Making scripts executable (chmod +x) in Bash Scripting - Time & Space Complexity

Time Complexity: Making scripts executable (chmod +x)

O(n)

Understanding Time Complexity

Let's explore how the time it takes to make a script executable changes as the number of scripts grows.

We want to know how the work increases when we use chmod +x on many files.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

for file in *.sh; do
  chmod +x "$file"
done
    

This script loops over all files ending with .sh and makes each one executable.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The chmod +x command inside the loop.
• How many times: Once for each .sh file found.

How Execution Grows With Input

As the number of script files grows, the total time grows in a similar way.

Input Size (n)	Approx. Operations
10	10 times running chmod +x
100	100 times running chmod +x
1000	1000 times running chmod +x

Pattern observation: The work grows directly with the number of files; double the files, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to make scripts executable grows linearly with the number of files.

Common Mistake

[X] Wrong: "Running chmod +x on many files takes the same time as on one file."

[OK] Correct: Each file needs its own permission change, so more files mean more time.

Interview Connect

Understanding how simple loops affect execution time helps you explain script performance clearly and confidently.

Self-Check

"What if we used a single chmod +x *.sh command instead of a loop? How would the time complexity change?"

Practice

1. What does the command chmod +x script.sh do in bash scripting?

easy

A. Makes the script script.sh executable so it can be run directly.

B. Deletes the script script.sh from the system.

C. Copies the script script.sh to another location.

D. Changes the owner of the script script.sh.

Solution

1. Step 1: Understand the chmod command

   The chmod command changes file permissions in Linux and Unix systems.

2. Step 2: Interpret the +x option

   The +x option adds execute permission, allowing the file to be run as a program.

3. Final Answer:

   Makes the script script.sh executable so it can be run directly. -> Option A

4. Quick Check:

   chmod +x means add execute permission [OK]

Hint: Remember +x means add execute permission [OK]

Common Mistakes:

• Confusing execute permission with deleting files
• Thinking chmod copies files
• Mixing up ownership change with permission change

2. Which of the following is the correct syntax to make a script named run.sh executable?

easy

A. chmod x run.sh

B. chmod x+ run.sh

C. chmod -x run.sh

D. chmod +x run.sh

Solution

1. Step 1: Recall the correct order of options in chmod

   The correct syntax to add execute permission is chmod +x filename.

2. Step 2: Check each option

   chmod +x run.sh matches the correct syntax exactly; others have incorrect order or missing symbols.

3. Final Answer:

   chmod +x run.sh -> Option D

4. Quick Check:

   Correct syntax is chmod +x filename [OK]

Hint: Use chmod +x filename to add execute permission [OK]

Common Mistakes:

• Placing + and x in wrong order
• Using -x which removes execute permission
• Omitting the + sign

3. Given a script hello.sh without execute permission, what will be the output of the command sequence below?

ls -l hello.sh
chmod +x hello.sh
ls -l hello.sh

medium

A. Both ls -l commands show execute permission.

B. The first ls -l shows no execute permission; the second shows execute permission added.

C. The first ls -l shows execute permission; the second removes it.

D. Both ls -l commands show no execute permission.

Solution

1. Step 1: Understand initial permissions

   Initially, hello.sh has no execute permission, so ls -l shows no 'x' in permissions.

2. Step 2: Apply chmod +x and check again

   After running chmod +x hello.sh, execute permission is added, so the second ls -l shows 'x' in permissions.

3. Final Answer:

   The first ls -l shows no execute permission; the second shows execute permission added. -> Option B

4. Quick Check:

   Adding +x updates permissions visible in ls -l [OK]

Hint: Check permissions before and after with ls -l [OK]

Common Mistakes:

• Assuming execute permission is present initially
• Thinking chmod +x removes execute permission
• Not noticing permission changes in ls -l

4. You try to run a script myscript.sh but get a "Permission denied" error. Which command fixes this issue?

medium

A. chmod -x myscript.sh

B. chmod +r myscript.sh

C. chmod +x myscript.sh

D. chmod 000 myscript.sh

Solution

1. Step 1: Identify cause of "Permission denied"

   The error usually means the script lacks execute permission.

2. Step 2: Add execute permission

   Using chmod +x myscript.sh adds execute rights, allowing the script to run.

3. Final Answer:

   chmod +x myscript.sh -> Option C

4. Quick Check:

   Add execute permission to fix "Permission denied" [OK]

Hint: Add execute permission with chmod +x [OK]

Common Mistakes:

• Using -x which removes execute permission
• Setting permissions to 000 which denies all access
• Adding only read permission (+r) which doesn't allow execution

5. You have a script deploy.sh that you want to run directly by typing ./deploy.sh. However, it still says "Permission denied" after chmod +x deploy.sh. What could be the reason?

hard

A. The script is on a mounted filesystem that does not support execute permissions.

B. You forgot to add read permission with chmod +r deploy.sh.

C. You need to rename the script to deploy.exe.

D. You must run the script with bash deploy.sh instead of ./deploy.sh.

Solution

1. Step 1: Confirm execute permission is set

   Since chmod +x deploy.sh was run, the script should have execute permission.

2. Step 2: Consider filesystem limitations

   Some mounted filesystems (like FAT32 or network shares) do not support Unix execute permissions, causing "Permission denied" despite chmod +x.

3. Step 3: Evaluate other options

   Adding read permission is usually default; renaming to .exe is irrelevant in Linux; running with bash works but does not explain the permission error when running ./deploy.sh.

4. Final Answer:

   The script is on a mounted filesystem that does not support execute permissions. -> Option A

5. Quick Check:

   Filesystem may block execute permission despite chmod [OK]

Hint: Check if filesystem supports execute permissions [OK]

Common Mistakes:

• Assuming chmod always works on all filesystems
• Thinking read permission alone allows execution
• Confusing script name extension with execution ability