linux中这段命令是什么意思？“ [ -z "${HOME}" ] ; echo $?”

-z string 如果 string长度为零，则为真

$? 最后运行的命令的结束代码（返回值）

判断用户HOME路径的字符串长度为零，如果为零，输出最后运行的命令的结束代码

[-z "你好 linux"] 与命令 test -z 你好\ linux 是一致的。

意义为检测字符串“你好 linux”是否存在， 不存在则返回真， 否则返回假。

以下是Help test的帮助信息：

test: test [expr]

Exits with a status of 0 (true) or 1 (false) depending on

the evaluation of EXPR. Expressions may be unary or binary. Unary

expressions are often used to examine the status of a file. There

are string operators as well, and numeric comparison operators.

File operators:

-a FILETrue if file exists.

-b FILETrue if file is block special.

-c FILETrue if file is character special.

-d FILETrue if file is a directory.

-e FILETrue if file exists.

-f FILETrue if file exists and is a regular file.

-g FILETrue if file is set-group-id.

-h FILETrue if file is a symbolic link.

-L FILETrue if file is a symbolic link.

-k FILETrue if file has its `sticky' bit set.

-p FILETrue if file is a named pipe.

-r FILETrue if file is readable by you.

-s FILETrue if file exists and is not empty.

-S FILETrue if file is a socket.

-t FD True if FD is opened on a terminal.

-u FILETrue if the file is set-user-id.

-w FILETrue if the file is writable by you.

-x FILETrue if the file is executable by you.

-O FILETrue if the file is effectively owned by you.

-G FILETrue if the file is effectively owned by your group.

-N FILETrue 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.

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