Bash Cheatsheet

#!/bin/bash

VAR="world"
echo "Hello $VAR!" # => Hello world!

$ bash hello.sh

NAME="John"

echo ${NAME}    # => John (Variables)
echo $NAME      # => John (Variables)
echo "$NAME"    # => John (Variables)
echo '$NAME'    # => $NAME (Exact string)
echo "${NAME}!" # => John! (Variables)

NAME = "John"   # => Error (about space)

# This is a signle-line (inline) Bash comment.

: '
This is a
very neat comment
in bash
'

get_name() {
    echo "John"
}

echo "You are $(get_name)"

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

echo {A,B}.js

# => I'm in /path/of/current
echo "I'm in $(PWD)"

# Same as:
echo "I'm in `pwd`"

echo ${food:-Cake}  #=> $food or "Cake"

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

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

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

echo ${STR/foo/bar} # /path/to/bar.cpp

name="John"
echo ${name}           # => John
echo ${name:0:2}       # => Jo
echo ${name::2}        # => Jo
echo ${name::-1}       # => Joh
echo ${name:(-1)}      # => n
echo ${name:(-2)}      # => hn
echo ${name:(-2):2}    # => hn

length=2
echo ${name:0:length}  # => Jo

SRC="/path/to/foo.cpp"

BASEPATH=${SRC##*/}
echo $BASEPATH  # => "foo.cpp"


DIRPATH=${SRC%$BASEPATH}
echo $DIRPATH   # => "/path/to/"

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

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

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

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

ARRAY1=(foo{1..2}) # => foo1 foo2
ARRAY2=({A..D})    # => A B C D

# Merge => foo1 foo2 A B C D
ARRAY3=(${ARRAY1[@]} ${ARRAY2[@]})

# declare construct
declare -a Numbers=(1 2 3)
Numbers+=(4 5) # Append => 1 2 3 4 5

Fruits=('Apple' 'Banana' 'Orange')

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

for i in "${!Fruits[@]}"; do
  printf "%s\t%s\n" "$i" "${Fruits[$i]}"
done

Fruits=("${Fruits[@]}" "Watermelon")     # Push
Fruits+=('Watermelon')                   # Also Push
Fruits=( ${Fruits[@]/Ap*/} )             # Remove by regex match
unset Fruits[2]                          # Remove one item
Fruits=("${Fruits[@]}")                  # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}")  # Concatenate
lines=(`cat "logfile"`)                  # Read from file

function extract()
{
    local -n myarray=$1
    local idx=$2
    echo "${myarray[$idx]}"
}
Fruits=('Apple' 'Banana' 'Orange')
extract Fruits 2     # => Orangle

declare -A sounds

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

echo ${sounds[dog]} # Dog's sound
echo ${sounds[@]}   # All values
echo ${!sounds[@]}  # All keys
echo ${#sounds[@]}  # Number of elements
unset sounds[dog]   # Delete dog

for val in "${sounds[@]}"; do
    echo $val
done

for key in "${!sounds[@]}"; do
    echo $key
done

if [[ -z "$string" ]]; then
    echo "String is empty"
elif [[ -n "$string" ]]; then
    echo "String is not empty"
else
    echo "This never happens"
fi

if [[ X && Y ]]; then
    ...
fi

if [[ "$A" == "$B" ]]; then
    ...
fi

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

if (( $a < $b )); then
   echo "$a is smaller than $b"
fi

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

if [ "$1" = 'y' -a $2 -gt 0 ]; then
    echo "yes"
fi

if [ "$1" = 'n' -o $2 -lt 0 ]; then
    echo "no"
fi

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

for ((i = 0 ; i < 100 ; i++)); do
    echo $i
done

for i in {1..5}; do
    echo "Welcome $i"
done

for i in {5..50..5}; do
    echo "Welcome $i"
done

for number in $(seq 1 3); do
    if [[ $number == 2 ]]; then
        continue;
    fi
    echo "$number"
done

for number in $(seq 1 3); do
    if [[ $number == 2 ]]; then
        # Skip entire rest of loop.
        break;
    fi
    # This will only print 1
    echo "$number"
done

count=0
until [ $count -gt 10 ]; do
    echo "$count"
    ((count++))
done

i=1
while [[ $i -lt 4 ]]; do
    echo "Number: $i"
    ((i++))
done

i=3
while [[ $i -gt 0 ]]; do
    echo "Number: $i"
    ((i--))
done

i=3
while ((i--)); do
    echo "Number: $i"
done

while true; do
    # here is some code.
done

while :; do
    # here is some code.
done

while read line; do
    echo $line
done < file.txt

myfunc() {
    echo "hello $1"
}

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

myfunc "John"

myfunc() {
    local myresult='some value'
    echo $myresult
}

result="$(myfunc)"

myfunc() {
    return 1
}

if myfunc; then
    echo "success"
else
    echo "failure"
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

$((a + 200))      # Add 200 to $a

$(($RANDOM%200))  # Random number 0..199

(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory

command -V cd
#=> "cd is a function/alias/whatever"

python hello.py > output.txt   # stdout to (file)
python hello.py >> output.txt  # stdout to (file), append
python hello.py 2> error.log   # stderr to (file)
python hello.py 2>&1           # stderr to stdout
python hello.py 2>/dev/null    # stderr to (null)
python hello.py &>/dev/null    # stdout and stderr to (null)

python hello.py < foo.txt      # feed foo.txt to stdin for python

source "${0%/*}/../share/foo.sh"

DIR="${0%/*}"

case "$1" in
    start | up)
    vagrant up
    ;;

    *)
    echo "Usage: $0 {start|stop|ssh}"
    ;;
esac

trap 'echo Error at about $LINENO' ERR

traperr() {
    echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}

set -o errtrace
trap traperr ERR

printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga

printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"

printf "Print a float: %f" 2
#=> "Print a float: 2.000000"

while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
    -V | --version )
    echo $version
    exit
    ;;
    -s | --string )
    shift; string=$1
    ;;
    -f | --flag )
    flag=1
    ;;
esac; shift; done
if [[ "$1" == '--' ]]; then shift; fi

if ping -c 1 google.com; then
    echo "It appears you have a working internet connection"
fi

# Show this machine's public IP
curl myip.dog

# Save the IP for use in a script
public_ip=$(curl -fsS myip.dog)
printf '%s\n' "$public_ip"

if grep -q 'foo' ~/.bash_history; then
    echo "You appear to have typed 'foo' in the past"
fi

cat <<END
hello world
END

pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/foo

echo -n "Proceed? [y/n]: "
read ans
echo $ans

read -n 1 ans    # Just one character

git commit && git push
git commit || echo "Commit failed"

set -euo pipefail
IFS=$'\n\t'

args=("$@")
args+=(foo)
args+=(bar)
echo "${args[@]}"