部分显示文件内容命令之head，tail，

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

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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

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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

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head和tail命令总结