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How Bash reads a command line interview questions

Interview questions on how Bash reads a command line: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

17 questions: 6 warm-ups and 11 with model answers · Levels: Fresher to staff

Warm-up questions

Short questions with short answers, each answered in full.

Fresher to mid

In what order does Bash expand a command line?

Answered in the lesson How Bash reads a line: tokens and expansions
Fresher to mid

How do single quotes, double quotes and $'...' differ in Bash?

Answered in the lesson Quoting: single, double, backslash and $'...'
Fresher to mid

Why should you write "$var" instead of $var in Bash?

Answered in the lesson Word splitting and IFS: why "$var" needs quotes
Fresher to mid

What happens in Bash when a glob matches no files?

Answered in the lesson Globbing: wildcards, dotfiles, extglob, globstar
Fresher to mid

Why does echo {1..$n} not print a range in Bash?

Answered in the lesson Brace and tilde expansion
Fresher to mid

Why do shell scripts prefer printf to echo?

Answered in the lesson printf and echo: printing text reliably

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

Mid to senior

A deploy script builds a command in a variable, cmd="rsync -av --exclude='*.log' src/ host:/srv", and runs $cmd. The exclude is ignored and log files are copied. Why, and how do you fix it?
What it tests

Knowing that quotes inside a variable's value are not syntax, that expansions are split but not re-parsed, and the safe ways to keep a command: an array or a function.

First step

Trace it with bash -x: the trace shows rsync receiving --exclude='*.log' with the quotes inside the word, because quote removal applies only to quotes written in the line, not to quotes that come out of a variable.
Follow-ups
- How would you write it with an array?
- Why is eval a poor fix?
- How do you print the command for a log before running it?
From the lesson How Bash reads a line: tokens and expansions
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Fresher to mid

A backup loop, for f in $(find . -name '*.pdf'); do cp $f backup/; done, fails with "No such file" for some files and copies others. How do you see exactly what went wrong, and what do you change?
What it tests

Using a trace to see the words a command receives, recognising word splitting of command substitution, and replacing the loop with one that keeps names whole.

First step

Run the script with bash -x, or put set -x before the loop, and read the traced cp lines: a name with a space shows up as two separate words, which is word splitting of the unquoted $(find …) and of $f.
Follow-ups
- What is the safest loop for names that may contain newlines?
- Why is quoting "$f" alone not enough?
- How do you make the trace easier to read in a long script?
From the lesson How Bash reads a line: tokens and expansions
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

You run ssh web1 "grep 'error' $LOG | wc -l" and get the count of the wrong file. Then a teammate writes the same with single quotes outside and it works. Explain both, and how you pass a local value safely to a remote command.
What it tests

Understanding that a command line sent over ssh is parsed twice, by the local and the remote shell, and quoting for two shells with printf %q.

First step

Note which shell expands $LOG: in double quotes the local shell expands it before ssh runs, so the remote side gets the local value. In single quotes the text reaches the remote shell unchanged and $LOG is the remote variable.
Follow-ups
- How does printf %q help here?
- What breaks if the value holds a space or a quote?
- When would you send a script on standard input instead?
From the lesson Quoting: single, double, backslash and $'...'
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Fresher to mid

A script prints echo "Price: $5 per item" and shows "Price: per item". Another prints echo 'Hello, $USER' and shows the text $USER. Explain each and fix both.
What it tests

Knowing which characters double and single quotes protect, recognising a positional parameter, and choosing the quoting for each part of a message.

First step

Read each $ the way the shell does. Inside double quotes $5 is the fifth positional parameter, empty here; inside single quotes $USER is plain text. So the first needs the $ escaped or single-quoted, the second needs double quotes.
Follow-ups
- How would you write a message that holds both a literal $ and a variable?
- Which ShellCheck warnings would have caught these?
- Why does printf make the first one easier?
From the lesson Quoting: single, double, backslash and $'...'
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

You write a wrapper that adds --verbose and then runs the real tool with all the user's arguments. A user passes 'Q3 report.pdf' and the tool says two files are missing. What did the wrapper do wrong, and what is the correct line?
What it tests

Knowing the difference between "$@", "$", $@ and $, and passing arguments on without changing their boundaries.

