shell 笔记

str=‘abc 123 def abc‘
echo ${str/abc /def} # def123 def abc
echo ${str//abc/def} # def 123 def def
echo ${str//[0-9]/} # abc def abc

[ $var == "xxx" ]
[ $var != "xxx" ]
[ -n STRING ] # the length of STRING is nonzero
[ -z STRING ] # the length of STRING is zero

[ $num -eq 10 ]
[ $num -ne 10 ]
[ $num -lt 10 ]
[ $num -le 10 ]
[ $num -gt 10 ]
[ $num -ge 10 ]

[ -d $FILE_NAME ] # dir
[ -e $FILE_NAME ] # exist
[ -f $FILE_NAME ] # regular file
[ -L $FILE_NAME ] # symbol link
[ -r $FILE_NAME ]
[ -w $FILE_NAME ]
[ -x $FILE_NAME ]
[ -S $FILE_NAME ] # socket
[ -b $FILE_NAME ] # block
[ -c $FILE_NAME ] # char

shell 的 $((RANDOM)) 会返回一个随机值

if [ ${#} -eq 0 ]; then
echo $((RANDOM))
elif [ $1 == ‘--help‘ -o $1 == ‘-h‘ ]; then
echo "usage: random num1, num2 [num1, num2)"
elif [ ${#} -eq 1 ]; then
echo $((RANDOM%$1))
else
echo $((RANDOM%(($2-$1))+$1))
fi

echo $((100 / 3))

i=8
echo $((i + 12))

i=8
let i=5+3 # ‘=‘ 两侧不允许有空格
i=$((i+1))
let i++

# if
if [ xxx ]; then
xxx
elif [ xxx ]; then
xxx
else
xxx
fi

if [ xxx ] && [ xxx ]; then
xxx
fi

# while
while [ xxx ]; do
xxx
done

while read line; do
echo $line;
done < input.txt;

# for
for i in `seq 1 10`; do
echo $i;
done

for x in one two three four; do
echo number $x
done

# case
x="start"
case $x in
start)
echo ‘start‘
;;
stop)
echo "stop"
;;
*)
echo stop
;;
esac

x="restart"
case $x in
start|restart)
echo ‘start‘
;;
stop)
echo "stop"
;;
*)
echo stop
;;
esac

myvar="hello"
myfunc() {
myvar="one two three"
for x in $myvar; do
echo $x
done
}
myfunc # 执行函数
echo $myvar $x # one two three (说明变量是全局的)

myvar="hello"
myfunc() {
local x
local myvar="one two three"
for x in $myvar; do
echo $x
done
}
myfunc
echo $myvar $x # hello ($x 没有输出)

repeat() {
while true; do
$@ && return
sleep 5
done
}

子 shell 就是 () 里面的内容，相当于重新开启一个 shell。这样做的好处是 子 shell 里面做的事情不会影响到当前的 shell，比如定义变量，cd 到别的路径..

pwd;
(cd /bin; ls);
pwd;