Linux Survival Basics
Read-only, can be slowFind the Exact Log Line Before You Scroll
A log file contains likely failures, but you need line numbers so you can inspect context around the match.
Command
grep -inE 'error|failed|denied|timeout' /var/log/nginx/error.logBefore you run this
System impact: Read-only. Can create load on large logs, directories, filesystems, or process tables.
When not to use it: Do not assume these four words catch every failure; applications often log custom error language.
Expected output
Matching log lines prefixed by their line numbers.
System impact
Read-only, can be slow. Nothing changes. The command prints matching lines with line numbers for follow-up inspection.
May require elevated permissions on protected paths or service-owned files.
Scope this to the smallest useful path or service on busy systems.
Recovery / rollback: no state is changed.
When to use it
Use this when a first-pass grep finds failures and you need the surrounding context next.
When not to use it
Do not assume these four words catch every failure; applications often log custom error language.
Watch this command run
Command transcript
This sanitized transcript shows the commands and output shape without exposing host details.
$ grep -inE 'error|failed|denied|timeout' /var/log/nginx/error.log
2:2026/06/25 10:01:42 [error] 11#11: *7 connect() failed (111: Connection refused) while connecting to upstream
4:2026/06/25 10:03:16 [error] 11#11: *9 access forbidden by rule, client: 203.0.113.44
5:2026/06/25 10:04:33 [error] 11#11: *10 upstream timed out while reading response header$ grep -inE 'error|failed|denied|timeout' /var/log/nginx/error.log | tail -20
2:2026/06/25 10:01:42 [error] 11#11: *7 connect() failed (111: Connection refused) while connecting to upstream
4:2026/06/25 10:03:16 [error] 11#11: *9 access forbidden by rule, client: 203.0.113.44
5:2026/06/25 10:04:33 [error] 11#11: *10 upstream timed out while reading response header$ sed -n '2,5p' /var/log/nginx/error.log
2026/06/25 10:01:42 [error] 11#11: *7 connect() failed (111: Connection refused) while connecting to upstream
2026/06/25 10:02:05 [warn] 11#11: upstream server temporarily disabled
2026/06/25 10:03:16 [error] 11#11: *9 access forbidden by rule, client: 203.0.113.44
2026/06/25 10:04:33 [error] 11#11: *10 upstream timed out while reading response headerView commands shown
These are the commands shown in the sanitized transcript.
Commands shown
grep -inE 'error|failed|denied|timeout' /var/log/nginx/error.loggrep -inE 'error|failed|denied|timeout' /var/log/nginx/error.log | tail -20sed -n '2,5p' /var/log/nginx/error.log
next steps
Related commands
Find Errors Before Reading Every Log Line
The error was in the log. The problem was finding it without reading noise.
grep -iE 'error|failed|denied|timeout' /var/log/nginx/error.log | tail -40
Show Only Recent Errors
The log had old failures too. I only cared about the newest ones.
grep -iE 'error|failed|denied|timeout' /var/log/nginx/error.log | tail -10
Watch Logs Without Opening the Whole File
The app was failing now. Opening a giant log file was the wrong move.
tail -n 80 -f /var/log/nginx/error.log
Find Warnings in Apt Terminal Logs
The package installed, but the terminal log may still contain the warning that matters.
grep -Ei 'warning|error|failed|dpkg' /var/log/apt/term.log
Find Which Package Owns a File
That binary came from somewhere. dpkg can tell you where.
dpkg-query -S /usr/sbin/nginx
Study mapping
Use this as independent command practice: read the notes, predict the output, then compare it with the example before using a real shell.
Useful for
- LPIC-1 style command-line practice
- LFCS style performance tasks
- Linux+ style troubleshooting review
Independent study support only. No affiliation, endorsement, exam dumps, or real exam questions.