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Log, Configuration, and Diff Troubleshooting

This note collects small shell patterns for inspecting logs, comparing configuration files, and narrowing failures.

Start with Recent Evidence

tail -n 100 service.log
grep -n "ERROR" service.log | tail -n 20
grep -n "WARN" service.log | tail -n 20

This gives a quick view of recent activity and the latest error locations.

Extract a Context Window

When a line number matters:

nl -ba service.log | sed -n '240,280p'

When a pattern matters:

grep -n -C 3 "connection refused" service.log

Use -C for surrounding context, -A for lines after a match, and -B for lines before a match.

Compare Configuration Files

diff -u app.conf app.conf.new

Look for:

  • changed ports or addresses
  • changed file paths
  • changed feature flags
  • missing comments that documented assumptions
  • whitespace-only changes in sensitive formats

For directory comparisons:

diff -ru config.before config.after

Confirm File Type and Permissions

ls -l app.conf
stat app.conf
file app.conf

This catches simple problems:

  • wrong owner
  • unexpected permissions
  • symlink surprises
  • binary or encoded content where plain text was expected

Build a Small Report

{
  date
  hostname
  echo "Recent errors:"
  grep -n "ERROR" service.log | tail -n 20
  echo
  echo "Config diff:"
  diff -u app.conf app.conf.new
} > troubleshooting-report.txt

The useful habit is to keep evidence, commands, and conclusions close together.

Safety Notes

  • Inspect before editing.
  • Copy or version-control important config before changing it.
  • Prefer diff -u over memory when reviewing changes.
  • Avoid pasting private hostnames, IPs, tokens, or logs into public notes.