shell - test expression

shell 测试语句

Here is a partial list of the conditions that test can evaluate. Since test is a shell builtin, use "help test" to see a complete list.

Expression	Description
-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.
-L file	True if file is a symbolic link.
-r file	True if file is a file readable by you.
-w file	True if file is a file writable by you.
-x file	True if file is a file executable by you.
file1 -nt file2	True if file1 is newer than (according to modification time) file2
file1 -ot file2	True if file1 is older than file2
-z string	True if string is empty.
-n string	True if string is not empty.
string1 = string2	True if string1 equals string2.
string1 != string2	True if string1 does not equal string2.

Before we go on, I want to explain the rest of the example above, since it also reveals more important ideas.