Shell_scripting.md

🔹 Square Brackets [ ]

• Use: Test conditions (same as test command).
• Example:

  if [ -f /etc/passwd ]; then
      echo "File exists"
  fi

• DevOps use case: Checking if config/log files exist before deploying or restarting services.

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🔹 Double Square Brackets [[ ]]

• Use: Advanced test syntax (Bash/Ksh only). Safer than [ ].

• Supports regex =~, logical operators && ||, and no word-splitting issues.

• Example:

  if [[ $USER =~ ^dev.* ]]; then
      echo "Dev user detected"
  fi

  [!NOTE]
  in if block Use [ ] for portable POSIX scripts (works in sh in -gt like condition) in if block Use[[ ]] for Bash/Ksh scripts (safer, more features in both manner like <= and -le). In if block Use (( )) for arithmetic.

• Production use case: Validating usernames, branch names, or environment variables in CI/CD pipelines.

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🔹 Curly Braces { }

• Use:
  1. Variable expansion → ${var} avoids ambiguity.
  2. Brace expansion → Generate strings/sequences.
• Examples:

  echo "Hello ${USER}!"
  echo file{1..3}.log   # file1.log file2.log file3.log

• Production use case: Creating multiple log files, batch renaming, or looping over ranges in automation scripts.

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🔹 Parentheses ( )

• Use:
  1. Subshell → Commands inside run in a new shell.
  2. Arrays → Define Bash arrays.
• Examples:

  (cd /tmp && ls)   # runs in subshell, doesn’t affect current dir
  arr=(one two three)
  echo ${arr[1]}    # two

• Production use case:
  • Running isolated commands without changing current environment.
  • Managing arrays of servers or configs in deployment scripts.

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🔹 Double Parentheses (( ))

• Use: Arithmetic evaluation.
• Example:

  count=0
  ((count++))
  echo $count   # 1

• Production use case: Counters in retry loops, monitoring scripts, or resource checks.

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✅ Interview-style Question Examples

1. Condition check: “How would you verify if a log file exists before parsing it?” → [ -f logfile ].
2. Regex validation: “Check if a branch name starts with feature/.” → [[ $branch =~ ^feature/ ]].
3. Batch file creation: “Generate 10 numbered config files.” → touch config{1..10}.yaml.
4. Subshell isolation: “Run a command in /tmp without leaving current directory.” → (cd /tmp && ls).
5. Retry logic: “Increment a counter until a service responds.” → ((count++)).

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Here’s a cheat sheet of 10 real-world DevOps/production interview questions focused on bracket usage in Linux shell scripting, with short answers:

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🔹 Bracket Interview Questions

1. File existence check
   Q: How do you check if /etc/passwd exists?
   A: if [ -f /etc/passwd ]; then echo "exists"; fi

2. String pattern match
   Q: Validate if $branch starts with feature/.
   A: if [[ $branch =~ ^feature/ ]]; then echo "valid"; fi

3. Multiple file creation
   Q: Create 10 numbered log files in one command.
   A: touch log{1..10}.txt

4. Variable expansion safety
   Q: Print $HOME followed by _backup.
   A: echo "${HOME}_backup"

5. Subshell isolation
   Q: List files in /tmp without leaving current directory.
   A: (cd /tmp && ls)

6. Array usage
   Q: Store three server names and print the second.
   A:

   servers=(app1 app2 app3)
   echo ${servers[1]}

7. Arithmetic increment
   Q: Increase a retry counter by 1.
   A: ((retry++))

8. Logical AND condition
   Q: Check if $user is admin AND $env is prod.
   A: if [[ $user == "admin" && $env == "prod" ]]; then echo "ok"; fi

9. Brace expansion with ranges
   Q: Generate filenames configA, configB, configC.
   A: echo config{A..C}

10. Numeric comparison
    Q: Check if $count is greater than 5.
    A: if (( count > 5 )); then echo "high"; fi

Here’s a mini practice test with 5 timed-style questions on bracket usage in Linux shell scripting, designed like real DevOps interview scenarios. Try answering them quickly as if you’re in an interview:

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Here’s a 20-question full mock test on Linux shell scripting bracket usage, styled like real DevOps/production interview scenarios. Each question is practical, concise, and tests logic you’d actually use in CI/CD, monitoring, or support scripts.

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📝 20-Question Mock Test – Brackets in Shell Scripting

🔹 File & Condition Checks

1. Check if /etc/hosts exists before appending a new entry.
2. Verify if /var/log/app.log is non-empty.
3. Ensure $USER is not equal to root.
4. Test if $PORT is greater than 1024.
5. Confirm that $env is either dev or test.

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🔹 Regex & String Validation

6. Validate if a branch name starts with feature/.
7. Ensure a filename ends with .yaml.
8. Check if $email contains @company.com.
9. Verify if $version matches semantic versioning (X.Y.Z).
10. Confirm $hostname starts with prod-.

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🔹 Brace Expansion

11. Generate 10 numbered log files (log1.log … log10.log).
12. Create configs named configA, configB, configC.
13. Expand {dev,test,prod} into environment names.
14. Produce a sequence of numbers from 5 to 15.
15. Generate filenames server1.conf to server3.conf.

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🔹 Parentheses (Subshells & Arrays)

16. Run ls inside /tmp without leaving current directory.
17. Store three IPs in an array and print the second.
18. Execute (cd /var && ls) and confirm your current directory remains unchanged.
19. Use an array to loop over servers app1, app2, app3.
20. Run two commands in a subshell: change directory and list files.

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✅ Answer Key (Short Syntax)

1. [ -f /etc/hosts ]
2. [ -s /var/log/app.log ]
3. [ "$USER" != "root" ]
4. (( PORT > 1024 ))
5. [[ $env == "dev" || $env == "test" ]]
6. [[ $branch =~ ^feature/ ]]
7. [[ $file == *.yaml ]]
8. [[ $email == *@company.com ]]
9. [[ $version =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]
10. [[ $hostname =~ ^prod- ]]
11. touch log{1..10}.log
12. touch config{A..C}
13. echo {dev,test,prod}
14. echo {5..15}
15. echo server{1..3}.conf
16. (cd /tmp && ls)
17. ips=(10.0.0.1 10.0.0.2 10.0.0.3); echo ${ips[1]}
18. (cd /var && ls); pwd
19. servers=(app1 app2 app3); for s in "${servers[@]}"; do echo $s; done
20. (cd /etc && ls)

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Example 1

read N
sum=0
count=$N
while(( $N>= 1))
do
read x
sum=$(( sum + x ))
#arr+=($x)
(( N-- ))
done
#avg=$(( sum / count )) not working
printf "%0.3f" "$( echo "scale=4; $sum / $count " |bc)"