Unix

# anything written here is ignored

Exit
# Exit: command not found

echo Hello world! # Hello world!

exit --help
# exit: exit [n]
#     Exit the shell.
#
#     Exits the shell with a status of N.  If N is omitted, the exit status
#     is that of the last command executed.

type echo # echo is a shell builtin

# type command with option -a
type -a echo
# echo is a shell builtin
# echo is /bin/echo

# To enable or disable shell builtins
enable
# List builtins
compgen -b

# Define function
function greet() { echo "hello"; }

# Create alias
alias my_alias='command_1; command_2'
# Destory alias
unalias

man -k TERM     # Equivalent to command "apropos TERM"

echo   The   triple   spaces   are   shrinken   .
# The triple spaces are shrinken .

echo ~
# /home/YOUR_USERNAME

echo "~   $USER    ."
# ~   YOUR_USERNAME    .

echo '~   $USER    .'
# ~   $USER    .

# Show path to the current directory
pwd         # /home/USER/Desktop/DIR

# Show full path to directory or file.
realpath /  # /
realpath ~  # /home/USER
realpath .  # /home/USER/Desktop/DIR
realpath .. # /home/User/Desktop

cd /home/username/path/to/the/directory

cd   # Plain cd moves to the home directory i.e. works same as cd ~
cd - # Goes to the direcotry you was previously in.

# Create new directory
mkdir NEW_DIRECTORY
mkdir -p ./path/to/end/dir # create every dir that does not exist on the path

# Copy files and directories
cp file1 file2  # Copy file1 to file2
cp -r dir1 dir2 # Copy all from dir1 to dir2

# Move file to another place
mv /old/path/to/file1 /new/path/to/file1
# Rename file1 as file2 and check if not overwriting
mv -i file1 file2
# Create new directory and put everything in
mv ./!(dir1) ./dir1/

# Rename directories tutorial_01,tutorial_02,tutorial_03 to t_01, t_02, t_03
for var in 0{1,2,3}; do mv tutorial_$var t_$var; done
# Replace space with underscore
for file in *; do mv "$file" `echo $file | tr ' ' '_'` ; done
# Add word before extension (file_whatever.png -> file_whatever_historical.png)
for file in *; do mv "$file" $(echo ${file%%.*}_historical.png); done

# Delete files and directories
rm file # Remove file
rm -r   # Remove directory (recursively with all files)

# Shows all files of given directory
ls ./path/to/the/directory

# Show structure of all files and dictionaries from your dictionary
tree

# Shows counts of lines, words and bytes
wc FILE_NAME
# show number of lines (-l) words (-w) and bytes (-c)
wc -l FILE_NAME

# Recursively shows sizes (disk usage) of directories and files
du ./path/to/the/directory
# Show size of the DIRECTORY and its direct children (2 ~ levels down etc.)
du -d 1 ./path/to/the/DIRECTORY
# Display info in human readable form (-h) only for the top folder (-s)
du -hs FILE_NAME
# Display all file and directory sizes
du -a

# show file statistics
stat file1

zip new file1 file2 file3

unzip new.zip
# to NEW_DIR
unzip new.zip -d PATH/NEW_DIR

# -c = create
# -v = verbose
# -z = gzip / gz / zip
# -f = following files

tar -cvzf new.tar.gz file1 file2 file3

# -x = extract

tar -xvzf oldFile.tar.gz

# -j = bz2
# -C = where to extract

tar -xvjf oldFile.tar.bz2 -C /path/Directory

# Ways to create soft links
ln -s FILENAME LINKNAME
cp -s FILENAME LINKNAME

ln -s /usr/share/code_like_hell/bin/code_like_hell /usr/bin/ch

# Hard link creations
ln FILENAME LINKNAME
cp -l FILENAME LINKNAME

# Question Mark
? # Matches one character within one directory.

# Single Asterisk
* # Matches zero or more characters within one directory.

## Double Asterisk
** # Matches matches zero or more characters across multiple directories.

# Square Brackets Range
[a-dsu] # Matches one character from the range i.e. a,b,c,d,s,u
[3-7a-g]

# Square Brackets Group
[:digit:] # Equals to [0-9]
[:alpha:] # Equals to [a-zA-Z]
[:alnum:] # Equals to [0-9a-zA-Z]
[:upper:] # Equals to [A-Z]
[:lower:] # Equals to [a-z]

# Not containing anything from bracket
[!a] # Matches one character that is not letter a. Equivalent of [^ ] in RegEx.

# Examples
ls ?[[:digit:]]*
ls ?[4-6]

# Create new (empty) file
touch path/to/my/newFileName.anything

# Output every line from file
cat file

# Output every line from file in reverse order
tac file

# Output first 5 lines from file in reverse order
head file
# Output first n lines from file in reverse order
head -n 3 file

