Shell scripting · Lesson 26 of 31 · 8 minutes

Script input guard

Use test checks to prove required files and directories exist.

Open the lab Course map

Mastery 0/3Run itExplain itApply it

learner@clairos:~ $

↑ history · Tab complete · Ctrl+C cancel · Esc, Tab to leaveExit 0

Your objectives

Hints are always available. Wrong turns never cost XP.

1
Check the script file
Run test -f scripts/deploy.sh.
Hint & worked example
Evaluate a file or string condition through its exit status rather than printed output.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
test -f scripts/deploy.sh
```
Remember: A successful test usually prints nothing; inspect $? or chain a command.
2
Check the output directory
Run test -d lab.
Hint & worked example
Evaluate a file or string condition through its exit status rather than printed output.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
test -d lab
```
Remember: A successful test usually prints nothing; inspect $? or chain a command.
3
Inspect the guarded input
Run stat scripts/deploy.sh.
Hint & worked example
Inspect a file or directory metadata record, including ownership, permissions, and size.
The command outcome must match the task, including successful access or the explicitly requested failure.
```
stat scripts/deploy.sh
```
Remember: Metadata and file contents answer different questions.

Understand the pattern

Guarding script-style work with explicit preconditions helps operators explain what they know before they change anything.

test: Evaluate a file or string condition through its exit status rather than printed output.
A successful test usually prints nothing; inspect $? or chain a command.
stat: Inspect a file or directory metadata record, including ownership, permissions, and size.
Metadata and file contents answer different questions.

Worked example and expected result

$ test -f scripts/deploy.sh

(no output; exit 0)

Exit status: 0. The command completed successfully in the training fixture.

$ test -d lab

(no output; exit 0)

Exit status: 0. The command completed successfully in the training fixture.

$ stat scripts/deploy.sh

Path: /home/learner/scripts/deploy.sh
Type: file
Mode: -rw-r--r--
Owner: learner
Group: learners
Size: 85 bytes

Exit status: 0. The command completed successfully in the training fixture.

Try a variation

Repeat this workflow in the sandbox with a different scratch filename. Keep the system fixtures unchanged and compare the observed output with your expectation before moving on.

Explain it

In “Script input guard”, which interpretation is safe?A successful test usually prints nothing; inspect $? or chain a command.A command appearing in history proves it completed successfully.A successful first check guarantees that every dependent service is healthy.

Apply it elsewhere

Before adapting the test workflow from “Script input guard” to another server, what should you do?Paste the training paths unchanged into production.Skip output inspection whenever the command is familiar.Confirm the current directory, target identity, permissions, and the installed command behavior first.

Keep the momentum.

Complete the lab, then explain and apply the pattern.

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