man 文档的一些示例¶
man 文档非常详细。当然，详细的后果，就是我们在阅读的时候很容易变得一头雾水。必须承认的是，尽管 man 文档有一些有中文翻译（安装 manpages-zh），但是阅读它们，仍然是一件非常痛苦的事情。况且中文翻译有一些也已经十分老旧（例如，execve() 文档的最新翻译在 2003 年 5 月，而现在已经过去 17 年了），且有一些错误（例如，man 的文档中介绍 -a|-b 的含义的时候，将 "cannot be used together" 翻译为了「可以一起使用」）。下面会给出一些 man 文档以及其内容结构的解释，以帮助你更好地理解，man 文档到底在做什么长篇大论。
数字的含义¶
在查阅资料时，你常常会见到如 chmod(1) 或 mount(8) 这样的表达方式。正如上面所述，数字表示这个条目所属的章节。
但是为什么要加数字呢？这是因为不同的章节可能有相同的条目，例如 chmod 既是一条命令，又是一个系统调用。当你要查看 chmod 命令的 man 文档时，你会去看 chmod(1)；而当你要查看 chmod 系统调用的 man 文档时，你会去看 chmod(2)。这时候条目后面数字的作用就体现出来了，你可以放心的说 chmod(1) 内部使用了 chmod(2) 这样的话而不必担心造成困惑。
命令行工具：以 su 为例¶
SU(1) User Commands SU(1)
（第一行的标题是 User Commands，两边的 SU(1) 代表这篇文档是关于 `su` 的，在文档第一卷。
第一卷与 Shell 命令和程序有关。更多信息可以查看 man man 中 DESCRIPTION 一节的内容。）
NAME（名字，包含了我们的主角的名字 `su`，和它的简要介绍。）
su - run a command with substitute user and group ID
SYNOPSIS（概要，包含了命令参数的结构。）
su [options] [-] [user [argument...]]
（可能你会对这样的内容感到迷惑。但在概要中显示的内容格式有一个固定的格式模版。例如，
[-ab] 代表参数是可选的，而 -a|-b 代表这两个参数是互斥的。同样可以查看 man man
的介绍。）
DESCRIPTION（介绍，大致描述了它能够做什么事情）
su allows to run commands with a substitute user and group ID.
When called without arguments, su defaults to running an interactive
shell as root.
For backward compatibility, su defaults to not change the current di‐
rectory and to only set the environment variables HOME and SHELL (plus
USER and LOGNAME if the target user is not root). It is recommended to
always use the --login option (instead of its shortcut -) to avoid side
effects caused by mixing environments.
This version of su uses PAM for authentication, account and session
management. Some configuration options found in other su implementa‐
tions, such as support for a wheel group, have to be configured via
PAM.
su is mostly designed for unprivileged users, the recommended solution
for privileged users (e.g. scripts executed by root) is to use non-set-
user-ID command runuser(1) that does not require authentication and
provide separate PAM configuration. If the PAM session is not required
at all then the recommend solution is to use command setpriv(1).
（看完这段话之后，你可能会感到非常非常迷惑：PAM 是什么？最后一段到底在说啥？这种感觉是
很正常的。从文档的立场来讲，它不得不面面俱到，描述所有可能的情况，而有些情况你根本不会遇
到。
不过，就算我们忽略一些问题（先不管 PAM 是啥），我们仍然可以了解到很多信息，比如说：
- 推荐使用 `su -` 而不是 `su`，这可以防止（当前用户和切换目标用户不同的）环境混起来
带来问题。
- `root` 用户更推荐使用 `runuser` 代替 `su`，因为不需要认证为其他用户，且在 PAM
方面更好。）
OPTIONS（选项，包含了所有参数的语法和介绍）
-c, --command=command
Pass command to the shell with the -c option.
-f, --fast
Pass -f to the shell, which may or may not be useful, depending
on the shell.
-g, --group=group
Specify the primary group. This option is available to the root
user only.
