使用历史命令：
history，打完以后前面会有顺序号的比如
1 cd
2 ls
3 pwd
如果需要重新执行cd命令则可以执行 !3 命令
命令补全功能，比如你要执行history命令，可以打上histo+键
命令帮助信息、man
man history
例如查 wait 这个命令
[root@jcwkyl /]# man -k wait
部分输出如下：
wait (1p) - await process completion
wait (2) - wait for process to change state
wait (3p) - wait for a child process to stop or terminate
wait3 [wait4] (2) - wait for process to change state, BSD style
wait4 (2) - wait for process to change state, BSD style
wait [builtins ] (1) - bash built-in commands, see bash(1)
waitid (3p) - wait for a child process to change state
waitpid [wait] (2) - wait for process to change state
waitpid [wait] (3p) - wait for a child process to stop or terminate
上述命令 man –k 等同于 apropos 命令。会看到输出的第行分三部分，最中间的部分就表示这个命令在哪一类。例如：
wait (2) - wait for process to change state
就表示这个一个系统调用。因为分类 2 是系统调用的分类。
man 文档分为 9 大类 ：
Section The human readable name
1 User commands that may be started by everyone.
2 System calls, that is, functions provided by the kernel.
3 Subroutines, that is, library functions.
4 Devices, that is, special files in the /dev directory.
5 File format descriptions, e.g. /etc/passwd .
6 Games, self-explanatory.
7 Miscellaneous, e.g. macro packages, conventions.
8 System administration tools that only root can execute.
9 Another (Linux specific) place for kernel routine documentation.
n (Deprecated) New documentation, that may be moved to a more appropriate section.
o (Deprecated) Old documentation, that may be kept for a grace period.
l (Deprecated) Local documentation referring to this particular system.
了解这些，在看一个命令的 man 文档的 see also 部分时就能猜到这个命令大概是哪一类的，是一个 bash 命令，还是一个系统调用，还是其它，等等，心里会有个数。
曾经自己在 linux 下写一个 socket 程序，因为记不太清楚 sockaddr_in 数据结构的定义，但是又 man 不到，最后在 linuxforum 论坛上找到了答案：应该用 man 7 ip 来看。