Logical operators (-a, -o, !) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the file 'data.txt' exists and is readable.

Bash Scripting

if [ -e data.txt [1] ]; then echo "File exists and is readable"; fi

Drag options to blanks, or click blank then click option'

A-a -r data.txt

B-a -e data.txt

C-a -f data.txt

D-o -r data.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-o' instead of '-a' for AND operation.

Forgetting to include the file test after '-a'.

Using '-f' instead of '-r' when checking readability.

The '-a' operator means AND, so '-a -r data.txt' checks if the previous condition (file exists with '-e') and the file is readable are both true.

2fill in blank

medium

Complete the code to check if the variable 'count' is either 0 or 10.

Bash Scripting

if [ "$count" -eq 0 [1] "$count" -eq 10 ]; then echo "Count is 0 or 10"; fi

Drag options to blanks, or click blank then click option'

A==

B-a

C-o

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-a' which means AND instead of OR.

Using '==' which is not valid in this context.

Using '!' which negates a condition.

The '-o' operator means OR, so it checks if either condition is true.

3fill in blank

hard

Fix the error in the code to check if the directory 'logs' does NOT exist.

Bash Scripting

if [ [1] -e logs ]; then echo "Logs directory missing"; fi

Drag options to blanks, or click blank then click option'

A-z

B-o

C-a

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-o' or '-a' which are logical AND/OR, not negation.

Using '-z' which checks for empty strings, not existence.

The '!' operator negates the test, so '! -e logs' means the file or directory does NOT exist.

4fill in blank

hard

Complete the code to check if the file 'config.cfg' exists AND is NOT empty.

Bash Scripting

if [ -e config.cfg [1] -s config.cfg ]; then echo "Config file exists and is not empty"; fi

Drag options to blanks, or click blank then click option'

A-a

B-o

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-o' which means OR instead of AND.

Confusing '-s' (non-empty) with empty check; '! -s' would be for empty or non-existent.

Use '-a' for AND to combine the existence check ('-e') and the non-empty check ('-s' means size greater than zero).

5fill in blank

hard

Fill all three blanks to create a condition that checks if variable 'user' is NOT 'admin' AND file 'access.log' exists OR variable 'mode' equals 'debug'.

Bash Scripting

if [ ! "$user" = [1] [2] -e access.log [3] "$mode" = "debug" ]; then echo "Condition met"; fi

Drag options to blanks, or click blank then click option'

A"admin"

B-a

C-o

D"root"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong strings for user comparison.

Mixing up '-a' and '-o' operators.

Not quoting variables properly.

The condition uses '!' to negate user='admin', '-a' for AND with file existence, and '-o' for OR with mode='debug'.

Practice

1. Which logical operator in bash scripting means AND when used inside [ ]?

easy

A. &&

B. -o

C. !

D. -a

Solution

1. Step 1: Understand bash logical operators inside [ ]

   Inside single brackets, -a means AND, -o means OR, and ! means NOT.

2. Step 2: Identify the AND operator

   The operator -a is used to combine two conditions with AND logic.

3. Final Answer:

   -a -> Option D

4. Quick Check:

   AND operator = -a [OK]

Hint: Remember: -a is AND, -o is OR, ! is NOT inside [ ] [OK]

Common Mistakes:

• Confusing -a with -o
• Using && inside [ ] which is invalid
• Thinking ! means AND

2. Which of the following is the correct syntax to check if file file.txt exists AND is readable in bash?

easy

A. [ -e file.txt -o -r file.txt ]

B. [ -e file.txt && -r file.txt ]

C. [ -e file.txt -a -r file.txt ]

D. [ ! -e file.txt -a -r file.txt ]

Solution

1. Step 1: Understand the condition to check

   We want to check if file.txt exists AND is readable, so both conditions must be true.

2. Step 2: Choose correct logical operator and syntax

   Inside [ ], -a means AND, so [ -e file.txt -a -r file.txt ] is correct. Using -o means OR, which is wrong here. Using && inside [ ] is invalid syntax.

3. Final Answer:

   [ -e file.txt -a -r file.txt ] -> Option C

4. Quick Check:

   AND check uses -a inside [ ] [OK]

Hint: Use -a for AND inside [ ], not && or -o for AND [OK]

Common Mistakes:

• Using -o instead of -a for AND
• Using && inside [ ] which causes syntax error
• Negating conditions incorrectly with !

3. What will be the output of this script snippet?

if [ -f myfile.txt -o ! -r myfile.txt ]; then echo "Check passed"; else echo "Check failed"; fi

Assuming myfile.txt exists but is NOT readable.

medium

A. Check passed

B. Check failed

C. Syntax error

D. No output

Solution

1. Step 1: Analyze conditions with given file state

   -f myfile.txt is true because file exists. ! -r myfile.txt is true because file is not readable.

2. Step 2: Evaluate OR (-o) logic

   Since OR means either condition true passes, and both are true here, the whole condition is true.

3. Final Answer:

   Check passed -> Option A

4. Quick Check:

   True OR True = True [OK]

Hint: OR (-o) passes if any condition is true [OK]

Common Mistakes:

• Thinking ! -r means readable
• Confusing -o with -a
• Expecting syntax error due to !

4. Identify the error in this bash condition:

if [ !-f myfile.txt -a -r myfile.txt ]; then echo "Valid"; fi

medium

A. Missing spaces around ! operator

B. Incorrect use of -a inside [ ]

C. No error, condition is correct

D. Missing closing bracket ]

Solution

1. Step 1: Check spacing around ! operator

   In bash, ! must have a space after it inside [ ]. The code [ ! -f myfile.txt -a -r myfile.txt ] is correct only if spaces are present. If missing, it causes error.

2. Step 2: Confirm other syntax parts

   The use of -a inside [ ] is valid. The closing bracket is present. So no error there.

3. Final Answer:

   Missing spaces around ! operator -> Option A

4. Quick Check:

   ! needs spaces inside [ ] [OK]

Hint: Always put spaces around ! inside [ ] [OK]

Common Mistakes:

• Using && or || inside [ ]
• Forgetting spaces around operators
• Assuming -a is invalid inside [ ]

5. You want to write a bash condition that checks if a variable var is NOT empty AND file1.txt exists OR file2.txt is readable. Which condition is correct?

hard

A. [ -n "$var" -a ( -e file1.txt -o -r file2.txt ) ]

B. [ -n "$var" -a -e file1.txt -o -r file2.txt ]

C. [ -z "$var" -a -e file1.txt -o -r file2.txt ]

D. [ -n "$var" -o -e file1.txt -a -r file2.txt ]

Solution

1. Step 1: Understand the logic to implement

   We want: var NOT empty AND file1 exists OR file2 readable, i.e., (NOT empty AND file1 exists) OR file2 readable.

2. Step 2: Check each option's logic and syntax

   A: Parentheses for grouping invalid inside [ ], syntax error.
   B: [ -n "$var" -o -e file1.txt -a -r file2.txt ] wrong logic: mixes OR then AND.
   C: [ -z "$var" -a -e file1.txt -o -r file2.txt ] uses -z (empty), wrong for NOT empty.
   D: [ -n "$var" -a -e file1.txt -o -r file2.txt ] correct: left-to-right ((non-empty -a exists) -o readable).

3. Final Answer:

   [ -n "$var" -a -e file1.txt -o -r file2.txt ] -> Option B

4. Quick Check:

   -a/-o left-to-right no grouping needed [OK]

Hint: -a/-o associate left-to-right inside [ ] [OK]

Common Mistakes:

• Trying to group with ( ) invalid in [ ]
• Mixing -o and -a without understanding associativity
• Using -z for non-empty check