我正在读 ls 的 man，最后它谈到了 ls 的退出状态。它说：
Exit status:
0 if OK,
1 if minor problems (e.g., cannot access subdirectory),
2 if serious trouble (e.g., cannot access command-line argument).
但问题是我不明白他们的意思：
无法访问命令行参数
我从未遇到过无法访问传递给程序的参数的情况，并且我在网上搜索有关这种特定情况的信息，但除此之外我无法获得很多信息这个网站 这对我来说并不是很清楚，而且我无法重现该错误。我不确定我是否错过了理解 MAN 页面
答案1
对于 GNU ls，请使用源 Luke：http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/ls.c;h=bf0c5941d7de699fc5a85d44461ef29192216d9d;hb=HEAD
你会发现很多情况下返回码都是2，有的很容易触发，如下图。
首先你可以读到：
802 /* Exit statuses. */
803 enum
804 {
805 /* "ls" had a minor problem. E.g., while processing a directory,
806 ls obtained the name of an entry via readdir, yet was later
807 unable to stat that name. This happens when listing a directory
808 in which entries are actively being removed or renamed. */
809 LS_MINOR_PROBLEM = 1,
810
811 /* "ls" had more serious trouble (e.g., memory exhausted, invalid
812 option or failure to stat a command line argument. */
813 LS_FAILURE = 2
814 };
因此您已经可以看到，值 2 涵盖的情况比文档中编写的情况更多。
然后，如果您在代码中进一步搜索，LS_FAILURE您会发现不同的情况：
情况1
1896 case 'w':
1897 if (! set_line_length (optarg))
1898 die (LS_FAILURE, 0, "%s: %s", _("invalid line width"),
1899 quote (optarg));
1900 break;
set_line_length将根据xstrtoumax给定宽度的返回方式做出反应。如果你仔细查看它的源代码，你可以得到一些边缘情况：
$ ls -w -1 >& /dev/null
$ echo $?
2
$ ls -w 1 >& /dev/null
$ echo $?
0
案例2
1964 case 'T':
1965 tabsize = xnumtoumax (optarg, 0, 0, SIZE_MAX, "",
1966 _("invalid tab size"), LS_FAILURE);
1967 break;
与之前的案例类似：
$ ls -T 1 >& /dev/null
$ echo $?
0
$ ls -T -1 >& /dev/null
$ echo $?
2
案例3
2106 default:
2107 usage (LS_FAILURE);
因此，如果您提供无效参数，这就是默认错误代码。看这个例子：
$ ls --unknown-option >& /dev/null
$ echo $?
2
案例4
2198 if (strchr (p1 + 1, '\n'))
2199 die (LS_FAILURE, 0, _("invalid time style format %s"),
2200 quote (p0));
当您提供无效的时间格式（有两个）时，就会发生这种情况\n：
$ ls -l --time-style=+%T >& /dev/null ; echo $?
0
$ ls -l --time-style=+%T$'\n' >& /dev/null ; echo $?
0
$ ls -l --time-style=+%T$'\n'%T >& /dev/null ; echo $?
0
$ ls -l --time-style=+%T$'\n'%T$'\n' >& /dev/null ; echo $?
2
案例5
2218 /* The following is a manual expansion of argmatch_valid,
2219 but with the added "+ ..." description and the [posix-]
2220 prefixes prepended. Note that this simplification works
2221 only because all four existing time_style_types values
2222 are distinct. */
2223 fputs (_("Valid arguments are:\n"), stderr);
2224 char const *const *p = time_style_args;
2225 while (*p)
2226 fprintf (stderr, " - [posix-]%s\n", *p++);
2227 fputs (_(" - +FORMAT (e.g., +%H:%M) for a 'date'-style"
2228 " format\n"), stderr);
2229 usage (LS_FAILURE);
使用无效的时间格式名称时触发：
$ LANG=C ls -l --time-style=whatever
ls: invalid argument 'whatever' for 'time style'
Valid arguments are:
- [posix-]full-iso
- [posix-]long-iso
- [posix-]iso
- [posix-]locale
- +FORMAT (e.g., +%H:%M) for a 'date'-style format
Try 'ls --help' for more information.
$ echo $?
