linux - Shell 脚本中 -n、-z、-x、-L、-d 等的含义是什么？

Question

我经常在文件 .sh 中找到条件语句中的 Shell 脚本，如下所示:

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

那些-n、-z、-x、-L、-d是函数，或者它的名称如何以及它的用途是什么？

Answer 1

恕我直言，最好的方法是您可以简单地对所有这些细节进行人工测试。那里解释得很好。以下是手册页的文本。对于 BASH 条件表达式，请查找链接 https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html也有一次。

   -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 man test 中的表达式，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，请使用 man bash 它也提供如下信息。

CONDITIONAL EXPRESSIONS Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons. Expressions are formed from the following unary or binary primaries. If any file argument to one of the primaries is of the form /dev/fd/n, then file descriptor n is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

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

than the link itself.

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