Shell Script Cheatsheet

#!/usr/bin/env bash

NAME="John"
echo "Hello $NAME!"

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

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

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

get_name() {
  echo "John"
}

echo "You are $(get_name)"

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

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

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

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##*.}     # 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)

${#FOO}	# Length of $FOO

# Single line comment
: '
This is a
multi line
comment
'

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

cat file.txt | while read line; do
  echo $line
done

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 arguments
$@	All arguments, starting from first
$1	First argument
$_	Last argument of the previous command

myfunc() {
  return 1
}

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

[[ -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 ]] # 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"
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[@]}           # 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)

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

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

$((a + 200))      # Add 200 to $a
$((RANDOM%=200))  # Random number 0..200

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"

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"

cat <<END
hello world
END

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

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