This EX200 practice question tests your understanding of create simple shell scripts. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer wants to create a script that accepts a directory path as an argument and creates a timestamped backup of that directory. If no argument is provided, it should back up the current directory. How should the script handle the argument?
dir=${1:-.}
D is correct. ${1:-.} uses $1 if set, otherwise '.'.
dir=${@:-.}
Why wrong: C is wrong because $@ expands to all arguments, not just the first.
dir=${0:-.}
Why wrong: A is wrong because $0 is the script name, not the first argument.
dir=${?:-.}
Why wrong: B is wrong because $? is the exit status of the last command.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
dir=${1:-.}
Option A is correct because `${1:-.}` uses the default value substitution syntax in bash: if parameter `$1` (the first positional argument) is unset or null, it expands to `.` (the current directory). This ensures the script backs up the supplied directory path or defaults to the current directory when no argument is provided, exactly matching the requirement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
dir=${1:-.}
Why this is correct
D is correct. ${1:-.} uses $1 if set, otherwise '.'.
Related concept
Read the scenario before looking for a memorised answer.
dir=${@:-.}
Why it's wrong here
C is wrong because $@ expands to all arguments, not just the first.
dir=${0:-.}
Why it's wrong here
A is wrong because $0 is the script name, not the first argument.
dir=${?:-.}
Why it's wrong here
B is wrong because $? is the exit status of the last command.
Common exam traps
Red Hat often tests the distinction between positional parameters (`$1`, `$2`, etc.) and special variables (`$@`, `$0`, `$?`), and the trap here is that candidates confuse `$1` with `$0` (the script name) or incorrectly assume `$@` works as a single default value, leading to option B or C.
Trap categories for this question
Command / output trap
B is wrong because $? is the exit status of the last command.
Detailed technical explanation
The `${parameter:-word}` syntax is defined in the POSIX shell specification and is a form of parameter expansion that provides a default value when the parameter is unset or null. In bash, `$1` is the first positional parameter, and using `:-` ensures the script gracefully handles missing arguments without requiring explicit `if` statements. A real-world scenario is a cron job that calls this script with a directory argument; if the argument is accidentally omitted, the script safely backs up the current working directory instead of failing.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Create simple shell scripts — This question tests Create simple shell scripts — Read the scenario before looking for a memorised answer..
The correct answer is: dir=${1:-.} — Option A is correct because `${1:-.}` uses the default value substitution syntax in bash: if parameter `$1` (the first positional argument) is unset or null, it expands to `.` (the current directory). This ensures the script backs up the supplied directory path or defaults to the current directory when no argument is provided, exactly matching the requirement.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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