-G, --supp-group=group
Specify a supplemental group. This option is available to the
root user only. The first specified supplementary group is also
used as a primary group if the option --group is unspecified.
-, -l, --login
Start the shell as a login shell with an environment similar to
a real login:
o clears all the environment variables except TERM and
variables specified by --whitelist-environment
o initializes the environment variables HOME, SHELL,
USER, LOGNAME, and PATH
o changes to the target user's home directory
o sets argv[0] of the shell to '-' in order to make the
shell a login shell
-m, -p, --preserve-environment
Preserve the entire environment, i.e. it does not set HOME,
SHELL, USER nor LOGNAME. This option is ignored if the option
--login is specified.
-P, --pty
Create pseudo-terminal for the session. The independent terminal
provides better security as user does not share terminal with
the original session. This allow to avoid TIOCSTI ioctl termi‐
nal injection and another security attacks against terminal file
descriptors. The all session is also possible to move to back‐
ground (e.g. "su --pty - username -c application &"). If the
pseudo-terminal is enabled then su command works as a proxy be‐
tween the sessions (copy stdin and stdout).
This feature is EXPERIMENTAL for now and may be removed in the
next releases.
-s, --shell=shell
Run the specified shell instead of the default. The shell to
run is selected according to the following rules, in order:
o the shell specified with --shell
o the shell specified in the environment variable SHELL,
if the --preserve-environment option is used
o the shell listed in the passwd entry of the target
user
o /bin/sh
If the target user has a restricted shell (i.e. not listed in
/etc/shells), the --shell option and the SHELL environment vari‐
ables are ignored unless the calling user is root.
--session-command=command
Same as -c but do not create a new session. (Discouraged.)
-w, --whitelist-environment=list
Don't reset environment variables specified in comma separated
list when clears environment for --login. The whitelist is ig‐
nored for the environment variables HOME, SHELL, USER, LOGNAME,
and PATH.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
（这里按照字典序介绍了所有的参数。对于想要详尽了解命令使用的人来说没有什么问题，而对于想
要快速找到自己想要的参数的人来说，阅读这样的文档确实会让人一头雾水。
你可以按下 /，然后输入搜索关键词快速搜索，按 n 查看下一个搜索项，按 N 查看上一个搜索项。
至于专有名词，你可能不得不去了解它们的含义。查看相关的文档，或者在互联网上搜索。）
SIGNALS（信号，信号的含义详见进程一章）
Upon receiving either SIGINT, SIGQUIT or SIGTERM, su terminates its
child and afterwards terminates itself with the received signal. The
child is terminated by SIGTERM, after unsuccessful attempt and 2 sec‐
onds of delay the child is killed by SIGKILL.
（你会注意到，不是所有程序手册都有这一节。不同的程序，文档的结构可能不一样。）
CONFIG FILES（配置文件）
su reads the /etc/default/su and /etc/login.defs configuration files.
The following configuration items are relevant for su(1):
FAIL_DELAY (number)
Delay in seconds in case of an authentication failure. The number
must be a non-negative integer.
ENV_PATH (string)
Defines the PATH environment variable for a regular user. The de‐
fault value is /usr/local/bin:/bin:/usr/bin.
ENV_ROOTPATH (string)
ENV_SUPATH (string)
Defines the PATH environment variable for root. The default value
is /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin.
ALWAYS_SET_PATH (boolean)
If set to yes and --login and --preserve-environment were not spec‐
ified su initializes PATH.
The environment variable PATH may be different on systems where /bin
and /sbin are merged into /usr.
EXIT STATUS（退出状态）
su normally returns the exit status of the command it executed. If the
command was killed by a signal, su returns the number of the signal
plus 128.
