bash 中的各种标志

Question

我多次看到诸如

if [-n ${Parameter}]

或

echo -n <string>

之类的标志。有时，我看到过类似

-r

等标志的使用。但是，我不知道如何在网络上搜索这些标志的含义。有人可以给我发送一些链接，让我可以理解它们的含义，或者一些关于我应该如何在谷歌上搜索它们的一般说明吗？谢谢！

Answer 1

您当然可以在互联网上查看文档https://ss64.com/bash/if.html

或者，如果您可以访问 Linux 或 Mac 计算机，只需查看已安装的文档即可！

例如尝试

man if

。

此外，

man

可以有多个页面用于同一查询，例如

man open

将在我的机器上显示openvt的手册，并且是命令行可执行文件。但是写

man 2 open

会给你C open函数的手册。 所以默认情况下，man 会给你 bash/命令行手册，然后是 C 函数。 所以

man open

可以写成

man 1 open

如果您无法访问互联网或者您想要使用的工具版本与“正常”版本不同，这非常有用。例如，我认为 sed 与 linux 和 mac 不同。所以他们有不同的手册。

当然有一个男人的男人...:)

man man

我忘了谈论帮助，大多数（和体面的）程序都有 -h or/and --help。大多数时候，手册页会显示更多信息。

Answer 2

更直接的答案：

 -a    file exists  This is identical in effect to -e. It has been "deprecated," [1] and its use is discouraged.
 -b    file is a block device
 -c    file is a character device  [ -b "/dev/sda2" ]
 -d    file is a directory
 -e    file exists
 -f    file is a regular file (not a directory or device file)
 -G    group-id of file same as yours
 -g    set-group-id (sgid) flag set on file or directory
          If a directory has the sgid flag set, then a file created within that directory belongs to the group
          that owns the directory, not necessarily to the group of the user who created the file.
 -h    file is a symbolic link
 -k    sticky bit1 set
 -L    file is a symbolic link
 -N    file modified since it was last read
 -O    you are owner of file
 -p    file is a pipe  [ -p /dev/fd/0 ]
 -r    file has read permission (for the user running the test)
 -S    file is a socket
 -s    file is not zero size
 -t    file (descriptor) is associated with a terminal device.
 -u    set-user-id2 (suid) flag set on file
 -w    file has write permission (for the user running the test)
 -x    file has execute permission (for the user running the test)

 -nt   file f1 is newer than f2   f1 -nt f2
 -ot   file f1 is older than f2   f1 -ot f2
 -ef   files f1 and f2 are hard links to the same file   f1 -ef f2
    
 !     "not" -- reverses the sense of the tests above (returns true if condition absent).

参考：https://ss64.com/bash/syntax-file-operators.html