Shell手册

Jul 5, 2020 · shell  ·
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This is a quick reference to getting started with Bash scripting.

1#!/usr/bin/env bash
2
3NAME="John"
4echo "Hello $NAME!"
bash
1NAME="John"
2echo $NAME
3echo "$NAME"
4echo "${NAME}!"
bash
1NAME="John"
2echo "Hi $NAME"  #=> Hi John
3echo 'Hi $NAME'  #=> Hi $NAME
bash
1echo "I'm in $(pwd)"
2echo "I'm in `pwd`"
3# Same
bash

See Command substitution

1git commit && git push
2git commit || echo "Commit failed"
bash
1get_name() {
2  echo "John"
3}
4
5echo "You are $(get_name)"
bash

See: Functions

1if [[ -z "$string" ]]; then
2  echo "String is empty"
3elif [[ -n "$string" ]]; then
4  echo "String is not empty"
5fi
bash

See: Conditionals

1set -euo pipefail
2IFS=$'\n\t'
bash

See: Unofficial bash strict mode

1echo {A,B}.js
bash
Expression Description
{A,B} Same as A B
{A,B}.js Same as A.js B.js
{1..5} Same as 1 2 3 4 5

See: Brace expansion

1name="John"
2echo ${name}
3echo ${name/J/j}    #=> "john" (substitution)
4echo ${name:0:2}    #=> "Jo" (slicing)
5echo ${name::2}     #=> "Jo" (slicing)
6echo ${name::-1}    #=> "Joh" (slicing)
7echo ${name:(-1)}   #=> "n" (slicing from right)
8echo ${name:(-2):1} #=> "h" (slicing from right)
9echo ${food:-Cake}  #=> $food or "Cake"
bash
1length=2
2echo ${name:0:length}  #=> "Jo"
bash

See: Parameter expansion

 1STR="/path/to/foo.cpp"
 2echo ${STR%.cpp}    # /path/to/foo
 3echo ${STR%.cpp}.o  # /path/to/foo.o
 4echo ${STR%/*}      # /path/to
 5
 6echo ${STR##*.}     # cpp (extension)
 7echo ${STR##*/}     # foo.cpp (basepath)
 8
 9echo ${STR#*/}      # path/to/foo.cpp
10echo ${STR##*/}     # foo.cpp
11
12echo ${STR/foo/bar} # /path/to/bar.cpp
bash
1STR="Hello world"
2echo ${STR:6:5}   # "world"
3echo ${STR: -5:5}  # "world"
bash
1SRC="/path/to/foo.cpp"
2BASE=${SRC##*/}   #=> "foo.cpp" (basepath)
3DIR=${SRC%$BASE}  #=> "/path/to/" (dirpath)
bash
Code Description
${FOO%suffix} Remove suffix
${FOO#prefix} Remove prefix
--- ---
${FOO%%suffix} Remove long suffix
${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 
1# Single line comment
bash
1: '
2This is a
3multi line
4comment
5'
bash
Expression Description
${FOO:0:3} Substring (position, length)
${FOO:(-3):3} Substring from the right
Expression Description
${#FOO} Length of $FOO 
1STR="HELLO WORLD!"
2echo ${STR,}   #=> "hELLO WORLD!" (lowercase 1st letter)
3echo ${STR,,}  #=> "hello world!" (all lowercase)
4
5STR="hello world!"
6echo ${STR^}   #=> "Hello world!" (uppercase 1st letter)
7echo ${STR^^}  #=> "HELLO WORLD!" (all uppercase)
bash
Expression Description
${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.

1for i in /etc/rc.*; do
2  echo $i
3done
bash
1for ((i = 0 ; i < 100 ; i++)); do
2  echo $i
3done
bash
1for i in {1..5}; do
2    echo "Welcome $i"
3done
bash
1for i in {5..50..5}; do
2    echo "Welcome $i"
3done
bash
1cat file.txt | while read line; do
2  echo $line
3done
bash
1while true; do
2  ···
3done
bash
1myfunc() {
2    echo "hello $1"
3}
bash
1# Same as above (alternate syntax)
2function myfunc() {
3    echo "hello $1"
4}
bash
1myfunc "John"
bash
1myfunc() {
2    local myresult='some value'
3    echo $myresult
4}
bash
1result="$(myfunc)"
bash
1myfunc() {
2  return 1
3}
bash
1if myfunc; then
2  echo "success"
3else
4  echo "failure"
5fi
bash
Expression Description
$# Number of arguments
$* All positional arguments (as a single word)
$@ All positional arguments (as separate strings)
$1 First argument
$_ Last argument of the previous command

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

See Special parameters.

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.