First step

Look at how the wrapper passes the arguments: $@ or $* unquoted splits 'Q3 report.pdf' into two words. The correct form is tool --verbose "$@", which passes every argument on as exactly one word.
Follow-ups
- When is "$*" the right choice?
- How would you add an argument at the front or remove the first one?
- How do you test the wrapper?
From the lesson Word splitting and IFS: why "$var" needs quotes
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

A nightly job must delete .log files older than seven days in /var/app/logs and its subfolders. A teammate proposes cd /var/app/logs; rm **/*.log. What risks do you see, and what would you write instead?
What it tests

Judging glob behaviour (globstar, no match, dotfiles), the danger of a failed cd, and choosing find for age-based deletion with a dry run first.

First step

List what can go wrong before writing anything: if cd fails, rm runs in the wrong folder; without globstar ** is just *; with no match the literal pattern reaches rm; and a glob cannot select by age at all, so find is the right tool.
Follow-ups
- How do you test it safely before the first real run?
- What if a file name begins with a dash?
- How would you log what was deleted?
From the lesson Globbing: wildcards, dotfiles, extglob, globstar
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Senior to staff

Your build script uses shopt -s globstar extglob and GLOBSORT=mtime. It must also run on Macs whose /bin/bash is 3.2.57. What breaks there, and how do you make the script reliable on both?
What it tests

Knowing which glob features each Bash version has, detecting the version, and choosing between requiring a newer Bash and writing portable code.

First step

Check each feature against its release: extglob exists since 2.02, but globstar needs 4.0 and GLOBSORT 5.3, so on 3.2.57 shopt -s globstar fails and GLOBSORT is silently ignored. Decide whether the script may require a newer Bash.
Follow-ups
- How does the script detect the version?
- What portable command replaces ** here?
- How do you sort by modification time without GLOBSORT?
From the lesson Globbing: wildcards, dotfiles, extglob, globstar
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Fresher to mid

You must create folders q1 to q4 under data/raw and data/clean, and copy settings.yml to a backup before editing it. Show the commands with brace expansion, and say what you check before running them on a server.
What it tests

Using nested braces and the {,.bak} idiom correctly, and previewing an expansion with echo before running it.

First step

Write the expansions and preview them with echo first: echo mkdir -p data/{raw,clean}/q{1..4} shows the eight paths, and echo cp settings.yml{,.bak} shows the two names, before anything is created or copied.
Follow-ups
- What happens if the script runs under sh?
- How would you do the same when the years come from variables?
- Why does mkdir need -p here?
From the lesson Brace and tilde expansion
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Fresher to mid

A config file holds the line path=~/data, read by a script into a variable and used as cd "$path". The script fails with "No such file or directory", although ~/data exists. Explain and fix it.
What it tests

Knowing that tilde expansion happens only on words in the command line, never on values read from files or inside quotes, and replacing the tilde safely.

First step

Trace what the variable holds: the tilde came from a file, so it is a plain character, and the shell never expands a tilde inside a variable's value. cd receives the literal ~/data. Replace a leading ~/ with $HOME/ in the value.
Follow-ups
- Why is eval not the answer?
- How do you handle ~otheruser in the file?
- What would you write in the config file instead?
From the lesson Brace and tilde expansion
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Fresher to mid

A report prints name, count and amount with echo and tabs, and the columns drift whenever a name is long. Amounts such as 12.5 print with one decimal. Rewrite the output with printf and explain each part of the format.
What it tests

Using printf widths, alignment and precision to build a stable table, and handling long values and money correctly.

