,$2,$NF}' file.txt (prints 1st, 2nd, last line)
tar tar caf myfile.tar.gz myDirectory myDirectory is compressed to tar.gz folder
zip / unzip compress or decompress
gzip compresses file with GNU Zip, more info gzip -c filename > filename.gz gzip -d filename.gz or gunzip (decompress)
last all logins and reboots in the system
lpr line printer - sends file to printer, '# 5' prints 5 pgs lpr -P myprinter myfile # 5
mail create/send an email from Linux mail prog `echo "body of email"



Enhanced version of vi. 2 modes - insert (enter text) and command (del text, copy/paste and other ops)

cmd task example
gvim open editor in new window
vim run in existing shell window vim or vim
i switch to insert mode then type any text
ESC switch to command mode / end command in progress
: switch to command line mode
:wq save and quit
:q! quit w/o saving
:w save / save as :w filename
l or -> move right
h or <- move left
k or up arrow move up
j or down arrow move down
w move to next word
b move to prev word
0 move to beg of line
$ move to end of line
^f move down one screen
^b move up one screen
gg move to beg of doc
G move to end of doc
x del next charac
X del prev charac
de delete next word
db del prev word
dd del current line
D del end of line
:help get help / view manual
:snytax on turns on syntax highlighting
vim ~/.vimrc make certain settings permanent (type w/o colon) syntax on


shortcut what it does
Tab tab completion - autocompletes file or dir name
ctrl + shift + c copies from clipboard
ctrl + shift + v pastes from clipboard
ctrl + c stops/cancels program running
ctrl + d terms program / ex. after typing text (cat > newfile.txt)
ctrl + - / ctrl + larger/smaller text
ctrl + u cuts all text before cursor, ctrl + y to paste cropped text
ctrl + k cuts all text after cursor, ctrl + y to paste cropped text
ctrl + a moves cursor to beginning of line
ctrl + e moves cursor to end of line
ctrl + r and then type 1st few letters from previous cmds - searches for cmds used, press enter to run or Esc to end search
ctrl + left / right arrow moves left / right 1 word
up / down arrow scroll through previous commands
clear or ctrl + l clears screen
exit exits terminal

File permissions

View files and perms with 'ls -l'. File perms are in 1st col.

  • Example: drwxr-xr-x. 1st charac is the file type. d = dir or '-' for file

    • r = readable means can view file, w = writable means can edit/del file, x = executable means can execute and access file/dir.
    • rwx means full access and a dash means no access given.
  • There are 3 sets - user, group, others (UGO).

    • 1st set/1st 4 chars - 'drwx' in this dir, user/owner has full rwx perms.
    • 2nd set/5-7th chars - 'r-x' - this group can read (r) and execute (x) but not write (w).
    • 3rd set/8-10th chars - 'r-x' - all other users can read (r) and execute (x) but not write (w).
    • r = 4 bits, w = 2 bits, x = 1 bit = 4+2+1 = 7 bits / rwx = 777 = full access / 774 = all perms to user and group, read perms to others / 766 = user has rwx, all others and group rw only (4+2=6)

For more info - chmod Command in Linux and commands with examples

  • chmod - change perms/mode, root user or file owner only. | chmod 774 file.txt
  • UGO (text) method instead of numeric (777)
  • References: (u) user, (g) group, (o) others
  • Operators: - means removes, + means adds, = means sets/assigns
  • Modes: (r) read, (w) write, (x) execute, (a) all


  1. chmod u-w file.txt (removes w perm for user)
  2. chmod u+x, o+x file.txt (user and others can execute to file.txt)
  3. chmod ugo=rx file.txt (everyone has rx perms)
  4. chmod a=rx file.txt (same as ugo, all users have rx perms)
  5. chmod ugo+rwx file.txt (all perms to all users)

Tables from LinkedIn Learning course References | Read (4) | Write (2) | Execute (1) | Result --- | --- | --- | --- | --- | Users | r | w | x | 7 Groups | r | - | x | 5 Others | r | - | - | 4

