Common commands for viewing logs in Linux

Common commands for viewing logs in Linux
1. Common command for viewing logs: tail:-n indicates the display line number; equivalent to nl command; example: tail-100f test. log monitors 100 lines of logs in real time.

Tail-n 10 test. log query the logs of the last 10 lines at the end of the log;

Tail-n + 10 test. log query all logs after 10 rows;

Head:

This is the opposite of tail. tail is the number of lines of logs after reading. The example is as follows:

Head-n 10 test. log query the first 10 lines of logs in the log file;

Head-n-10 test. log query all logs except the last 10 lines in the log file;

Cat:

Tac is a reverse query of cat words. The example is as follows:

Cat-n test. log | grep "debug" query the keyword log

2. Application Scenario 1: View by row number --- filter logs near keywords

1) cat-n test. log | grep "debug" gets the line number of the key log

2) cat-n test. log | tail-n + 92 | head-n 20 select the middle line of the keyword. Then, view the logs of the first 10 rows and the last 10 rows of the Keyword:

Tail-n + 92 indicates querying logs after 92 rows

Head-n 20 indicates that the first 20 records are queried in the previous query results.

3. Application Scenario 2: Query logs by date

Sed-n'/16:17:20/,/16:17:36/P' test. log

Note: The preceding two dates must be printed logs; otherwise, they are invalid;

Grep '2017-12-17 16:17:20 'test. log to check whether the time point exists in the log.

4. Application Scenario 3: log Content is too large to be viewed on the screen.

(1) Use the more and less commands,

For example, cat-n test. log | grep "debug" | more is printed by page. Click the Space key to flip the page.

(2) Use> xxx.txt to save it to a file, and then you can pull the file for analysis.

For example, cat-n test. log | grep "debug"> debug.txt