Linux and the CLI Notes

pwd Displays current path location
ls Displays folders and files within the current directory
- .. Performs ls on parent directory
- . References current directory, no changes
- ~ Perform ls on home directory
- / Perform ls on root of the path
- -l Display more information, such as permissions and size
- -a Display hidden files (prefixed with .)
cd Change current working directory
- .. Change to parent directory
- . References current directory, no changes
- ~ Change to home directory
- / Change to root path directory

Tips

Up/Down arrow keys to revisit old commands
Tab for autocomplete
Ctrl + R for reverse searching commands, Left/Right arrow keys to return to CLI with selected command
~/.bash_history is a file appending commands from terminal, beware of sensitive information
!! Runs previous command
clear Moves cursor to top of terminal to reduce clutter
Ctrl + Shift + C and Ctrl + Shift + V Copy/paste in terminal, standard shortcut is already assigned

Shortcuts

Ctrl + A Moves cursor to beginning of line
Ctrl + E Moves cursor to end of line
Ctrl + K Deletes everything after cursor, the contents are 'yanked'
Ctrl + U Deletes everything before cursor, the contents are 'yanked'
Ctrl + Y The 'yanked' contents are 'pasted'
Ctrl + L Clear the screen

Signals

Signals are notifications sent to programs, program decides what to do with it but generally abides
Ctrl + C Sends SIGINT to interrupt a program
Ctrl + D Sends SIGQUIT, not used as much but can end sessions on bash servers
SIGTERM No shortcut but ued by kill command to terminate programs
SIGKILL No shortcut but used by kill -9 or kill -SIGKILL to stop a program's runtime

Text editors

nano {file}
- Commands listed at bottom of interface, ^ represents Ctrl and paired with a letter to form a shortcut
vim {file}
- Esc Enter command mode (default mode)
- i Enter insert mode from command mode
- :w Save file
- :q Quit file
- :wq Save and quit file
- :q! Quit file without saving

Interacting with files

less Enter read mode for a file
man Displays manual, most programs have one
cat Reads file and outputs to terminal
head or tail Read first 10 or last 10 lines of a files
- -n={number} to specify number of lines
mkdir Creates a new folder.
mkdir -p Creates a nest of folders
touch Creates a new empty file
- If file exists then it manipulates the 'last modified' and 'last access' times
rm Remove files
- -r Remove empty folders
- -rf Remove all types of files without needing confirmation
- WARNING: Do not run rm -rf /, deletes everything from root of the system
cp Copy
- Copy contents from file to file
- Copy file to directory
- -R Recursively copy directory to another directory
mv Move file or folders to another location
tar Group files into a single file (tarball, similar to zip file)
- -z Compress the files
- **-zcf name.tar.gz {file} ... {folder} Compress and combine files/folders
- **-xzf name.tar.gz -C {dest-folder} Extract

Wildcards and expansions

Manipulate multiple files at once
* Represents the remainder of a file name
? Represents exactly one character
[ ] Represents limited number of characters
- Ex. ls file[1-3].txt or use ^ (not) ls file^[1-3].txt
Some expansions:
- {a..z} a to z
- {z..a} z to a, reverse order
- {0..100..2} Every even number between 0 to 100 inclusive
- {100..0..5} Every 5th number fromm 100 to 0 inclusive
- {a..z}{1..5} a1, ..., a5, b1, ..., b5, c1, ..., c5, etc

Streams and pipes

Standard output - stdout
- 1> Redirect output and overwrite the file
- 1>> Redirect output and append to the file
Standard error - stderr
- 2> Redirect error output instead of standard output
- 2>> Redirect error output and append to file

Both the standard output and error streams can be redirected at the same time, can be useful if you want to separate error messages and normal output

/dev/null - Basically the void
Any output redirected to /dev/null is discarded
Standard input - stdin
- < Redirect contents of a file to a program
Pipes can redirect the output of one program as input to another
- | Program on the left of pipe will direct its output to the program on the right
- Ex. ps aux | grep 'ps aux'

Users, groups and permissions

Users are stored in /etc/passwd
whoami Displays current user of the system
Superuser
- sudo Switch user and do, perform command as root user then switch back to normal user
- sudo su Switch to user
- sudo useradd Creates new user
- sudo passwd Adds password to user
Permissions of files and folders follow the format:
- d rwx rwx rwx
- The first character is either 'd' or '-' for directory/file
- The groups represent permissions for users that own the file, groups that own the file and everyone else respectively
- 'r' - Read
- 'w' - Write
- 'x' - Execute
- '-' - No permissions
Can directly assign permissions:
- Non-binary method:
  - Ex. sudo chmod u=rwx, g=rwx, o=rwx {file}
- Binary method:
  - Ex. sudo chmod 777 {file}
  - Add 4, 2 or 1 for adding read, write or executable permissions respectively, else just set to 0
  - Can also use + or - to apply/remove permissions from all groups
  - Ex. chmod +x {file} Adds executable permissions for all groups