References Read Write Execute Result
Users + + + u+rwx
Groups = - - g=r
Others - - - o-rwx
All a+rwx

Bash scripting

  • Begin with #!/bin/bash (means it's executable, saved as .sh file by default). / #!/bin/bash -x - debugging (remove later) / place in script, then run file in terminal:
set -x 
(what you want to debug) 
  • add scripts to your bin dir - /home/username/bin (exe in the bin only available to you, can't run with sudo) or /usr/local/bin (avail to all users, can run sudo)
  • New scripts are NOT executable. chmod 755 or chmod +x to make it executable.
  • Commands work exactly the same on the command line as they do within a script.
  • To run file bash or ./bashfile in the terminal. (./ = to run script)
  • Comments -- single line # your comment -- multi-line
  :'your comment in between single quotes
  next line
  another line'

= - to set variable, $ - call var | myvar="yo world" echo $myvar - outputs 'yo world' when your run script -i = removes case sensitivity / -n = line number


if statements

if [ statement ] 
  echo 'your message' 

  echo 'another message'
if (( ... ))
  echo "..."
  echo "..."


  • && = and, -a
  • || = or, -o

if ["$myVar" -gt 18] && ["$age" -lt 40]
  echo "true"
  echo "not true"

Can also use:

  • ["$myVar" -gt 18 -a "$age" -lt 40]
  • [ ... -o ...]
  • [..] OR [..]
  • [ ... || ... ]
  • [ ... ] || [ ... ]

user input

echo "Enter your name:"
read name
echo "Enter your age:"
read age
echo "Hello" $name, "you are" $age "years old"

Read - for user input. Run script, and user inputs name, age. Notes from

read -p "Enter your username: " username
read -sp "Enter your password: " password

echo -e "\nYour username is $username and Password is $password"
  • -p prompt a message, displays on screen
  • -sp - doesn't display on screen



RSA 2048-bit encryption, which is comparative to a 617 digit long password. More secure than passwords.

SSH keys are made up of a private key and a public key. Never share private key. If key is exposed, generate a new ssh key pair.

Can choose to either lock your private key with a passphrase. creating a key pair without a passphrase is more convenient and potentially essential for certain scripts and automation tasks. But it's also less secure. Generate a separate key pair for each service or connection you want to use, adding a passphrase only for critical services.

By default you private key is stored at /Users/myname/.ssh/id_rsa and your public key The private key - id_rsa - should never be shared

Public keys ( are meant to be shared or placed on a remote server. It's stored at /Users/myname/.ssh/ The private key is stored at /Users/myname/.ssh/id_rsa.

To generate a key - ssh-keygen

~/.ssh/id_rsa ~/ - public key goes into server "authorized_keys" file

To connect to server - ssh [email protected] or ssh [email protected], enter the servers password to authenticate (not local machine)

To add public key and enable pw-less login ssh-copy-id [email protected]

Create a config file and alias cd .ssh ls to view files, and touch config vi config

Host dev
  HostName <domain or ip>
  Port 22
  User <username>
  IdentityFile ~/.ssh/id_rsa

You can now SSH into the systems with this command ssh dev

SSH Tunneling Access resources on remote server or allow acccess to your local resources to someone else. (Creating a tunnel to ssh to a machine and forward data from one port to another). Need a ssh client (work) and ssh server (home)

Setup public SSH Server Local Port Forwarding - access remote resources that you can't access, internal remote database, RDP ssh -L local_port:remote_address:remote_port [email protected] Ex. ssh -L 8888:

Remote Port Forwarding - other ppl to have access to local resources they don't have access to, eg. local web server ssh -R remote_port:local_address:local_port [email protected] Ex. ssh -R 8888: The ssh server config /etc/ssh/sshd_config should have a property GatewayPorts yes

Github Go to Settings, then ssh and gpg keys, add new, add your public key (ls to view files, cat, copy full key and paste in gh. Cloning with ssh - git clone <[email protected] link>


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