# Output last 5 lines from file in reverse order
tail file
# Monitor a file as that other software writes to
tail -f

# 0<      1> >>   2>
# 1> where_to     2>&1
# &> where_to

# Take output of command and writes it to the file
# COMMAND > FILE
echo Hello world! > file_example.txt

# Take output of command and writes it at the end of file
# COMMAND >> APPENDED_FILE
echo Hello another world! >> file_example.txt

# any_command | sort, uniq, less, head
# tee
# file

# Sort output
sudo du -a | sort -n -r | head -n 20

mkfifo pipe2
ls > pipe2
cat < pipe2

# Show differences of files FILE_A, FILE_B (e.g. "< UNIQUE LINE OF FILE_A")
diff FILE_A FILE_B

# Unified format - more information about the files, changes have a context
diff -u FILE_A FILE_B

# Compare file with a pipe redirection
SOME_COMMAND | diff FILE_A -

# Show non-printing characters
diff FILE_A FILE_B | cat -t

# Show differences side by side
diff --side-by-side --suppress-common-lines FILE_A FILE_B

# See the difference line by line with more command
diff -y --suppress-common-lines FILE_A FILE_B | more

# Get number of different lines
diff -y --suppress-common-lines FILE_A FILE_B | wc -l

cmp file1 file2

sudo apt install PACKAGE_NAME

sudo dpkg -i PACKAGE_NAME.deb
# attempt to fix corrupted dependencies
sudo apt-get install -f

# Most of the programs have implemented option --version
COMMAND_NAME --version

apt-cache search KEYWORD

dpkg-query -l 'someth'

adduser USERNAME
deluser USERNAME
groupadd GROUPNAME
goupdel GROUPNAME

usermod -a -G GROUP USER

less /etc/group

[sudo] passwd [username]

su USERNAME

# Show number of open shells
echo $SHLVL

su

sudo su

sudo COMMAND

ls -la
#drwxrwxrwx NUMBER USER USER NUMBER DATE NAME_OF_THE_FILE

/etc/shadow

setuid bit chmod u+s ... 4777
setgid bit chmod g+s ... 2777
sticky bit chmod  +t ... 1777

umask 0224 d- 777 f- 666

chown USER[:GROUP] OBJECT

bg

fg

pwd | xclip -selection clipboard

echo "scale=2; 3/2" | bc

echo "obase=10; ibase=2; 1101" | bc

date            # Get current date
date -r FILE    # Get last modification date of a file
date +%T.%N     # Specify the datetime format (start with + for datetime format use %)

scp -r compute  USER@78.128.250.10:/home/USER/computing/

# How to use screen from terminal
screen      # Create screen
screen -d   # Detach from screen
screen -r   # Reattach to screen
screen -ls  # List all screens

# Kill screen
screen -X -S SESSION_ID_FROM_LS kill

sshfs $USER@remote.example.com:/home/$USER/code ~/remote_code

user_allow_other

sshfs -o allow_other user@myserver:/home/user/myprojects ~/mount/myprojects

fusermount -u ~/mount/myprojects

echo ${...}

# Export can be used to export variables or functions.
export
# With the -p option, it prints exported variables and functions
export -p

set | grep ''
var_a=123a

# CONSTANTS
$USER; $PATH; $SHLVL; $SHELL;

# variables
name=value [[:alnum:]]

unset ... (local)
declare [-i; -r] ...

# Open (some) shell
bash # open bash
dash # open dash
sh   # open shell

# Tell current shell language
echo $0
# See shell level
echo $SHLVL
# exit shell i.e. go to the previous enviroment
exit

bash    child[sub]   interactive / uninteractiv - | read

startup

~/.bashrc

HISTSIZE=1000
HISTFILESIZE=10000

echo $HISTFILE

echo 'export HISTIGNORE="ls:tree:cat:tail:head:bash"' >> ~/.bashrc

history !111

# Example: Remove spaces from filename
ls # name\ with\ spaces.txt
for file in *; do mv "$file" `echo $file | tr ' ' '_'` ; done
ls # name_with_spaces.txt

tr [OPTIONS] SET1 [SET2]

# Characters specified by enumeration
echo "characters" | tr abc 123
# Characters specified by range
echo "characters" | tr a-z 1-3
echo "chArACtERs" | tr a-z A-Z
# Characters specified by POSIX
echo "chArACtERs" | tr [:lower:] [:upper:]
# First range is shorter - nothing special
echo "aaabbbccc" | tr a-b 1-3
# First range is longer - characters from SET1 without partner are replaced with last form SET2
echo "aaabbbccc" | tr a-c 1-2

# remove characters
echo "aaabbbccc" | tr -d ab
# replace repetitions with one occurance
echo "aaabbbccc" | tr -s ab
echo "abcabcabc" | tr -s ab