Linux Commands – echo, clear, sleep
🧠 FAANG-Level Interview Q&A with Examples & Explanations
🔹 1. echo – Text Output & Debugging
# | Question | Answer / Example | Explanation |
1 | What does echo do in shell scripting? | echo "Hello, FAANG" | Prints strings or variables to stdout |
2 | How to print a variable value? | name="Alice"; echo $name | Outputs value of name |
3 | How to print without newline? | echo -n "Processing..." | -n omits the trailing newline |
4 | How do you print special characters like newline? | echo -e "Line1\nLine2" | -e enables interpretation of escape sequences |
5 | How to append text to a file using echo? | echo "Log entry" >> file.txt | >> appends instead of overwriting |
6 | What’s the difference between echo $VAR and echo "$VAR"? | Quoting preserves whitespace | Prevents word splitting in variables |
7 | How to print a command’s output using echo? | echo "Now: $(date)" | Command substitution with $() |
8 | How do you send echo output to stderr? | echo "error" 1>&2 | Useful for logging errors separately |
9 | How do you print $PATH literally? | echo "\$PATH" | Escape $ with \ |
10 | What are echo's limitations? | Lacks formatting | Use printf when you need precision (e.g., decimal alignment) |
🔹 2. clear – Terminal UI Control
# | Question | Answer / Example | Explanation |
11 | What does the clear command do? | clear | Clears the terminal screen |
12 | How can clear be used in a script for better UX? | clear; echo "Select option:" | Provides clean interface before input |
13 | What’s an alternative to clear? | printf "\033c" | ANSI escape code resets terminal |
14 | Can clear be used in cron jobs? | ❌ Not useful | Cron has no terminal |
15 | How do you use clear with interactive shell menus? | Use before select or read | Keeps prompt clean and readable |
16 | How to alias clear to cls? | alias cls='clear' | Custom shortcut |
17 | How is clear helpful in loops? | while true; do clear; date; sleep 1; done | Used to create dynamic terminal dashboards |
18 | What’s the benefit of using clear in login scripts? | clear; echo "Welcome!" | Clean welcome screen for users |
19 | How to clear terminal from Python/Bash script? | os.system('clear') or clear in .sh | Cross-language scripting support |
20 | What’s a scenario where clear might reduce productivity? | In log capture scripts | Hides terminal scrollback history |
🔹 3. sleep – Execution Control
# | Question | Answer / Example | Explanation |
21 | What does sleep 5 do? | Pauses for 5 seconds | Delays script execution |
22 | Can sleep handle fractions? | sleep 0.5 | Yes, it supports floating-point |
23 | What does sleep 1m mean? | Sleeps for 1 minute | Supports suffixes like s, m, h |
24 | How to use sleep in retry logic? | `for i in {1..5}; do cmd | |
25 | How to delay a background task? | sleep 10 && task.sh & | Defers execution asynchronously |
26 | What’s the purpose of using sleep between loop iterations? | Throttling, pacing, or animation | Useful in polling, dashboards |
27 | Can sleep be interrupted? | Yes, via CTRL+C or signal | Responds to SIGINT, etc. |
28 | How is sleep used in systemd service files? | ExecStartPre=/bin/sleep 5 | Adds startup delay before main service |
29 | How to simulate a slow network request? | curl ...; sleep 3; curl ... | Introduces delay between dependent calls |
30 | When is sleep a bad idea in production scripts? | When hardcoded, without checks | Can cause indefinite delays in failure scenarios |
🔸 4. Combined Real-World Scenarios
# | Question | Use Case | Example |
31 | Create a live terminal clock | Bash UI | while true; do clear; date; sleep 1; done |
32 | Log build start timestamp | CI/CD | echo "Build started at $(date)" >> build.log |
33 | Display progress dots | UX | for i in {1..5}; do echo -n "."; sleep 1; done |
34 | Add delay between steps | Pipeline | step1 && sleep 10 && step2 |
35 | Print environment at runtime | Debugging | echo "PATH=$PATH" |
36 | Clear screen on login | User experience | Add clear to .bash_profile |
37 | Retry failed health check | Infra validation | until curl -s localhost:8080; do sleep 5; done |
38 | Delay script to wait for DB | Startup script | sleep 15; ./start-app.sh |
39 | Print conditional status | Logging | if [ $? -ne 0 ]; then echo "Failed"; fi |
40 | Time an operation | Performance | start=$(date +%s); ... ; end=$(date +%s); echo $((end - start)) |
🔹 5. Edge Cases & Trick Questions
# | Question | Answer / Example | Explanation |
41 | What happens on sleep 0? | No delay | Effectively a no-op |
42 | What happens on sleep abc? | Error: invalid time interval | Non-numeric input is invalid |
43 | Can echo output binary? | Yes, but limited | Use printf for control |
44 | What does echo -e "123\r456" do? | Overwrites output | \r returns cursor to line start |
45 | How to delay execution without sleep? | read -t 5 | Waits for input for 5 seconds |
46 | Why avoid echo for structured output? | Use printf instead | Better formatting control |
47 | How to simulate clear effect without using clear? | printf "\033c" | ANSI reset sequence |
