Master Log Analysis: Fast Linux Commands to Pinpoint Errors

Why Log Analysis Matters

Viewing server and application logs is a fundamental skill for developers because logs are the most reliable way to identify where errors occur. When a server crashes, knowing how to pinpoint the problem can save you from taking the blame.

Quick Ways to View Logs Dynamically

tail -f catalina.out

Continuously follows the end of the log file. cat catalina.out Displays the entire log from the beginning.

You can redirect output to a new file for focused inspection:

cat -n catalina.out | grep "717892466" > nanjiangtest.txt

Basic tail / head Commands

# Show the last <em>number</em> lines
tail -n number catalina.out
# Show from line <em>number</em> to the end
tail -n +number catalina.out
# Show the first <em>number</em> lines
head -n number catalina.out
# Show all lines except the last <em>number</em>
head -n -number catalina.out

Method 1 – Find Line Numbers with grep

First obtain the line numbers containing a keyword, then view surrounding lines: cat -n test.log | grep "keyword" Example output (line numbers highlighted):

# cat -n catalina.out | grep 717892466
13230539   [11:07 17:47:11] INFO nanjiang:Edit Old Article:717892466-2020-11-07 17:47:11
13230593   [11:07 17:47:15] INFO nanjiang Save Article ID IS:717892466
...

Then combine with tail and head to see a context window:

cat -n catalina.out | tail -n +13230539 | head -n 10

Method 2 – Search Within a Time Range

grep '11:07 18:29:20' catalina.out
grep '11:07 18:31:11' catalina.out

Or use sed to extract a block between two timestamps:

sed -n '/11:07 18:29:20/,/11:07 18:31:11/p' catalina.out

Method 3 – Count Occurrences of a Specific String

grep '1175109632' catalina.out | wc -l

Result:

154

Method 4 – Search the Last number Lines for a Keyword

tail -n 20 catalina.out | grep 'INFO Takes:1'

Method 5 – Highlight Matches with Color

tail -n 20 catalina.out | grep 'INFO Takes:1' --color

Method 6 – Show Matching Lines with Two Context Lines

tail -n 20 catalina.out | grep 'INFO Takes:1' --color -A2

Method 7 – Paginated Viewing with more / less

tail -n 2000 catalina.out | grep 'INFO Takes:1' --color -A2 | more
tail -n 2000 catalina.out | grep 'INFO Takes:1' --color -A2 | less

Additional Navigation Shortcuts (for less )

ctrl + F

– forward one screen ctrl + B – backward one screen ctrl + D – forward half screen ctrl + U – backward half screen j – move down one line k – move up one line G – go to the last line g – go to the first line q or ZZ – exit