First step

Decide the columns first: the name left-aligned in a fixed width, the count right-aligned, the amount right-aligned with two decimals. That is printf '%-20s %5d %10.2f\n' "$name" "$count" "$amount", with a header in the same widths.
Follow-ups
- What happens when a name is longer than 20 characters?
- How do you add a total without floating-point errors?
- How would you print the same table as CSV?
From the lesson printf and echo: printing text reliably
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes
Mid to senior

A logging function is log() { printf "$(date +%T) $1\n"; }. A message such as "disk 95% full" prints garbage, and a message with a backslash prints oddly. What is wrong, and how do you write log() safely?
What it tests

Recognising data used as a printf format, rewriting with a fixed format and %s, and keeping the function's behaviour.

First step

Point at the format: the message is part of printf's first argument, so its % and backslashes are read as instructions. Move the message into an argument: printf '%s %s\n' "$(date +%T)" "$1".
Follow-ups
- How do you log several arguments at once?
- How can printf produce the time without running date?
- Should the log go to standard output or standard error?
From the lesson printf and echo: printing text reliably
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 2 minutes

The lessons of this module

How Bash reads a line: tokens and expansions Free lesson, Pro answers: 5 minutes
Quoting: single, double, backslash and $'...' Free lesson, Pro answers: 5 minutes
Word splitting and IFS: why "$var" needs quotes Free lesson, Pro answers: 5 minutes
Globbing: wildcards, dotfiles, extglob, globstar Free lesson, Pro answers: 5 minutes
Brace and tilde expansion Free lesson, Pro answers: 5 minutes
printf and echo: printing text reliably Free lesson, Pro answers: 5 minutes

All Bash & the POSIX Shell interview questions · The Bash & the POSIX Shell track

Warm-up questions

In what order does Bash expand a command line?

How do single quotes, double quotes and $'...' differ in Bash?

Why should you write "$var" instead of $var in Bash?

What happens in Bash when a glob matches no files?

Why does echo {1..$n} not print a range in Bash?

Why do shell scripts prefer printf to echo?

Questions with model answers

A deploy script builds a command in a variable, cmd="rsync -av --exclude='*.log' src/ host:/srv", and runs $cmd. The exclude is ignored and log files are copied. Why, and how do you fix it?

A backup loop, for f in $(find . -name '*.pdf'); do cp $f backup/; done, fails with "No such file" for some files and copies others. How do you see exactly what went wrong, and what do you change?

You run ssh web1 "grep 'error' $LOG | wc -l" and get the count of the wrong file. Then a teammate writes the same with single quotes outside and it works. Explain both, and how you pass a local value safely to a remote command.

A script prints echo "Price: $5 per item" and shows "Price: per item". Another prints echo 'Hello, $USER' and shows the text $USER. Explain each and fix both.

You write a wrapper that adds --verbose and then runs the real tool with all the user's arguments. A user passes 'Q3 report.pdf' and the tool says two files are missing. What did the wrapper do wrong, and what is the correct line?

A nightly job must delete .log files older than seven days in /var/app/logs and its subfolders. A teammate proposes cd /var/app/logs; rm **/*.log. What risks do you see, and what would you write instead?

Your build script uses shopt -s globstar extglob and GLOBSORT=mtime. It must also run on Macs whose /bin/bash is 3.2.57. What breaks there, and how do you make the script reliable on both?

You must create folders q1 to q4 under data/raw and data/clean, and copy settings.yml to a backup before editing it. Show the commands with brace expansion, and say what you check before running them on a server.

A config file holds the line path=~/data, read by a script into a variable and used as cd "$path". The script fails with "No such file or directory", although ~/data exists. Explain and fix it.

A report prints name, count and amount with echo and tabs, and the columns drift whenever a name is long. Amounts such as 12.5 print with one decimal. Rewrite the output with printf and explain each part of the format.

A logging function is log() { printf "$(date +%T) $1\n"; }. A message such as "disk 95% full" prints garbage, and a message with a backslash prints oddly. What is wrong, and how do you write log() safely?