# 是否存在且是文件
test -f /etc/passwd
# 等价 [ -f /etc/passwd ]

# 是否存在且是目录
test -d /etc
# 等价 [ -d /etc ]

# 是否存在
test -e /etc
# 等价 [ test -e /etc ]

# 字符串长度是否为0（为0则表达式成立）
test -z hello
# 等价 [ test -z hello ]

# 判断数字
[ 5 -gt 4 ]
# 等价 [[ 5 > 4 ]]
# -gt >
# -lt <
# -eq =
# -ge >=
# -le <=

# 判断字符串
[ "abc" = "abc" ]
# 区分大小写
[ "abc" = "Abc" ]

if [ 测试条件成立 ] 或者使用命令返回0值
then
  执行命令
fi

if [ $UID = 0 ]
then
  echo "you are root"
fi

if pwd
then
  echo "pwd is running"
fi

if [ 测试条件成立 ]
then
  执行命令
else
  执行条件不成功的命令
fi

#!/bin/bash

# if-else的使用
if [ $USER = root ] ;then
  echo "you are root"
  echo $UID
else
  echo "you are other"
  echo $UID
fi

if [ 测试条件成立 ]
then
  执行命令
elif [ 测试条件成立 ]
then
  执行命令
else
  执行命令
fi

#!/bin/bash

# if-elif-else的使用
if [ $USER = root ] ; then
  echo "you are root"
elif [ $USER = chendong ] ; then
  echo "you are chendong"
else
  echo "you are other"
fi

#!/bin/bash

if [ $UID = 0 ] ;then
  echo "please run"
  if [ -x /root/shell/5.sh ] ;then
    bash /root/shell/5.sh
  fi
else
  echo "switch root user"
fi

case "$变量" in
  "情况1")
    执行命令  ;;
  "情况2")
    执行命令  ;;
  *)
    执行命令  ;;
esac

#!/bin/bash

# 注意：判断条件中的字符串一定要使用双引号
case "$1" in
  "start"|"START")
    echo "start application"  
    echo "applicaion is running" 
    ;;
  "stop"|"STOP")
    echo "stop application"  
    echo "applicaion is stopped" 
    ;;
  *)
    echo "unknow option"
    echo "Usage: $0 {start|stop}"  
    ;;
esac