Master Fast Log Error Detection with Tail, Grep, and Sed Commands

Quick Log Viewing

Use tail -f catalina.out to continuously monitor a log file as new entries are written.

Open a log from the beginning with cat catalina.out.

Basic Tail/Head Commands

tail -n number catalina.out

– display the last number lines. tail -n +number catalina.out – show all lines starting from line number . head -n number catalina.out – display the first number lines. head -n -number catalina.out – display all lines except the last number lines.

Method 1 – Find Line Numbers by Keyword

First locate the line numbers containing a keyword, then view surrounding lines:

cat -n catalina.out | grep "keyword"
# Example output shows line numbers like 13230539

Combine the line number with tail and head to extract a specific range:

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

Explanation: tail -n +13230539 – start from line 13,230,539. head -n 10 – then keep only the first 10 lines of that result.

Method 2 – Query Logs Within a Time Range

First verify that the timestamps exist in the current log file, then use grep or sed to extract the interval:

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

Extract the block between the two timestamps with sed:

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

Method 3 – Count Occurrences of a Specific String

Count how many times a pattern appears:

grep '1175109632' catalina.out | wc -l
# Output: 154

Method 4 – Search the Last N Lines for a Keyword

Combine tail with grep to find a keyword in the most recent lines:

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

Method 5 – Highlight Keyword in the Last N Lines

Use --color to color‑highlight matches:

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

Method 6 – Show Context Around a Match

Display two lines of context before and after each match:

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

Method 7 – Paginate Results with more or less

Pipe the filtered output to a pager for easier navigation:

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

Navigation Shortcuts in less

Ctrl+F

– move forward one screen. Ctrl+B – move backward one screen. Ctrl+D – move forward half a screen. Ctrl+U – move backward half a screen. j – move forward one line. k – move backward one line. G – jump to the end of the file. g – jump to the beginning of the file. q or ZZ – exit the pager.

These commands enable efficient navigation and inspection of large log files, helping developers quickly pinpoint errors and understand system behavior.