Bash scripting cheat sheet

#!/usr/bin/env bash

name="John"
echo "Hello $name!"

name="John"
echo $name 
echo "$name"
echo "${name}!"

wildcard="*.txt"
options="iv"
cp -$options $wildcard /tmp

name="John"
echo "Hi $name"  #=> Hi John
echo 'Hi $name'  #=> Hi $name

echo "I'm in $(pwd)"
echo "I'm in `pwd`"  # obsolescent

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

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

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

echo {A,B}.js

{A,B}   Same as A B
{A,B}.js    Same as A.js B.js
{1..5}  Same as 1 2 3 4 5
{{1..3},{7..9}}     Same as 1 2 3 7 8 9

name="John"
echo "${name}"
echo "${name/J/j}"    #=> "john" (substitution)
echo "${name:0:2}"    #=> "Jo" (slicing)
echo "${name::2}"     #=> "Jo" (slicing)
echo "${name::-1}"    #=> "Joh" (slicing)
echo "${name:(-1)}"   #=> "n" (slicing from right)
echo "${name:(-2):1}" #=> "h" (slicing from right)
echo "${food:-Cake}"  #=> $food or "Cake"

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

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

str="Hello world"
echo "${str:6:5}"    # "world"
echo "${str: -5:5}"  # "world"

src="/path/to/foo.cpp"
base=${src##*/}   #=> "foo.cpp" (basepath)
dir=${src%$base}  #=> "/path/to/" (dirpath)
dir=${src%/*}     #=> "/path/to" (dirpath)

prefix_a=one
prefix_b=two
echo ${!prefix_*}  # all variables names starting with `prefix_`
prefix_a prefix_b

name=joe
pointer=name
echo ${!pointer}
joe

${foo%suffix}   Remove suffix

${foo#prefix}   Remove prefix

${foo%%suffix}  Remove long suffix

${foo/%suffix}  Remove long suffix

${foo##prefix}  Remove long prefix

${foo/#prefix}  Remove long prefix

${foo/from/to}  Replace first match

${foo//from/to}     Replace all

${foo/%from/to}     Replace suffix

${foo/#from/to}     Replace prefix

# Single line comment

: '
This is a
multi line
comment
'

${foo:0:3}  Substring (position, length)

${foo:(-3):3}   Substring from the right

str="HELLO WORLD!"
echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
echo "${str,,}"  #=> "hello world!" (all lowercase)

str="hello world!"
echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)

${foo:-val}     $foo, or val if unset (or null)
${foo:=val}     Set $foo to val if unset (or null)
${foo:+val}     val if $foo is set (and not null)
${foo:?message}     Show error message and exit if $foo is unset (or null)

#Omitting the : removes the (non)nullity checks, e.g. ${foo-val} expands to val if unset otherwise $foo.

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

#With step size

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

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

while true; do
  ···
done

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)

$#  Number of arguments

$*  All positional arguments (as a single word)

$@  All positional arguments (as separate strings)

$1  First argument

$0  The name of the shell or the shell script (filename). Set by the shell itself

$_  Last argument of the previous command

$?  Status of the most recently executed foreground-pipeline (exit/return code)

$!  The process ID (PID) of the most recently executed background pipeline (like started with command &)

$$  The process ID (PID) of the shell. In an explicit subshell it expands to the PID of the current "main shell", not the subshell. This is different from $BASHPID!

#Note: $@ and $* must be quoted in order to perform as described. Otherwise, they do exactly the same thing (arguments as separate strings).

#Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.

