源自：https://devhints.io/bash
此网站的其他内容也不错，例如https://devhints.io/docker

Go to previous directory

cd -

Subshells

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

Conditionals execution

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

Conditionals

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

Function arguments

$#  Number of arguments
$*  All arguments
$@  All arguments, starting from first
$1  First argument
}
echo "You are $(get_name)"

Function return values

get_name() {
  echo "John" # return
}
echo "You are $(get_name)"

Brace expansion

echo {a,b}.js
echo {a..z}.js

String quotes

NAME="Hohn"
echo "Hi $NAME"
echo 'Hi $NAME'

Shell execution

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

Length

${#FOO} Length of $FOO

Default values

${FOO:-val} $FOO, or val if not set
${FOO:=val} Set $FOO to val if not set
${FOO:+val} val if $FOO is set
${FOO:?message} Show error message and exit if $FOO is not set

Loops

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

Printf

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

Special variables

$?  Exit status of last task
$!  PID of last background task
$$  PID of shell

File conditions

[ -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

Defining arrays

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)

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