1.shell学习之常用语句

#!/bin/bash

for var in $*
do
   echo "$var"           #for循环打印所有参数
done

if [ "$filename" == "$1" ]; then     #执行两条命令
echo "File $filename exists.";fi

case "变量" in
  "变量1")
      ...
  ;;          #这里的双分号类似于break

  "变量2")
      ...
  ;; 

  *)          #匹配剩下的变量n
     ...
  ;; 
esac        #case结束语句

#!/bin/base

variable=xyz

case "$variable" in
abc) echo "\$variable = abc" ;;
xyz) echo "\$variable = xyz" ;; 
esac

#!/bin/bash

a="this is a"
b="this is b"

echo '${a}'
echo "${b}"

${a}
this is b

echo $(date "+ %Y/%m/%d %H:%M:%S")     #运行date命令
echo `date "+ %Y/%m/%d %H:%M:%S"`      #运行date命令
c=$(echo hello)                        #打印: c=hello

A=123

echo $AB      #错误,将打印空的一行,因为运行时默认会去找变量值AB的解释
echo ${A}B    #正常打印:123B

:
echo $?    # 打印上个命令的返回值,为0

: > data.xxx         #如果该文件不存在,则会创建, 与cat /dev/null >data.xxx的作用相同

a=1
(a=2; echo "a = $a")    #打印a=2
echo "a = $a"           #打印a=1,说明变量a并没有改变

arr=(1 2 3)             #等价于array[0]=1 array[1]=2 array[2]=3

for(( i=0;i<${#arr[@]};i++)) 
do
echo ${arr[i]};
done;

echo $(date "+ %Y/%m/%d %H:%M:%S")    #运行date命令

for(( i=0;i<5;i++));do
echo $i;done

echo $((16#ff))    #打印255

if (("$a" < "$b"))
if (("$a" <= "$b"))
if (("$a" > "$b"))
if (("$a" >= "$b"))

 

a=1
{
   a=2
   echo "a = $a"   #打印a=2
} 
echo "a = $a"      #打印a=2,说明已被修改

#!/bin/bash
{
echo 'hello'
echo 'this is test file'
echo 'end'
}  > ./test

echo {1..10}        #打印1 2 3 4 5 6 7 8 9 10
for var in {a..z}
do
echo $var           #打印a~z字母
done

#!/bin/bash

testfile="${PWD}/test"

if [ -e "$testfile" ];then
      echo " $testfile is exist"
fi

//-eq 判断等于(equal)
if [ "$a" -eq "$b" ]

//-ne 判断不等于(no equal)
if [ "$a" -ne "$b" ]

//-gt 大于(greater than)
if [ "$a" -gt "$b" ]

//-ge 大于等于
if [ "$a" -ge "$b" ]

//-lt 小于(less than)
if [ "$a" -lt "$b" ]

//-le 小于等于
if [ "$a" -le "$b" ]

a="abcde"
b="abc"

if [ -n "$a" -o "$a" \> "$b" ] #判断a非0,且a>b
then
echo "a > b"
fi

a=11
b=11
c="abc"
e="abcd"

if [[ "$a" == "$b" && "$c" < "$e" ]]
then
echo "a = b,c < e"
fi

A=~
B=~+
echo $A            #打印home目录
echo $B            #打印当前目录

sleep 1     #延时1秒
sleep 0.5   #延时0.5秒
sleep 1m    #睡眠1分
sleep 1h    #睡眠1小时

a= 1 2 3     #将会出错,正确的用法应该是a="1 2 3"