bash - 解释这个 bash 脚本

Question

谁能告诉我这个脚本会做什么？第 3 行的 -z 是什么？

  Filename=File.txt
  X=`ls /home/$Filename`
  if [ -z "$X" ]; then
  exit
  fi

Answer 1

Filename=File.txt

该$Filename变量现在具有值File.txt。

X=`ls /home/$Filename`

该$X变量现在包含命令的输出ls /home/File.txt。如果文件存在，它将包含一些东西，否则它将是一个空字符串。

if [ -z "$X" ]; then

如果变量的值$X是一个空字符串（意思是/home/File.txt不存在，否则$X会包含一些东西），那么：

  exit
fi

---

这是一种很尴尬的写法：

if ! [ -e "/home/File.txt" ]; then exit; fi

-e如果路径存在，则返回 true。您还可以检查文件 ( -f)、目录 ( -d)、符号链接 ( -L) 等。查看man [更多可与[. 您还可以-z在那里找到：

-z string 如果 的长度string为零，则为真。

Answer 2

脚本的作用是

创建一个名为 Filename 的变量 Filename 的值是 File.txt 执行命令 ls File.txt 将结果分配给变量 X 然后检查变量 X 中值的长度是否为零 如果它为零脚本将退出 所以基本上你的程序所做的是，它检查文件是否存在

而不是所有这些行，你可以使用

if [ -a "$Filename"]

此链接将对您有所帮助

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html

Answer 3

它测试是否/home/File.txt存在，如果不存在，则exit. 但这不是一个理想的解决方案，最好这样写：

if ! test -e /home/File.txt; then exit; fi

---

$ help test
test: test [expr]
    Evaluate conditional expression.

    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 and numeric comparison operators as well.

    The behavior of test depends on the number of arguments.  Read the
    bash manual page for the complete specification.

    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.
      -v VAR     True if the shell variable VAR is set
      ! 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.

    Exit Status:
    Returns success if EXPR evaluates to true; fails if EXPR evaluates to
    false or an invalid argument is given.