48 | How to show spinner using sleep? | spin='-|/'; while true; do ...; sleep 0.1; done | Visual loading animation |
49 | Why use echo with >> in deployment? | Logging progress | Tracks step-by-step execution |
50 | How to use all three (echo, clear, sleep) together? | Interactive UI | clear; echo "Loading..."; sleep 3 |
📦 Summary
Command | Best Use Cases |
echo | Logging, debugging, output control |
clear | UI, clean prompts, loop dashboards |
sleep | Timing control, retry logic, pacing |
FAANG-Level Tricky Pathname Questions & Edge Cases (with Solutions)
# | Category | Question | Edge/Trick | ✅ Solution / Explanation |
1 | Absolute vs Relative | What’s the difference between /etc/passwd and passwd? | One is fully qualified, the other depends on $PATH or pwd | /etc/passwd is absolute; passwd may fail unless present in current or $PATH directories |
2 | cd .. | What does cd .. do from /? | Root has no parent — cd .. stays in / | No error, but no movement occurs |
3 | Double slashes | What is the result of cd ////usr///bin? | Multiple slashes are collapsed | It’s equivalent to cd /usr/bin |
4 | . and .. | What does ls ./../././../etc resolve to? | Dot and dot-dot normalize to /etc | Simplifies to /etc through normalization |
5 | Symlink loop | What if a symlink points to itself? | Causes infinite resolution loop | Commands like readlink -f or ls -l will error |
6 | ~ Expansion | What does echo "~user" do? | Quoted tilde prevents expansion | Outputs ~user, not the home dir |
7 | Trailing Slash | Difference between /var/log and /var/log/? | Usually none, but some scripts check trailing / | rsync behaves differently depending on presence of trailing / |
8 | Spaces in Paths | What happens with cd /home/user/My Documents? | Fails if not quoted | Must use cd "/home/user/My Documents" |
9 | $0 in Scripts | What does $0 represent in a script? | Path to script — may be relative or absolute | Use readlink -f "$0" for reliable full path |
10 | Quoting Errors | What does cd "$DIR" do if DIR="file 1"? | Works correctly — quoting prevents split | cd $DIR (unquoted) would break |
11 | $PWD vs pwd | Are $PWD and pwd always the same? | No — $PWD is shell-internal, pwd may resolve symlinks | Use pwd -P to get physical path |
12 | Subdir Access | Can you access /home/user/secret/ if secret/ has ---x--x--x perms? | You can cd but not ls | Execute-only allows traversal, not listing |
13 | Cron Path Error | Why might script.sh work in shell but fail in cron? | Cron lacks full $PATH and relative dirs | Use absolute paths always in cron jobs |
14 | . in $PATH | Is adding . to $PATH safe? | Dangerous — allows execution of unintended binaries in pwd | Never include . in $PATH on production systems |
15 | Loop Symlinks | What if A → B and B → A? | Circular symlinks — cause resolution failure | Tools like realpath detect and fail |
16 | readlink -f | What does readlink -f ./../dir return? | Canonical absolute path | Resolves symlinks and cleans relative components |
17 | basename | What does basename /usr/bin/env return? | Just env | Removes directory part |
18 | dirname | What does dirname /home/user/file.txt return? | /home/user | Returns everything but the last component |
19 | $HOME unset | What if cd ~ fails? | If $HOME is unset or invalid | cd ~ will error or go nowhere |
20 | Symlink to Parent | What if you cd symlink where symlink → ..? | You move up one level | Follows the target path just like any normal .. |
🔍 Real-World Debug Scenarios
# | Scenario | Problem | Root Cause | Fix |
21 | Bash script using ../env.sh fails in Jenkins | Relative path fails | Jenkins changes working directory | Use source "$(dirname "$0")/../env.sh" |
22 | source ~/setup.sh fails in cron | ~ is not expanded | Cron runs in minimal shell | Use $HOME/setup.sh or hardcode full path |
23 | ./run.sh gives “No such file” but file exists | Wrong shell / CRLF | File has DOS line endings | Use dos2unix run.sh |
24 | Symbolic link to moved file broken | ln -s oldpath link → file moved | Link doesn't auto-update | Use ln -s $(realpath file) if predictable |
25 | Dockerfile COPY ./config /etc/config fails | Wrong build context | Path not visible in build context | Use relative paths from Docker build dir |
🔧 Bonus Edge Behavior (Often Misunderstood)
Edge Case | Command | Behavior |
cd ~nonexistentuser | Fails | No such user exists |
cd /dev/null | Works but you're stuck | /dev/null is a character device, not a dir |
ls -- | Lists files named - | -- stops flag parsing |
rm -rf /some/path/.* | Dangerous! | Removes .. too, deleting parent dir recursively |
find . -name '../*' | Won’t match | find operates within current tree only |
🧠 Summary Tips for FAANG Interviews
Topic | Insight |
Always use readlink -f | Ensures canonical path |
Avoid hardcoded relative paths | Breaks in CI/CD, containers |
Prefer quoting "${PATH}" | Avoids word-splitting bugs |
Know difference between logical and physical paths | $PWD vs pwd -P |
Understand file permissions at directory level | x vs r on dirs |
