What is the linux log file?

In Linux, log files refer to "log files", which record important events including user login information, system startup and security information, service information, etc. How to view the log file: 1. Use the "tail -n number of lines log file name" command; 2. Use the "head -n number of lines log file name" command, etc.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

In Linux, log file refers to "log file", which is an important system information file, which records many important system events, including user login information, system startup information, and system security information. , email-related information, various service-related information, etc. Some of this information is very sensitive, so in Linux these log files can only be read by the root user.

log files are stored in the /var/log/ directory, which is the storage location for system log files; in addition, system services installed using RPM packages will also be installed by default Record the log in the /var/log/ directory (the service log installed by the source code package is stored in the directory specified by the source code package).

Linux log file description

/var/log/message 系统启动后的信息和错误日志，是Red Hat Linux中最常用的日志之一 /var/log/secure 与安全相关的日志信息 /var/log/maillog 与邮件相关的日志信息 /var/log/cron 与定时任务相关的日志信息 /var/log/spooler 与UUCP和news设备相关的日志信息 /var/log/boot.log 守护进程启动和停止相关的日志消息 /var/log/wtmp 该日志文件永久记录每个用户登录、注销及系统的启动、停机的事件 How to view the log under Linux?

There are many commands to view logs in Linux: tail, cat, tac, head, echo, etc. Here are some commonly used methods.

1, tail

This is my most commonly used viewing method

命令格式: tail[必要参数][选择参数][文件] -f 循环读取 -q 不显示处理信息 -v 显示详细的处理信息 -c3614180660cacc92788ac4f21b0f27ec 显示的字节数 -n1dcc53fc8cc7b2478796660caa67152b 显示行数 -q, --quiet, --silent 从不输出给出文件名的首部 -s, --sleep-interval=S 与-f合用,表示在每次反复的间隔休眠S秒 The usage is as follows:

tail -n 10 test.log 查询日志尾部最后10行的日志; tail -n +10 test.log 查询10行之后的所有日志; tail -fn 10 test.log 循环实时查看最后1000行记录(最常用的) Generally still It will be used in conjunction with grep, for example:

tail -fn 1000 test.log | grep '关键字' If the amount of data queried at one time is too large, you can turn pages to view, for example:

tail -n 4700 aa.log |more -1000 可以进行多屏显示(ctrl + f 或者 空格键可以快捷键) 2, head

It is the opposite of tail. Head is to look at the previous rows of logs.

head -n 10 test.log 查询日志文件中的头10行日志; head -n -10 test.log 查询日志文件除了最后10行的其他所有日志; For other parameters of head, please refer to tail

3, cat

cat is displayed continuously on the screen from the first line to the last line

Display the entire file at once:

cat filename Create a file from the keyboard:

cat > filename Merge several files For a file:

cat file1 file2 > file 只能创建新文件,不能编辑已有文件. Append the contents of one log file to another:

cat -n textfile1 > textfile2 Clear a log file:

cat : >textfile2 Note: > means create, >> means append. Don't get confused.

cat Other parameters reference tail

4、more

more## The # command is a text filter based on the vi editor. It displays the content of a text file on a page-by-page basis in full-screen mode, and supports keyword positioning operations in vi. more There are several shortcut keys built into the list. Commonly used ones include H (get help information), Enter (scroll down one line), space (scroll down one screen), Q (Quit command). The more command reads the file from front to back, so the entire file is loaded on startup.

This command displays one screen of text at a time, stops when the screen is full, and a prompt message appears at the bottom of the screen, giving the percentage of the file that has been displayed so far: –More–(XX% )

more syntax: more File name

Enter Down n Line, needs to be defined, the default is 1 line

Ctrl f Scroll down one screen

Space bar scroll down one screen

Ctrl b Return to the previous screen

= Output the line number of the current line

:f Output the file name and line number of the current line

v Call the vi editor

! command to call Shell and execute the command

q Exit more

##5. sed This command can search for a specific section of the log file, based on Query a range of time, you can query according to the line number and time range

According to the line number

sed -n '5,10p' filename 这样你就可以只查看文件的第5行到第10行。 According to the time period

sed -n '/2014-12-17 16:17:20/,/2014-12-17 16:17:36/p' test.log

6, less When the less command queries logs, the general process is like this

less log.log

shift + G 命令到文件尾部 然后输入 ？加上你要搜索的关键字例如 ？1213

按 n 向上查找关键字

shift+n 反向查找关键字 less与more类似，使用less可以随意浏览文件，而more仅能向前移动，不能向后移动，而且 less 在查看之前不会加载整个文件。 less log2013.log 查看文件 ps -ef | less ps查看进程信息并通过less分页显示 history | less 查看命令历史使用记录并通过less分页显示 less log2013.log log2014.log 浏览多个文件 Commonly used command parameters:

less与more类似，使用less可以随意浏览文件，而more仅能向前移动，不能向后移动，而且 less 在查看之前不会加载整个文件。 less log2013.log 查看文件 ps -ef | less ps查看进程信息并通过less分页显示 history | less 查看命令历史使用记录并通过less分页显示 less log2013.log log2014.log 浏览多个文件 常用命令参数： -b 3d31a9198ac4151a99e626fc6f77157a 设置缓冲区的大小 -g 只标志最后搜索的关键词 -i 忽略搜索时的大小写 -m 显示类似more命令的百分比 -N 显示每行的行号 -o 479d34ed3e826e8b14bd2c55e47269c5 将less 输出的内容在指定文件中保存起来 -Q 不使用警告音 -s 显示连续空行为一行 /字符串：向下搜索"字符串"的功能 ?字符串：向上搜索"字符串"的功能 n：重复前一个搜索（与 / 或 ? 有关） N：反向重复前一个搜索（与 / 或 ? 有关） b 向后翻一页 h 显示帮助界面 q 退出less 命令

Generally, I check the logs in conjunction with other commands used history // 所有的历史记录

history | grep XXX // 历史记录中包含某些指令的记录

history | more // 分页查看记录

history -c // 清空所有的历史记录

!! 重复执行上一个命令

查询出来记录后选中 : !323 Related recommendations: "

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