2
案例6
2669 static void
2670 set_exit_status (bool serious)
2671 {
2672 if (serious)
2673 exit_status = LS_FAILURE;
2674 else if (exit_status == EXIT_SUCCESS)
2675 exit_status = LS_MINOR_PROBLEM;
2676 }
这种情况（严重 = true）可能会在多种情况下发生，例如，如果某处存在循环：
2747 /* If we've already visited this dev/inode pair, warn that
2748 we've found a loop, and do not process this directory. */
2749 if (visit_dir (dir_stat.st_dev, dir_stat.st_ino))
2750 {
2751 error (0, 0, _("%s: not listing already-listed directory"),
2752 quotef (name));
2753 closedir (dirp);
2754 set_exit_status (true);
2755 return;
2756 }
根据争论，这种情况也可能发生在许多其他情况下。
file_failure第一个参数是传递给的布尔值set_exit_status
子情况A
2710 /* Read directory NAME, and list the files in it.
2711 If REALNAME is nonzero, print its name instead of NAME;
2712 this is used for symbolic links to directories.
2713 COMMAND_LINE_ARG means this directory was mentioned on the command line. */
...
2725 if (!dirp)
2726 {
2727 file_failure (command_line_arg, _("cannot open directory %s"), name);
2728 return;
2729 }
例如：
$ ls /thatDOESnotEXIST >& /dev/null
$ echo $?
2
子情况B
2736 /* If dirfd failed, endure the overhead of using stat. */
2737 if ((0 <= fd
2738 ? fstat (fd, &dir_stat)
2739 : stat (name, &dir_stat)) < 0)
2740 {
2741 file_failure (command_line_arg,
2742 _("cannot determine device and inode of %s"), name);
这是某种无法访问的目录（例如远程目录）。
子情况C
2771 if (print_hyperlink)
2772 {
2773 absolute_name = canonicalize_filename_mode (name, CAN_MISSING);
2774 if (! absolute_name)
2775 file_failure (command_line_arg,
2776 _("error canonicalizing %s"), name);
或者
3189 if (print_hyperlink)
3190 {
3191 f->absolute_name = canonicalize_filename_mode (full_name,
3192 CAN_MISSING);
3193 if (! f->absolute_name)
3194 file_failure (command_line_arg,
3195 _("error canonicalizing %s"), full_name);
或者
3450 static void
3451 get_link_name (char const *filename, struct fileinfo *f, bool command_line_arg)
3452 {
3453 f->linkname = areadlink_with_size (filename, f->stat.st_size);
3454 if (f->linkname == NULL)
3455 file_failure (command_line_arg, _("cannot read symbolic link %s"),
3456 filename);
3457 }
这些是某种损坏的硬/软链接。
子情况D
2836 else if (errno != 0)
2837 {
2838 file_failure (command_line_arg, _("reading directory %s"), name);
或者
2851 if (closedir (dirp) != 0)
2852 {
2853 file_failure (command_line_arg, _("closing directory %s"), name);
另一种情况是无法读取目录内容（如果在命令行上提供）
子情况E
3235 if (err != 0)
3236 {
3237 /* Failure to stat a command line argument leads to
3238 an exit status of 2. For other files, stat failure
3239 provokes an exit status of 1. */
3240 file_failure (command_line_arg,
3241 _("cannot access %s"), full_name);
当尝试匹配文件时会发生这种情况，例如：
$ ls '*DOESnotEXIST*' >& /dev/null
$ echo $?
2
答案2
是的，man ls（对于 GNU ls）包含：
2 如果出现严重问题（例如，无法访问命令行参数）。
也许，这个词access应该读作stat。由于不存在而无法进行 stat ( ) 的参数（文件）stat filename将生成错误代码2。
简而言之，对于 GNU ls：命令如下：
ls NONE_existent_file # will result in exit 2
尝试：
ls nofile
ls: cannot access nofile: No such file or directory
echo $?
2
或者尝试：
$ ls $(date); echo "exit status: $?"
ls: cannot access 'Wed': No such file or directory
ls: cannot access 'Oct': No such file or directory
ls: cannot access '24': No such file or directory
ls: cannot access '02:42:02': No such file or directory
ls: cannot access 'UTC': No such file or directory
ls: cannot access '2018': No such file or directory
exit status: 2