Shell Script 中 -n、-z、-x、-L、-d 等的含义是什么？

Question

我经常.sh在条件 sintaxis 中的文件Shell Script 中找到这样的：

if [ -n "condition" ]; then ...
if [ -z "condition "]; then ...
if [ -x "condition" ]; then ...
if [ -L "condition" ]; then ...
if [ -d "condition" ]; then ...

这些 -n, -z, -x, -L, -d是函数或者它是如何命名的，它的目的是什么？

Answer 1

恕我直言，最好的方法是您可以简单地man test处理所有这些细节。那里有很好的解释。以下是手册页的文本。对于 BASH 条件表达式，也请查找链接https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html一次。

   -b FILE
          FILE exists and is block special

   -c FILE
          FILE exists and is character special

   -d FILE
          FILE exists and is a directory

   -e FILE
          FILE exists

   -f FILE
          FILE exists and is a regular file

   -g FILE
          FILE exists and is set-group-ID

   -G FILE
          FILE exists and is owned by the effective group ID

   -h FILE
          FILE exists and is a symbolic link (same as -L)

   -k FILE
          FILE exists and has its sticky bit set

   -L FILE
          FILE exists and is a symbolic link (same as -h)

   -O FILE
          FILE exists and is owned by the effective user ID

   -p FILE
          FILE exists and is a named pipe

   -r FILE
          FILE exists and read permission is granted

   -s FILE
          FILE exists and has a size greater than zero

   -S FILE
          FILE exists and is a socket

   -t FD  file descriptor FD is opened on a terminal

   -u FILE
          FILE exists and its set-user-ID bit is set

   -w FILE
          FILE exists and write permission is granted

   -x FILE
          FILE exists and execute (or search) permission is granted

对于其中的表达式，man test给出：

   ( EXPRESSION )
          EXPRESSION is true

   ! EXPRESSION
          EXPRESSION is false

   EXPRESSION1 -a EXPRESSION2
          both EXPRESSION1 and EXPRESSION2 are true

   EXPRESSION1 -o EXPRESSION2
          either EXPRESSION1 or EXPRESSION2 is true

   -n STRING
          the length of STRING is nonzero

   STRING equivalent to -n STRING

   -z STRING
          the length of STRING is zero

   STRING1 = STRING2
          the strings are equal

   STRING1 != STRING2
          the strings are not equal

   INTEGER1 -eq INTEGER2
          INTEGER1 is equal to INTEGER2

   INTEGER1 -ge INTEGER2
          INTEGER1 is greater than or equal to INTEGER2

对于条件表达式信息，man bash它也提供如下信息。

条件表达式 [[ 复合命令和 test 和 [ 内置命令使用条件表达式来测试文件属性并执行字符串和算术比较。表达式由以下一元或二元原色组成。如果任何一个主文件参数的格式为 /dev/fd/n，则检查文件描述符 n。如果主要参数之一的文件参数是 /dev/stdin、/dev/stdout 或 /dev/stderr 之一，则分别检查文件描述符 0、1 或 2。

   Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather 

比链接本身。

   When used with [[, The < and > operators sort lexicographically using the current locale.