Condition Description
[[ -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
Condition Description
[[ -o noclobber ]] If OPTIONNAME is enabled
--- ---
[[ ! EXPR ]] Not
[[ X && Y ]] And
`[[ X
Condition Description
[[ -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 
1# String
2if [[ -z "$string" ]]; then
3  echo "String is empty"
4elif [[ -n "$string" ]]; then
5  echo "String is not empty"
6else
7  echo "This never happens"
8fi
bash
1# Combinations
2if [[ X && Y ]]; then
3  ...
4fi
bash
1# Equal
2if [[ "$A" == "$B" ]]
bash
1# Regex
2if [[ "A" =~ . ]]
bash
1if (( $a < $b )); then
2   echo "$a is smaller than $b"
3fi
bash
1if [[ -e "file.txt" ]]; then
2  echo "file exists"
3fi
bash
1Fruits=('Apple' 'Banana' 'Orange')
bash
1Fruits[0]="Apple"
2Fruits[1]="Banana"
3Fruits[2]="Orange"
bash
1echo ${Fruits[0]}           # Element #0
2echo ${Fruits[-1]}          # Last element
3echo ${Fruits[@]}           # All elements, space-separated
4echo ${#Fruits[@]}          # Number of elements
5echo ${#Fruits}             # String length of the 1st element
6echo ${#Fruits[3]}          # String length of the Nth element
7echo ${Fruits[@]:3:2}       # Range (from position 3, length 2)
8echo ${!Fruits[@]}          # Keys of all elements, space-separated
bash
1Fruits=("${Fruits[@]}" "Watermelon")    # Push
2Fruits+=('Watermelon')                  # Also Push
3Fruits=( ${Fruits[@]/Ap*/} )            # Remove by regex match
4unset Fruits[2]                         # Remove one item
5Fruits=("${Fruits[@]}")                 # Duplicate
6Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
7lines=(`cat "logfile"`)                 # Read from file
bash
1for i in "${arrayName[@]}"; do
2  echo $i
3done
bash
1declare -A sounds
bash
1sounds[dog]="bark"
2sounds[cow]="moo"
3sounds[bird]="tweet"
4sounds[wolf]="howl"
bash

Declares sound as a Dictionary object (aka associative array).

1echo ${sounds[dog]} # Dog's sound
2echo ${sounds[@]}   # All values
3echo ${!sounds[@]}  # All keys
4echo ${#sounds[@]}  # Number of elements
5unset sounds[dog]   # Delete dog
bash
1for val in "${sounds[@]}"; do
2  echo $val
3done
bash
1for key in "${!sounds[@]}"; do
2  echo $key
3done
bash
1set -o noclobber  # Avoid overlay files (echo "hi" > foo)
2set -o errexit    # Used to exit upon error, avoiding cascading errors
3set -o pipefail   # Unveils hidden failures
4set -o nounset    # Exposes unset variables
bash
1shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
2shopt -s failglob    # Non-matching globs throw errors
3shopt -s nocaseglob  # Case insensitive globs
4shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
5shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
bash

Set GLOBIGNORE as a colon-separated list of patterns to be removed from glob matches.

Command Description
history Show history
shopt -s histverify Don't execute expanded result immediately
Expression Description
!$ 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>
Code Description
!! 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.

Code Description
!!: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.

1$((a + 200))      # Add 200 to $a
bash
1$(($RANDOM%200))  # Random number 0..199
bash
1(cd somedir; echo "I'm now in $PWD")
2pwd # still in first directory
bash
1python hello.py > output.txt   # stdout to (file)
2python hello.py >> output.txt  # stdout to (file), append
3python hello.py 2> error.log   # stderr to (file)
4python hello.py 2>&1           # stderr to stdout
5python hello.py 2>/dev/null    # stderr to (null)
6python hello.py &>/dev/null    # stdout and stderr to (null)
bash
1python hello.py < foo.txt      # feed foo.txt to stdin for python
bash
1command -V cd
2#=> "cd is a function/alias/whatever"
bash
1trap 'echo Error at about $LINENO' ERR
bash

or

1traperr() {
2  echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
3}
4
5set -o errtrace
6trap traperr ERR
bash
1case "$1" in
2  start | up)
3    vagrant up
4    ;;
5
6  *)
7    echo "Usage: $0 {start|stop|ssh}"
8    ;;
9esac
bash
1source "${0%/*}/../share/foo.sh"
bash
1printf "Hello %s, I'm %s" Sven Olga
2#=> "Hello Sven, I'm Olga
3
4printf "1 + 1 = %d" 2
5#=> "1 + 1 = 2"
6
7printf "This is how you print a float: %f" 2
8#=> "This is how you print a float: 2.000000"
bash
1DIR="${0%/*}"
bash
 1while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
 2  -V | --version )
 3    echo $version
 4    exit
 5    ;;
 6  -s | --string )
 7    shift; string=$1
 8    ;;
 9  -f | --flag )
10    flag=1
11    ;;
12esac; shift; done
13if [[ "$1" == '--' ]]; then shift; fi
bash
1cat <<END
2hello world
3END
bash
1echo -n "Proceed? [y/n]: "
2read ans
3echo $ans
bash
1read -n 1 ans    # Just one character
bash
Expression Description
$? Exit status of last task
$! PID of last background task
$$ PID of shell
$0 Filename of the shell script
$_ Last argrument of the previous command

See Special parameters.

1pwd # /home/user/foo
2cd bar/
3pwd # /home/user/foo/bar
4cd -
5pwd # /home/user/foo
bash
1if ping -c 1 google.com; then
2  echo "It appears you have a working internet connection"
3fi
bash
1if grep -q 'foo' ~/.bash_history; then
2  echo "You appear to have typed 'foo' in the past"
3fi
bash