Bash 知识点速查

#!/bin/bash
VAR="world"
echo "Hello $VAR!" # => Hello world!

执行脚本

$ bash hello.sh

NAME="John"
echo ${NAME}    # => John (变量)
echo $NAME      # => John (变量)
echo "$NAME"    # => John (变量)
echo '$NAME'    # => $NAME (字符串原样输出)
echo "${NAME}!" # => John! (变量)
NAME = "John"   # => Error (注意不能有空格)

# 这是一个内联 Bash 注释。

: '
这是一个
非常整洁的
bash 注释
'

多行注释使用 :' 打开和 ' 关闭

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

见：函数

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

见：条件句

echo {A,B}.js

见：大括号扩展

# => I'm in /当前/的/路径
echo "I'm in $(PWD)"
# Same as:
echo "I'm in `pwd`"

见：命令替换

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

见：参数扩展

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

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

# 声明构造
declare -a Numbers=(1 2 3)
Numbers+=(4 5) # 附加 => 1 2 3 4 5

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 [ "$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

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

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

while true; do
    # here is some code.
done

while :; do
    # here is some code.
done

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

myfunc() {
  echo "hello $1"
}

# 同上(替代语法)
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

$((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"

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

DIR="${0%/*}"

case "$1" in
    start | up)
    vagrant up
    ;;
    *)
    echo "Usage: $0 {start|stop|ssh}"
    ;;
esac

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"

if ping -c 1 google.com; then
    echo "看来您的互联网连接正常"
fi

if grep -q 'foo' ~/.bash_history; then
    echo "您过去似乎输入过“foo”"
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    # 只有一个字符

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

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

参见：非官方 bash 严格模式

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

将参数放入数组中，然后追加

启用调试模式，会把脚本中的每条命令的执行情况打印出来。它可以在整个会话或脚本上运行，也可以在脚本内以编程方式启用。

以调试模式运行脚本(整个脚本都会打印调试信息)：

$ bash -x myscript.sh

在bash脚本中打开调试(针对部分内容打印调试信息)。

#!/bin/bash
set -x   # Enable debugging
# some code here
set +x   # Disable debugging output.