Linux

Follow log files in real-time

tail -f log.txt &

echo 'New log entry' >> log.txt

echo 'Another entry' >> log.txt

echo 'ERROR: Connection failed' >> log.txt

jobs

kill %1

What This Does

• tail -f follows file updates in real-time. New lines appear as they’re written. Essential for monitoring logs. & runs in background. kill %1 stops it.tail -f shows initial content, then new lines appear as you echo them. Simulates real-time log monitoring. kill stops the tail process.

Pro Tips

1. tail -f monitors logs continuously (Ctrl+C to stop)
2. & runs command in background
3. jobs shows background processes
4. kill %1 stops job #1
5. Use for: tail -f /var/log/syslog (system logs)
6. tail -F follows even if file is rotated/recreated

Common Mistakes to Avoid

• Forgetting Ctrl+C to exit tail -f (it runs forever)
• Not using -f for log monitoring (missing real-time updates)

Search file contents with grep

grep ERROR log.txt

grep berry fruits.txt

grep -i error log.txt

grep -n ERROR log.txt

grep -v ERROR log.txt

What This Does

• grep searches file contents for patterns. -i ignores case. -n shows line numbers. -v inverts match (show non-matching lines). Powerful text filtering tool.
• First finds ERROR line. Second finds lines with ‘berry’. -i finds ‘ERROR’ case-insensitive. -n shows line numbers. -v shows lines without ERROR.

Pro Tips

1. grep is case-sensitive by default
2. -i for case-insensitive search
3. -n shows line numbers
4. -v inverts match (show what doesn't match)
5. -r searches directories recursively
6. grep ‘pattern’ file.txt (quotes if pattern has spaces)

Common Mistakes to Avoid

• Forgetting -i and missing matches due to case
• Not quoting patterns with spaces or special characters

Find files by name

find . -name '*.txt'

find . -name 'log*'

find ~ -name 'fruits.txt'

find . -type f

find . -type d

What This Does

• find searches directory trees. . means current directory. -name matches filenames. -type f finds files. -type d finds directories. * wildcard needs quotes to prevent shell expansion.
• First finds all .txt files in current dir. Second finds files starting with ‘log’. Third searches home for fruits.txt. Fourth lists all files. Fifth lists all directories.

Pro Tips

1. find . starts search in current directory
2. find ~ searches entire home directory
3. -name is case-sensitive
4. -iname for case-insensitive
5. -type f = files only
6. -type d = directories only
7. ALWAYS quote patterns: ‘*.txt’ not *.txt

Common Mistakes to Avoid

• Not quoting wildcards: find . -name *.txt (wrong!)
• Starting find in / (searches entire filesystem, very slow)
• Forgetting . means current directory

Find files by name

find . -name '*.txt'

find . -name 'log*'

find ~ -name 'fruits.txt'

find . -type f

find . -type d

What This Does

• find searches directory trees. . means current directory. -name matches filenames. -type f finds files. -type d finds directories. * wildcard needs quotes to prevent shell expansion.
• First finds all .txt files in current dir. Second finds files starting with ‘log’. Third searches home for fruits.txt. Fourth lists all files. Fifth lists all directories.

Pro Tips

1. find . starts search in current directory
2. find ~ searches entire home directory
3. -name is case-sensitive
4. -iname for case-insensitive
5. -type f = files only
6. -type d = directories only
7. ALWAYS quote patterns: ‘*.txt’ not *.txt

Common Mistakes to Avoid

• Not quoting wildcards: find . -name *.txt (wrong!)
• Starting find in / (searches entire filesystem, very slow)
• Forgetting . means current directory

Combine grep with other commands

cat log.txt | grep ERROR

ls -l | grep txt

find . -name '*.txt' -exec grep ERROR {} \;

What This Does

• (pipe) sends output of one command to input of next. cat

  grep searches file contents. ls

  grep filters directory listings. find -exec runs command on each result.

• First finds ERROR in log.txt. Second shows only .txt files from ls. Third finds all .txt files and searches each for ERROR.

Pro Tips

1. chains commands together

2. Output of left becomes input of right

3. Common: ls

   grep pattern

4. Common: cat file

   grep search

5. find -exec is powerful but complex (use carefully)

6. Can chain multiple pipes: cat file

   grep ERROR

   grep -v DEBUG

Common Mistakes to Avoid

• Confusing

  (pipe) with > (redirect)

• Complex -exec syntax (easier to use find

  xargs)

Practice real-world scenarios

echo -e 'DEBUG: Starting\nINFO: Processing\nERROR: Failed\nWARN: Retrying\nINFO: Success' > app.log

grep ERROR app.log

grep -E 'ERROR|WARN' app.log

tail -n 5 app.log | grep -v DEBUG

What This Does

• grep -E enables extended regex.

  combines pattern matching. Real DevOps: filter logs, find errors, exclude debug lines. These patterns solve daily tasks.

• First finds ERROR line. Second finds ERROR or WARN lines. Third shows last 5 lines excluding DEBUG entries.

Pro Tips

1. grep -E ‘pattern1

   pattern2’ matches either pattern

2. Pipe tail and grep for recent errors only
3. grep -v excludes unwanted lines

4. Real-world: tail -f /var/log/app.log

   grep ERROR (monitor for errors)

Common Mistakes to Avoid

• Not using -E for OR patterns (

  needs -E flag)

• Forgetting pipes are processed left-to-right

Clean up practice files

cd ~

rm -r search-practice

ls

What This Does

• Clean up our practice workspace. Always verify you’re in the right directory (pwd) before rm -r.search-practice directory and all contents removed. Clean slate.

Pro Tips

1. Always pwd before rm -r
2. Consider keeping practice directories for reference

Common Mistakes to Avoid

• Not checking pwd before rm -r