head
显示文件内容前面多少字节-c或者多少行-n的内容
[02:47:35 root@C8-3-55 ~]#head --help
用法：head [选项]... [文件]...
Print the first 10 lines of each FILE to standard output.
With more than one FILE, precede each with a header giving the file name.
如果没有指定文件，或者文件为"-"，则从标准输入读取。
必选参数对长短选项同时适用。
-c, --bytes=[-]NUM print the first NUM bytes of each file;
with the leading '-', print all but the last
NUM bytes of each file
-n, --lines=[-]NUM print the first NUM lines instead of the first 10;
with the leading '-', print all but the last
NUM lines of each file
-q, --quiet, --silent 不显示包含给定文件名的文件头
-v, --verbose 总是显示包含给定文件名的文件头
-z, --zero-terminated 以 NUL 字符而非换行符作为行尾分隔符
--help 显示此帮助信息并退出
--version 显示版本信息并退出
[02:50:02 root@C8-3-55 ~]#head -n 3 /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
如果参数为负，则去尾
例：-3表示从头开始显示到倒数第n-3行
[02:59:27 root@C8-3-55 ~]#head -n -3 ./f1.txt
1
2
3
4
5
6
7
tail
显示文件内容尾部多少字节或多少行的内容
[02:51:39 root@C8-3-55 ~]#tail --help
用法：tail [选项]... [文件]...
Print the last 10 lines of each FILE to standard output.
With more than one FILE, precede each with a header giving the file name.
如果没有指定文件，或者文件为"-"，则从标准输入读取。
必选参数对长短选项同时适用。
-c, --bytes=[+]NUM output the last NUM bytes; or use -c +NUM to
output starting with byte NUM of each file
-f, --follow[={name|descriptor}]
output appended data as the file grows;
an absent option argument means 'descriptor'
-F same as --follow=name --retry
-n, --lines=[+]NUM output the last NUM lines, instead of the last 10;
or use -n +NUM to output starting with line NUM
--max-unchanged-stats=N
with --follow=name, reopen a FILE which has not
changed size after N (default 5) iterations
to see if it has been unlinked or renamed
(this is the usual case of rotated log files);
with inotify, this option is rarely useful
--pid=PID with -f, terminate after process ID, PID dies
-q, --quiet, --silent never output headers giving file names
--retry keep trying to open a file if it is inaccessible
-s, --sleep-interval=N with -f, sleep for approximately N seconds
(default 1.0) between iterations;
with inotify and --pid=P, check process P at
least once every N seconds
-v, --verbose always output headers giving file names
-z, --zero-terminated 以 NUL 字符而非换行符作为行尾分隔符
--help 显示此帮助信息并退出
--version 显示版本信息并退出
[02:52:14 root@C8-3-55 ~]#tail -n 3 /etc/passwd
user98:x:8988:8988::/home/user98:/bin/bash
user99:x:8989:8989::/home/user99:/bin/bash
user100:x:8990:8990::/home/user100:/bin/bash
如果参数为添加了+号的数，则刨头
例：+3表示从尾开始显示到正数数第n-3行，或者从头第3行开始显示
[02:59:44 root@C8-3-55 ~]#tail -n -3 ./f1.txt
8
9
10
[03:03:54 root@C8-3-55 ~]#tail -n +3 ./f1.txt
3
4
5
6
7
8
9
10
tail的 -f选项
shell编程中tail比较常用的-f选项，可以跟踪文件的变化。
如果文件有追加内容，文件的尾部不断有新内容出现。
在生产中经常用来查看日志。
[03:04:40 root@C8-3-55 ~]#tail -f /var/log/messages
Mar 7 01:53:32 C8-3-55 systemd[1]: Starting Cleanup of Temporary Directories...
Mar 7 01:53:32 C8-3-55 systemd-tmpfiles[7254]: [/usr/lib/tmpfiles.d/mdadm.conf:1] Line references path below legacy directory /var/run/, updating /var/run/mdadm → /run/mdadm; please update the tmpfiles.d/ drop-in file accordingly.
Mar 7 01:53:32 C8-3-55 systemd[1]: Started Cleanup of Temporary Directories.
Mar 7 02:48:34 C8-3-55 systemd[1]: Starting dnf makecache...
Mar 7 02:48:34 C8-3-55 dnf[26938]: Repository AppStream is listed more than once in the configuration
Mar 7 02:48:34 C8-3-55 dnf[26938]: Repository extras is listed more than once in the configuration
Mar 7 02:48:34 C8-3-55 dnf[26938]: Repository PowerTools is listed more than once in the configuration
Mar 7 02:48:34 C8-3-55 dnf[26938]: Repository centosplus is listed more than once in the configuration
Mar 7 02:48:34 C8-3-55 dnf[26938]: 元数据缓存近期已刷新。
Mar 7 02:48:34 C8-3-55 systemd[1]: Started dnf makecache.
-f相当于跟踪了文件的描述符
查看日志和官方文档，是运维工作中解决问题的重要手段
head和tail应用实例
利用head和tail组合取出ens33配置文件中IP地址
[03:17:34 root@C8-3-55 ~]#ifconfig ens33
ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 10.0.0.55 netmask 255.0.0.0 broadcast 10.255.255.255
inet6 fe80::f7bf:c1dd:bbf2:865 prefixlen 64 scopeid 0x20<link>
ether 00:0c:29:c2:cc:4e txqueuelen 1000 (Ethernet)
RX packets 19645 bytes 1407622 (1.3 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 49644 bytes 5698480 (5.4 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
[03:18:55 root@C8-3-55 ~]#ifconfig ens33 | head -n 2 | tail -n 1
inet 10.0.0.55 netmask 255.0.0.0 broadcast 10.255.255.255
head和tail命令总结
