Revision and Interview Questions

Cheat sheet

Basics: #!/usr/bin/env bash, chmod +x, exit status 0 = success, $?, exit N; /bin/sh may be dash; macOS ships Bash 3.2. Variables: no spaces around =; export for children; local in functions; special $0 $1 $# $@ $? $$ $!. Quoting: double-quote every expansion ("$var", "$@", "${arr[@]}"); single quotes are literal. Expansion: ${var:-def}, ${var:?msg}, ${var#pre}, ${var%suf}, ${var//a/b}, ${#var}, $(cmd), $((math)). Tests: [[ ]] for strings, patterns, regex; (( )) for numbers; file tests -f -d -e -r -s. Flow: if, case, &&/||; for f in *.ext, while IFS= read -r line; do ...; done < file, avoid cat | while. Functions: "$@", shift, return for status, print for data. Arrays: indexed, associative (declare -A), mapfile -t. I/O: >, >>, 2>&1, &>, /dev/null, here-docs, <(cmd), xargs -0. Robustness: set -Eeuo pipefail and its caveats, trap ... EXIT/ERR/INT, mktemp, ${dir:?} before rm, flock, atomic mv, ShellCheck, bash -x. Automation: cron's minimal environment, systemd timers.

Common interview questions

Answer with a short example each time.

1. What is the difference between "$@" and "$*"?
2. Why should you quote variable expansions? Give an example of what breaks.
3. [ ] vs [[ ]] vs (( )): when do you use each?
4. What does set -euo pipefail do, and what are its pitfalls?
5. How do you read a file line by line safely?
6. How do you redirect both stdout and stderr to a file?
7. How do you make sure temporary files are cleaned up?
8. How do you return a value from a function?
9. Why might a script work in your terminal but fail in cron?
10. When would you rewrite a Bash script in another language?

Show the bug, then the fix

Interviewers love concrete examples: "rm $file breaks on 'my file.txt' because of word splitting; rm -- "$file" fixes it." A one-line bug and fix shows real experience.

त्वरित जाँच: Which line correctly passes all of a script's arguments, unchanged, to another command?

• other_cmd $*
• other_cmd $@
• other_cmd "$*"
• other_cmd "$@"