The first one: view the log of real-time changes

most commonly used:

tail -f app.log (the last 10 lines by default, equivalent to adding parameter -n 10)
tail -200f app.log (the last 200 lines, pushed forward at a certain moment)

Ctrl+c is to exit the tail command

Other cases:
tail -n 20 app.log (displays the last 20 lines of app.log)
tail -n +5 app.log (display files starting from line 5)
The second method: search for logs near keywords

The most commonly used: cat -n filename |grep “keywords”

Other cases:

cat app.log | grep -C 5 ‘Keywords’ (Display the line matching the string in the log and 5 lines before and after)

cat app.log | grep -B 5 ‘Keywords’ (display the matching string and the first 5 lines)

cat app.log | grep -A 5 ‘Keywords’ (display the matching string and the last 5 lines)
The third type: Enter the editor to find: vi (vim)

1. Enter vim editing mode: vi app.log
2. Enter “/keyword” and press enter to search
3. To find the next one, press “n” (n to find the previous error log N to find the next error log)

4.

ctrl+f: Scroll down one screen.

ctrl+b: Turn up one screen.

ctrl+d: scroll down half a screen.

ctrl+u: Turn up half a screen.

ctrl+e: Scroll down one line.

ctrl+y: Scroll up one line.

5, ctrl+o to return to the search place

Exit: After pressing the ESC key, and then enter the : number, vi will wait for us to enter the command at the bottom of the screen

wq! Save and exit;

q! Exit without saving;

Other cases:

/keyword Note: forward search, press n key to move the cursor to the next qualified place
?Keyword Note: For reverse search, press shift+n key to move the cursor to the next qualified one

The fourth type: View all keyword-related logs (more is similar to vi)

more app.log opens all by default, from front to back

Commonly used are H (get help information), Enter (roll down a line), space (scroll down one screen), Q (quit command)

1. more app.log | grep ‘keyword’
2. Press the Space key: display the content of the next screen of the text
3. Press the B key: display the content of the previous screen;
4. Press the H key: display the help screen, and there are relevant help information on the screen;
   The fifth is to view according to time

 cat xxx.log |sed -n '/2020-10-24 22:16:21/,/2020-10-24 22:16:59/p' abc.log