- A
dir=${1:-.}
Correct because `${1:-.}` uses default value substitution: if `$1` (the first positional argument) is unset or null, it expands to `.` (current directory), meeting the requirement.
- B
dir=${@:-.}
Why wrong: Incorrect because `${@:-.}` expands to the list of all positional parameters or `.` if none, but it does not isolate the first argument as required.
- C
dir=${0:-.}
Why wrong: Incorrect because `$0` is the script name, not the first argument. `${0:-.}` would default to the script name only if `$0` is unset, which is never the case.
- D
dir=${?:-.}
Why wrong: Incorrect because `$?` is the exit status of the last command, not a directory path. `${?:-.}` is not appropriate for capturing an argument.
Question 244 of 537
Positional Parameter Default Value: Using ${1:-.} in Bash
This EX200 practice question tests your understanding of create simple shell scripts. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer wants to create a script that accepts a directory path as an argument and creates a timestamped backup of that directory. If no argument is provided, it should back up the current directory. How should the script handle the argument?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
dir=${1:-.}
Option A is correct because `${1:-.}` uses the default value substitution syntax in bash: if parameter `$1` (the first positional argument) is unset or null, it expands to `.` (the current directory). This ensures the script backs up the supplied directory path or defaults to the current directory when no argument is provided, exactly matching the requirement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
dir=${1:-.}
Why this is correct
Correct because `${1:-.}` uses default value substitution: if `$1` (the first positional argument) is unset or null, it expands to `.` (current directory), meeting the requirement.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
dir=${@:-.}
Why it's wrong here
Incorrect because `${@:-.}` expands to the list of all positional parameters or `.` if none, but it does not isolate the first argument as required.
- ✗
dir=${0:-.}
Why it's wrong here
Incorrect because `$0` is the script name, not the first argument. `${0:-.}` would default to the script name only if `$0` is unset, which is never the case.
- ✗
dir=${?:-.}
Why it's wrong here
Incorrect because `$?` is the exit status of the last command, not a directory path. `${?:-.}` is not appropriate for capturing an argument.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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
Incorrect because `$?` is the exit status of the last command, not a directory path. `${?:-.}` is not appropriate for capturing an argument.
Detailed technical explanation
How to think about this question
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, `$1` is the first positional parameter, and using `:-` ensures the script gracefully handles missing arguments without requiring explicit `if` statements. A real-world scenario is a cron job that calls this script with a directory argument; if the argument is accidentally omitted, the script safely backs up the current working directory instead of failing.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the EX200 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this EX200 question test?
Create simple shell scripts — This question tests Create simple shell scripts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: dir=${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.
What should I do if I get this EX200 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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