Linux命令--test

官方文档： help test

File operators:
    
      -a FILE        True if file exists.
      -b FILE        True if file is block special.
      -c FILE        True if file is character special.
      -d FILE        True if file is a directory.
      -e FILE        True if file exists.
      -f FILE        True if file exists and is a regular file.
      -g FILE        True if file is set-group-id.
      -h FILE        True if file is a symbolic link.
      -L FILE        True if file is a symbolic link.
      -k FILE        True if file has its `sticky' bit set.
      -p FILE        True if file is a named pipe.
      -r FILE        True if file is readable by you.
      -s FILE        True if file exists and is not empty.
      -S FILE        True if file is a socket.
      -t FD          True if FD is opened on a terminal.
      -u FILE        True if the file is set-user-id.
      -w FILE        True if the file is writable by you.
      -x FILE        True if the file is executable by you.
      -O FILE        True if the file is effectively owned by you.
      -G FILE        True if the file is effectively owned by your group.
      -N FILE        True if the file has been modified since it was last read.
    
      FILE1 -nt FILE2  True if file1 is newer than file2 (according to
                       modification date).
    
      FILE1 -ot FILE2  True if file1 is older than file2.
    
      FILE1 -ef FILE2  True if file1 is a hard link to file2.
    
    String operators:
    
      -z STRING      True if string is empty.
    
      -n STRING
         STRING      True if string is not empty.
    
      STRING1 = STRING2
                     True if the strings are equal.
      STRING1 != STRING2
                     True if the strings are not equal.
      STRING1 < STRING2
                     True if STRING1 sorts before STRING2 lexicographically.
      STRING1 > STRING2
                     True if STRING1 sorts after STRING2 lexicographically.
    
    Other operators:
    
      -o OPTION      True if the shell option OPTION is enabled.
      ! EXPR         True if expr is false.
      EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
      EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.
    
      arg1 OP arg2   Arithmetic tests.  OP is one of -eq, -ne,
                     -lt, -le, -gt, or -ge.
    
    Arithmetic binary operators return true if ARG1 is equal, not-equal,
    less-than, less-than-or-equal, greater-than, or greater-than-or-equal
    than ARG2.

例子：

1. 判断文件是否存在

root@Director test]# test -f demo.sh 
[root@Director test]# test -f demo.sh  && echo true
true
[root@Director test]# test ! -f demo.sh && echo true
[root@Director test]# 

2. 判断字符串是否为空

[root@Director test]# var=hello
[root@Director test]# test -n $var && echo true
true
[root@Director test]# test -z $var && echo true
[root@Director test]# 

3.比较

[root@Director test]# var=a
[root@Director test]# test $var = a && echo true
true