Exit status generated by su itself:
1 Generic error before executing the requested command
126 The requested command could not be executed
127 The requested command was not found
（退出状态是什么？还记得你写 C 程序时在 `main()` 里面最后的 `return 0;` 吗？0 就是
你的程序的退出状态。一般来说，0 代表程序运行成功，而非 0 值代表程序没有完成它预期的功能，
或者用户输入了错误的选项。
你可以输入 `echo $?` 查看上一条命令的退出状态。）
FILES（相关文件）
/etc/pam.d/su default PAM configuration file
/etc/pam.d/su-l PAM configuration file if --login is specified
/etc/default/su command specific logindef config file
/etc/login.defs global logindef config file
NOTES（附录）
For security reasons su always logs failed log-in attempts to the btmp
file, but it does not write to the lastlog file at all. This solution
allows to control su behavior by PAM configuration. If you want to use
the pam_lastlog module to print warning message about failed log-in at‐
tempts then the pam_lastlog has to be configured to update the lastlog
file as well. For example by:
session required pam_lastlog.so nowtmp
SEE ALSO（另寻，包含了一些相关的命令。可以用 `man 号码 名称` 的格式查看这些命令的文档。）
setpriv(1), login.defs(5), shells(5), pam(8), runuser(8)
HISTORY（历史记录）
This su command was derived from coreutils' su, which was based on an
implementation by David MacKenzie. The util-linux has been refactored
by Karel Zak.
AVAILABILITY（「可用性」，即，我可以在哪里获得这个程序？）
The su command is part of the util-linux package and is available from
Linux Kernel Archive ⟨https://www.kernel.org/pub/linux/utils/util-
linux/⟩.
util-linux July 2014 SU(1)
程序库函数：以 strcmp() 为例¶
文档第三卷是程序库函数的信息，包括了 C 语言的标准库函数。
STRCMP(3) Linux Programmer's Manual STRCMP(3)
NAME
strcmp, strncmp - compare two strings
SYNOPSIS（概要，这里包含了你需要 include 的头文件，以及相关函数的声明。）
#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
（这里也会有需要的编译和链接参数，当然对 `strcmp()` 来说是不需要这些的。）
DESCRIPTION
The strcmp() function compares the two strings s1 and s2. It returns
an integer less than, equal to, or greater than zero if s1 is found,
respectively, to be less than, to match, or be greater than s2.
The strncmp() function is similar, except it compares only the first
(at most) n bytes of s1 and s2.
RETURN VALUE（返回值，有些函数会包含返回后参数的情况）
The strcmp() and strncmp() functions return an integer less than, equal
to, or greater than zero if s1 (or the first n bytes thereof) is found,
respectively, to be less than, to match, or be greater than s2.
ATTRIBUTES（属性）
For an explanation of the terms used in this section, see at‐
tributes(7).
┌────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────┼───────────────┼─────────┤
│strcmp(), strncmp() │ Thread safety │ MT-Safe │
└────────────────────┴───────────────┴─────────┘
（从 man 7 attributes 中，可以得到此章会使用的专有名词的解释。这里我们可以看到，
`strcmp()` 系列函数是线程安全的。）
CONFORMING TO（符合的标准，可以看到熟悉的 POSIX 和 C89、C99 标准）
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
SEE ALSO（另寻，表明这些库函数也和 `strcmp()` 相关）
bcmp(3), memcmp(3), strcasecmp(3), strcoll(3), string(3), strn‐
casecmp(3), strverscmp(3), wcscmp(3), wcsncmp(3)
COLOPHON（作者信息、文档来源等信息）
This page is part of release 4.16 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
2015-08-08 STRCMP(3)
系统调用：以 kill() 为例¶
文档第二卷是关于系统调用的信息。当然，很多系统调用都由 C 运行时库包装了一层，否则用起来很麻烦。一个目前还没有被包装的系统调用的例子是 copy_file_range()，你需要在你的代码里面使用 syscall() 去手动包装它，才能方便地使用。
KILL(2) Linux Programmer's Manual KILL(2)
NAME
kill - send signal to a process
SYNOPSIS
#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
kill(): _POSIX_C_SOURCE
（这里的话，出现了 "Macro Requirements"，表明 kill() 函数需要在 include 文件之前
作宏定义 _POSIX_C_SOURCE 来正常使用。当然，很多 Macro 都是默认包含的，可以在
feature_test_macros 文档中的 Default definitions, implicit definitions,
and combining definitions 一小节中找到）
DESCRIPTION
The kill() system call can be used to send any signal to any process
group or process.
