Shell 脚本：-z 和 -n 选项与 if

Question

我有一个 shell 脚本，其中有以下几行if [ -z "$xyz" ]和if [ -n "$abc" ]，但我不确定它们的目的是什么。谁能解释一下？

Answer 1

您可以在此处找到有关 bash 运算符的非常好的参考资料。如果您使用不同的外壳，只需搜索即可<my shell> operators找到所需的一切。在您的特定情况下，您正在使用：

-n
   string is not null.

-z
  string is null, that is, has zero length

为了显示：

$ foo="bar";
$ [ -n "$foo" ] && echo "foo is not null"
foo is not null
$ [ -z "$foo" ] && echo "foo is null"
$ foo="";
$ [ -n "$foo" ] && echo "foo is not null"
$ [ -z "$foo" ] && echo "foo is null"
foo is null

Answer 2

为了扩展terdon 的回答，我发现Tutorials Point 上的 Unix / Linux - Shell Basic Operators还包括与文件相关的运算符（以及其他有用的运算符）。

-b file     Checks if file is a block special file; if yes, then the condition becomes true.    [ -b $file ] is false.
-c file     Checks if file is a character special file; if yes, then the condition becomes true.    [ -c $file ] is false.
-d file     Checks if file is a directory; if yes, then the condition becomes true.     [ -d $file ] is not true.
-f file     Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true.  [ -f $file ] is true.
-g file     Checks if file has its set group ID (SGID) bit set; if yes, then the condition becomes true.    [ -g $file ] is false.
-k file     Checks if file has its sticky bit set; if yes, then the condition becomes true.     [ -k $file ] is false.
-p file     Checks if file is a named pipe; if yes, then the condition becomes true.    [ -p $file ] is false.
-t file     Checks if file descriptor is open and associated with a terminal; if yes, then the condition becomes true.  [ -t $file ] is false.
-u file     Checks if file has its Set User ID (SUID) bit set; if yes, then the condition becomes true.     [ -u $file ] is false.
-r file     Checks if file is readable; if yes, then the condition becomes true.    [ -r $file ] is true.
-w file     Checks if file is writable; if yes, then the condition becomes true.    [ -w $file ] is true.
-x file     Checks if file is executable; if yes, then the condition becomes true.  [ -x $file ] is true.
-s file     Checks if file has size greater than 0; if yes, then condition becomes true.    [ -s $file ] is true.
-e file     Checks if file exists; is true even if file is a directory but exists.  [ -e $file ] is true.

Answer 3

man test或man [将为您提供测试命令的所有选项。在这种情况下，-n 正在测试以查看 $abc 的内容是否具有非零长度，而 -z 正在测试以查看 $xyz 的内容是否是零长度字符串。