Environments

printenv Displays all environment variables, variables can be accessed with $
Set variables:
- Variables will revert after session ends
- {variable-name} = {value}
Set permanently:
- Configure ~/.bash_profile
```
if [ -f ~/.bashrc ]; then
	source ~/.bashrc
fi
```
- ~/.bashrc will be run whenever a new session is created which loads all environment variables
- Set variables in ~/.bashrc
- export {variable-name}={value}

Processes

ps Display running processes
ps aux Display all running processes on the system
{command} & Append & to run a command in the background
jobs List status of background processes currently running, use with -l to obtain process ID
- Listed jobs are assigned a number
- bg {number} Set to run in the background
- fg {number} Set to run in the foreground, will have to interrupt if you want to run any other commands

Exit codes

Any other code than 0 means fail, it can range from 0 to 256
Common exit codes:
- 0 : Successful
- 1 : General catch-all highlighting an error
- 2 : Bash internal error, you or program used bash incorrectly
- 126 : No permissions or file not executable
- 127 : Command not found
- 128 : Exit command had a problem, usually provided non-integer exit code
- 130 : Interrupted program with Ctrl + C
- 137 : Ended program with SIGKILL
- 255 : Out-of-bounds, tried to exit with code larger than 255

SSH

ssh-keygen -t rsa Create public and private key files stored in ~/.ssh, set a passphrase then confirm
ssh user@ip-address
ifconfig Displays network information

wget and curl

wget {url}
curl {url}
curl -I {url} Only checks if server is ready to respond
curl -X {verb} {url} HTTP verbs like POST, PUT, DELETE and PATCH
curl -d {contents} {url} Send a POST body
curl -b {cookies} {url} Send cookies with request
curl -c {cookies-file} {url} Put lots of cookies in a file and send with request
curl -L {url} Follows redirect links such as bit.ly or other URL shorteners
curl -H {header} {url} Include headers

Package management

apt is newer than apt-get, generally ues apt if possible
apt install {package}
apt autoremove Removed unused dependencies
apt update Update available packages apt users
apt list List of packages installed
apt list --upgradable List packages installed with an update available
apt upgrade Update all installed packages to the latest version
apt full-upgrade Autoremove and upgrade functionalities combined
Snaps are a new way to package apps
- Update automatically based off version difference
- Sandboxed from rest of system
- No need to review
snap install {package}
snap info {package} Display version details of package

Scripts

Shell scripts can run many commands at a time
. {bash-file} Runs script
Hashbang
- Execute without bash, run using ./{bash-file}
- Add to first line of file:
```
#! /bin/bash
```
PATH is a series of paths where programs are stored

Add programs to PATH:

cd ~
mkdir bin
mv <bash file> bin/<bash file>
PATH=~/bin:$PATH

To permanently keep custom PATH, add to ~/.bashrc similar to environment variables
```
PATH=~/bin:$PATH
```
In bash, you can assign values to variables like anything else:
```
DESTINATION=~/temp
mkdir -p $DESTINATION
```
Use read to retrieve user input and store into a variable. Can append -p to prompt user with a text for more description
```
read -p 'Enter your name: ' name
echo $name
```
Arguments can be passed into the program and stored in $1 for a single argument
- $0 will be the file, and any subsequent $2, $3, etc will store any other arguments if two or more were provided
```
./<bash file> <name>
```
```
name=$1
echo $name
```

Use special notation [ ] to test commands, used to evaluate conditional statements

read -p 'Enter your name: ' name

if [ -z $name ]; then
	echo 'Empty name'
fi

If, else if, else:

read -p 'Enter your name: ' name

if [ -z $name ]; then
	echo 'Empty name'
elif [ $name = 'Brian' ]; then
	echo 'Name is Brian'
else
	echo $name
fi

Case:

read -p 'Enter your name: ' name

case $name in
	'Brian')
		echo ':)'
		;;
	'Bob')
		echo ':('
		;;
	'John')
		echo ':|'
		;;
	*)
		echo $name
		;;
esac

For loops:

#! /bin/bash

names=(Brian Bob 'John Smith')

for name in ${names[*]}
do
	echo $name
done

echo "Number of names: ${#names[*]}"

While loops:

NUM_TO_GUESS=$(( $RANDOM % 10 + 1 ))
GUESSED_NUM=0

echo "Guess a number between 1 and 10"

while [ $NUM_TO_GUESS -ne $GUESSED_NUM ]
do
	read -p "Your guess: " GUESSED_NUM
done

echo "Congratulations!"

Some command operators:
- -eq Equals
- -ne Not equals
- = Returns 0 if true
- != Returns 1 if false
- -gt Greater than
- -le Less than
- -e {file} Checks file exists
- -w Checks file exists and writable
- -z Checks if variable is empty

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