If pid is positive, then signal sig is sent to the process with the ID
specified by pid.
If pid equals 0, then sig is sent to every process in the process group
of the calling process.
If pid equals -1, then sig is sent to every process for which the call‐
ing process has permission to send signals, except for process 1
(init), but see below.
If pid is less than -1, then sig is sent to every process in the
process group whose ID is -pid.
If sig is 0, then no signal is sent, but existence and permission
checks are still performed; this can be used to check for the existence
of a process ID or process group ID that the caller is permitted to
signal.
For a process to have permission to send a signal, it must either be
privileged (under Linux: have the CAP_KILL capability in the user name‐
space of the target process), or the real or effective user ID of the
sending process must equal the real or saved set-user-ID of the target
process. In the case of SIGCONT, it suffices when the sending and re‐
ceiving processes belong to the same session. (Historically, the rules
were different; see NOTES.)
RETURN VALUE
On success (at least one signal was sent), zero is returned. On error,
-1 is returned, and errno is set appropriately.
（一般来说，系统调用出错时，会返回非 0 值，且设置全局的 errno 值。
不同的 errno 值对应了不同的错误，可以查看 errno 的文档。）
ERRORS（错误解释）
EINVAL An invalid signal was specified.
EPERM The process does not have permission to send the signal to any
of the target processes.
ESRCH The process or process group does not exist. Note that an ex‐
isting process might be a zombie, a process that has terminated
execution, but has not yet been wait(2)ed for.
（这些大写字符串代表了 errno 中不同的错误值。可以使用 `perror()` 输出系统的报错信息。）
CONFORMING TO（符合的标准，可以看到这里没有 C89 这类值了，因为 `kill()` 不是标准 C 库函数。）
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
NOTES
The only signals that can be sent to process ID 1, the init process,
are those for which init has explicitly installed signal handlers.
This is done to assure the system is not brought down accidentally.
POSIX.1 requires that kill(-1,sig) send sig to all processes that the
calling process may send signals to, except possibly for some implemen‐
tation-defined system processes. Linux allows a process to signal it‐
self, but on Linux the call kill(-1,sig) does not signal the calling
process.
POSIX.1 requires that if a process sends a signal to itself, and the
sending thread does not have the signal blocked, and no other thread
has it unblocked or is waiting for it in sigwait(3), at least one un‐
blocked signal must be delivered to the sending thread before the
kill() returns.
Linux notes
Across different kernel versions, Linux has enforced different rules
for the permissions required for an unprivileged process to send a sig‐
nal to another process. In kernels 1.0 to 1.2.2, a signal could be
sent if the effective user ID of the sender matched effective user ID
of the target, or the real user ID of the sender matched the real user
ID of the target. From kernel 1.2.3 until 1.3.77, a signal could be
sent if the effective user ID of the sender matched either the real or
effective user ID of the target. The current rules, which conform to
POSIX.1, were adopted in kernel 1.3.78.
（很多系统调用都是 POSIX 标准，但是在 Linux 上可能会有一些地方不太一样。）
BUGS（问题，你需要绕开这些坑）
In 2.6 kernels up to and including 2.6.7, there was a bug that meant
that when sending signals to a process group, kill() failed with the
error EPERM if the caller did not have permission to send the signal to
any (rather than all) of the members of the process group. Notwith‐
standing this error return, the signal was still delivered to all of
the processes for which the caller had permission to signal.
SEE ALSO
kill(1), _exit(2), signal(2), tkill(2), exit(3), killpg(3),
sigqueue(3), capabilities(7), credentials(7), signal(7)
COLOPHON
This page is part of release 4.16 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 KILL(2)