Bash CheatSheet

Bash is the shell program, or command language interpreter, for the GNU operating system.

A shell program is typically an executable binary that takes commands that you type and (once you hit return), translates those commands into (ultimately) system calls to the Operating System API.

Bash can also read and execute commands from a file, called a shell script. Like most Unix shells, it supports filename globbing (wildcard matching), piping, here documents, command substitution, variables, and control structures for condition-testing and iteration. The keywords, syntax, dynamically scoped variables and other basic features of the language are all copied from sh. Other features, e.g., history, are copied from csh and ksh. Bash is a POSIX-compliant shell, but with a number of extensions.

The name is an acronym for the Bourne-Again SHell, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh, which appeared in the Seventh Edition Bell Labs Research version of Unix.

#!/bin/bash

VAR="bestforstudy.com"
echo "Hello World, $VAR!" 
# Output: Hello World, bestforstudy.com!

SITENAME="bestforstudy.com"

echo ${SITENAME}    # => bestforstudy.com
echo $SITENAME      # => bestforstudy.com
echo "$SITENAME"    # => bestforstudy.com
echo '$SITENAME'    # => $SITENAME
echo "${SITENAME}!" # => bestforstudy.com!

SITENAME = "bestforstudy.com" # => Error (about space)

# This is an inline Bash comment.


: '
    Multi-line comments
'

get_website_name() {
    echo "bestforstudy.com"
}


echo "You access $(get_name)"
# Output: You access bestforstudy.com

# Syntax if
if [ condition ] ; then
Command(s)
fi


# Syntax if-else statement
if [ condition ]; then
Command(s)
else
Command(s)
fi


# Synxtax if-elif-else statement
if [ condition ]; then
Command(s)
elif [ condition ]; then
Command(s)
..
else
Command(s)
fi


# Example 1
echo "Enter a number"
read n
if [ $n -lt 100 ]; then
printf "$n is less than 100\n"
fi


# Example 2
echo "Enter username"
read un
echo "Enter password"
read pw
if [[ "$un" == "admin" && "$pw" = "superuser" ]]; then
echo "Login Successful."
fi


# Example 3
if [[ -z "$string" ]]; then
    echo "String is empty"
elif [[ -n "$string" ]]; then
    echo "String is not empty"
fi

echo {A,B}.js

# Syntax
$( <COMMANDS> )
# Similar to
` <COMMANDS> `


# Example
echo "Your username is $(USERNAME)"
# is similar to
echo "Your username is `USERNAME`"

echo ${site:-bestforstudy}  
# Output: $site or "bestforstudy"


STR="/path/to/bestforstudy.cpp"
echo ${STR%.cpp}    # /path/to/bestforstudy
echo ${STR%.cpp}.o  # /path/to/bestforstudy.o
echo ${STR%/*}      # /path/to

echo ${STR##*.}     # cpp (extension)
echo ${STR##*/}     # bestforstudy.cpp (basepath)

echo ${STR#*/}      # path/to/bestforstudy.cpp
echo ${STR##*/}     # bestforstudy.cpp

echo ${STR/bestforstudy/google} # /path/to/google.cpp

name="Jack"

echo ${name}           
# Output: Jack

echo ${name:0:2}       
# Output: Ja

echo ${name::2}        
# Output: Ja

echo ${name::-1}       
# Output: Jac

echo ${name:(-1)}      
# Output: k

echo ${name:(-2)}      
# Output: ck

echo ${name:(-2):2}    
# Output: ck

length=2
echo ${name:0:length}  
# Output: Ja

SRC="/path/to/site/bestforstudy.com"

BASEPATH=${SRC##*/}   
echo $BASEPATH  
# Output: "bestforstudy.com"

DIRPATH=${SRC%$BASEPATH}
echo $DIRPATH   
# Output: "/path/to/site/"

STR="HELLO WORLD!"
echo ${STR,}   
# Output: hELLO WORLD!
echo ${STR,,}  
# Output: hello world!

STR="hello world!"
echo ${STR^}   
# Output: Hello world!
echo ${STR^^}  
# Output: HELLO WORLD!

