Shell Cheatsheet

变量

变量定义

var="hello"
echo $var ${var}-name

位置参数

param	desc
$0	script file
$1 .. $n	script params
$#	param count
$@ $*	all params
$?	most recent ret code
$!	most recent background job pid

$@ vs $*

# script.sh
function main() {
   echo 'I got ' $# ' args'
}
main $*
main $@
main "$*"
main "$@"

# output
$ sh script.sh 'a b c' d e
I got 5 args
I got 5 args
I got 1 args
I got 3 args

条件控制

注意项

• [ -z "" ] 前后都有空格
• [ "1" \> "" ] 因为 [ 是命令，后面跟着的都是它的参数，所以不要少加空格，且为了防止 > 被解析为重定向符号要转义
• [[ "1" > "" ]] 因为 [[ ]] 是 bash 的语法结构，所以不需要转义

逻辑测试

param	desc
(e)	([ -n "hello" ])
not e	[ ! -n "hello" ] 等价于 ! [ -n "" ]
e1 and e2	[ -n "" -a -n "" ] 等价于 [ -n "" ] && [ -n "" ]
e1 or e2	[ -n "" -o -n "" ] 等价于 [ -n "" ]		[ -n "" ]

文件测试

param	desc
-d FILE	is directory
-f FILE	is file
-e FILE	file or dir exists
-x FILE	excutable
f1 -nt f2	f1 newer then f2
f1 -ot f2	f1 older then f2
...	man test

字符串测试

param	desc
-z str	len(str) == 0
-n str	len(str) > 0
s1 = s2	s1 == s2
s1 != s2	s1 != s2
s1 > s2	s1 > s2
!(s1 < s2)	s1 >= s2
s1 < s2	s1 < s2
!(s1 > s2)	s1 <= s2
...	man test

数字测试

param	desc
n1 -eq n2	algebraically ==
n1 -ne n2	algebraically !=
n1 -gt n2	algebraically >
n1 -ge n2	algebraically >=
n1 -lt n2	algebraically <
n1 -le n2	algebraically <=
...	man test

直接测试

$ test expression && echo yes
$ [ expression ] && echo yes
$ [[ expression ]] && echo yes

条件判断

if [[ expression ]]; then
  # ...
elif [[ expression ]]; then
  # ...
else
  # ...
fi

条件选择

case VAR in
  var1) cmd1 ;;
  var2) cmd2 ;;
  # ...
  *) cmd ;;
esac

循环控制

for

for var in (list); do
  # ...
  break
  continue
done

for ((i=1, j=10; i <= 10; i++, j--)); do
  # ...
done

while

while ((1)); do
  # ...
  break
  continue
done

while true; do
  # ...
done

until

until [[ -n "" ]]; do
  # ...
  break
  continue
done

select DAY in Mon Tue Wed Thu Fri Sat Sun; do
case $DAY in
Mon) echo "Today is Monday" ;;
Tue) echo "Today is Tuesday" ;;
# ...
*) echo "Unkown day, stop" && break ;;
esac
done

# 函数

```shell
function foo() {
  local var=0
  return $var
}