Notes on Linux commands, Bash, SSH

practice Linux - repl.it, 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 \

Commands

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 blah.foo 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

Extras

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 oreilly.com 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 http://linuxpocketguide.com/sample.pdf
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"

advanced

• regular expressions
• aliases

vim

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 script.sh
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

Shortcuts

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

Examples:

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

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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) 
+x

• 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 bashfile.sh or chmod +x bashfile.sh to make it executable.
• Commands work exactly the same on the command line as they do within a script.
• To run file bash script.sh 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

Conditionals

if statements

if [ statement ] 
then 
  echo 'your message' 
  exit
fi

else
  echo 'another message'
fi  

if (( ... ))
then 
  echo "..."
else
  echo "..."
fi

operators

• && = and, -a
• || = or, -o

myVar=(var)

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

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

Resources

• Bash scripting tut
• BASH3 Boilerplate
• Bash Boilerplate

---

SSH

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

To generate a key - ssh-keygen

~/.ssh/id_rsa ~/.sshid_rsa.pub - 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 user@host

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 username@sshserver Ex. ssh -L 8888:192.168.1.3:8080 44.11.22.33

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 username@sshserver Ex. ssh -R 8888:10.0.0.3:8080 44.11.22.33 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 id_rsa.pub), copy full key and paste in gh. Cloning with ssh - git clone <[email protected] link>

Resources

• ssh tutorial
• ssh cheat sheet
• ssh tunneling explained
• ssh tunneling tutorial