ARR=(hello World)
echo "${ARR[@],}" 
# Output: hello world
echo "${ARR[@]^}" 
# Output: Hello World

# declare construct
declare -a Numbers=(1 2 3 4 5 6 7 8 9 10)

Sports=('Football' 'Baseball' 'Basketball')

Sports[0]="Football"
Sports[1]="Baseball"
Sports[2]="Basketball"

ARRAY2=(arr{1..2}) 
# Output: arr1 arr2

ARRAY3=({A..D})    
# Output: A B C D

Sports=('Football' 'Baseball' 'Basketball')

for e in "${Sports[@]}"; do
    echo $e
done

# loop with index
for i in "${!Sports[@]}"; do
  printf "%s\t%s\n" "$i" "${Sports[$i]}"
done

# Push value to array
Sports=("${Sports[@]}" "Swimming")     

# Another way to Push value to array
Sports+=('Swimming')                   

# Remove value in by regex if match
Sports=( ${Sports[@]/Ap*/} )             

# Remove one item
unset Sports[2]                          

# Duplicate
Sports=("${Sports[@]}")                  

# Concatenate
Sports=("${Sports[@]}" "${Veggies[@]}")  

# Read from file
lines=(`cat "logfileName"`)                  

function extract() {
    local -n myarray=$1
    local idx=$2
    echo "${myarray[$idx]}"
}

Sports=('Football' 'Baseball' 'Basketball')

extract Sports 1
# Result: Baseball

declare -A sounds

sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"

# Dog's sound
echo ${sounds[dog]}

# All values
echo ${sounds[@]}

# All keys
echo ${!sounds[@]}

# Number of elements
echo ${#sounds[@]}

# Delete dog
unset sounds[dog]

for val in "${sounds[@]}"; do
    echo $val
done
# Output: Show all sounds values


for key in "${!sounds[@]}"; do
    echo $key
done
# Output: Show all sounds key

if [ "$1" = 'y' -a $2 -gt 0 ]; then
    echo "The answer is YES"
fi


if [ "$1" = 'n' -o $2 -lt 0 ]; then
    echo "The answer is NO"
fi

# Conditions with String
read -p "Enter the string: " STR
if [[ -z "$STR" ]]; then
    echo "The input String is empty"
elif [[ -n "$STR" ]]; then
    echo "The input String is not empty"
else
    echo "This condition will never happens"
fi


# Combinations
if [[ X && Y ]]; then
    # Logic here ...
fi


# Equal
read -p "Enter first string: " VAR1
read -p "Enter second string: " VAR2
if [[ "$VAR1" == "$VAR2" ]]; then
    # Your logic here ...
fi


# Regex
if [[ '1. abc' =~ ([a-z]+) ]]; then
    echo ${BASH_REMATCH[1]}
fi


# Smaller
if (( $VAR1 < $VAR2 )); then
   echo "$VAR1 is smaller than $VAR2"
fi


# Exists
if [[ -e "file.txt" ]]; then
    echo "file exists"
fi

for i in /etc/rc.*; do
    echo $i
done

for ((i = 0 ; i < 200 ; i++)); do
    echo $i
done
# Output: 0 -> 199

for i in {1..5}; do
    echo "Welcome to bestforstudy.com: $i"
done

for i in {5..50..5}; do
    echo "Welcome to bestforstudy.com: $i"
done

i=1
while [[ $i -lt 10 ]]; do
    echo "Auto Increase index: $i"
    ((i++))
done
# Run this code to see the result ^^

i=30
while [[ $i -gt 0 ]]; do
    echo "Auto decrement index: $i"
    ((i--))
done
# Run this code to see the result

firstFunc() {
    echo "hello $1"
}


# Same define as above (alternate syntax)
function firstFunc() {
    echo "hello $1"
}


firstFunc "bestforstudy.com"

yourFunc() {
    local yourResult='Your result will be here'
    echo $yourResult
}

result="$(yourFunc)"

yourFunc() {
    return 1
}

if yourFunc; then
    echo "Success"
else
    echo "Fail"
fi

# Avoid overlay files
# (echo "hi" > foo)
set -o noclobber


# Used to exit upon error
# avoiding cascading errors
set -o errexit   


# Unveils hidden failures
set -o pipefail  


# Exposes unset variables
set -o nounset

# Non-matching globs are removed  
# ('*.foo' => '')
shopt -s nullglob   


# Non-matching globs throw errors
shopt -s failglob  


# Case insensitive globs
shopt -s nocaseglob 


# Wildcards match dotfiles 
# ("*.sh" => ".foo.sh")
shopt -s dotglob    


# Allow ** for recursive matches 
# ('lib/**/*.rb' => 'lib/a/b/c.rb')
shopt -s globstar