[[ -z STRING ]]     Empty string

[[ -n STRING ]]     Not empty string

[[ STRING == STRING ]]  Equal

[[ STRING != STRING ]]  Not Equal

[[ NUM -eq NUM ]]   Equal

[[ NUM -ne NUM ]]   Not equal

[[ NUM -lt NUM ]]   Less than

[[ NUM -le NUM ]]   Less than or equal

[[ NUM -gt NUM ]]   Greater than

[[ NUM -ge NUM ]]   Greater than or equal

[[ STRING =~ STRING ]]  Regexp

(( NUM < NUM ))     Numeric conditions

[[ -o noclobber ]]  If OPTIONNAME is enabled

[[ ! EXPR ]]    Not

[[ X && Y ]]    And

[[ X || Y ]]    Or

[[ -e FILE ]]   Exists

[[ -r FILE ]]   Readable

[[ -h FILE ]]   Symlink

[[ -d FILE ]]   Directory

[[ -w FILE ]]   Writable

[[ -s FILE ]]   Size is > 0 bytes

[[ -f FILE ]]   File

[[ -x FILE ]]   Executable

[[ FILE1 -nt FILE2 ]]   1 is more recent than 2

[[ FILE1 -ot FILE2 ]]   2 is more recent than 1

[[ FILE1 -ef FILE2 ]]   Same files

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

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

# Equal
if [[ "$A" == "$B" ]]

# Regex
if [[ "A" =~ . ]]

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

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

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

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

echo "${Fruits[0]}"           # Element #0

echo "${Fruits[-1]}"          # Last element

echo "${Fruits[@]}"           # All elements, space-separated

echo "${#Fruits[@]}"          # Number of elements

echo "${#Fruits}"             # String length of the 1st element

echo "${#Fruits[3]}"          # String length of the Nth element

echo "${Fruits[@]:3:2}"       # Range (from position 3, length 2)

echo "${!Fruits[@]}"          # Keys of all elements, space-separated

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

words=($(< datafile))                   # From file (split by IFS)

for i in "${arrayName[@]}"; do
  echo "$i"
done

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

#Iterate over values
for val in "${sounds[@]}"; do
  echo "$val"
done

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

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

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

set -o pipefail   # Unveils hidden failures

set -o nounset    # Exposes unset variables

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

shopt -s failglob    # Non-matching globs throw errors

shopt -s nocaseglob  # Case insensitive globs

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

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

history     Show history

shopt -s histverify     Don’t execute expanded result immediately

!$  Expand last parameter of most recent command

!*  Expand all parameters of most recent command

!-n     Expand nth most recent command

!n  Expand nth command in history

!<command>  Expand most recent invocation of command <command>

!!  Execute last command again

!!:s/<FROM>/<TO>/   Replace first occurrence of <FROM> to <TO> in most recent command

!!:gs/<FROM>/<TO>/  Replace all occurrences of <FROM> to <TO> in most recent command

!$:t    Expand only basename from last parameter of most recent command

!$:h    Expand only directory from last parameter of most recent command

!! and !$ can be replaced with any valid expansion.

!!:n    Expand only nth token from most recent command (command is 0; first argument is 1)

!^  Expand first argument from most recent command

!$  Expand last token from most recent command

!!:n-m  Expand range of tokens from most recent command

!!:n-$  Expand nth token to last from most recent command

!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc

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

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

declare -i count  # Declare as type integer
count+=1          # Increment

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

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 >output.txt 2>&1        # stdout and stderr to (file), equivalent to &>

python hello.py &>/dev/null             # stdout and stderr to (null)

echo "$0: warning: too many users" >&2  # print diagnostic message to stderr

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

diff <(ls -r) <(ls)            # Compare two stdout without files

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

trap 'echo Error at about $LINENO' ERR

or

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

set -o errtrace
trap traperr ERR

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

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

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

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

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

printf "This is how you print a float: %f" 2
#=> "This is how you print a float: 2.000000"

printf '%s\n' '#!/bin/bash' 'echo hello' >file
# format string is applied to each group of arguments
printf '%i+%i=%i\n' 1 2 3  4 5 9

-c  Operations apply to characters not in the given set

-d  Delete characters

-s  Replaces repeated characters with single occurrence

-t  Truncates

[:upper:]   All upper case letters

[:lower:]   All lower case letters

[:digit:]   All digits

[:space:]   All whitespace

[:alpha:]   All letters

[:alnum:]   All letters and digits

#Example

echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
WELCOME TO DEVHINTS

dir=${0%/*}

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

cat <<END
hello world
END

tr '[:lower:]' '[:upper:]' <<< "Will be uppercased, even $variable"

# loop on myfunc output lines
while read -r line; do
  echo "$line"
done < <(myfunc)

# compare content of two folders
diff <(ls "$dir1") <(ls "$dir2")

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

#The -r option disables a peculiar legacy behavior with backslashes.

read -n 1 ans    # Just one character

$?  Exit status of last task

$!  PID of last background task

$$  PID of shell

$0  Filename of the shell script

$_  Last argument of the previous command

${PIPESTATUS[n]}    return value of piped commands (array)

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

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

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