Day 04 · Linux

Script Execution Permissions

By Published Linux~1h to complete

Technical Overview

In Linux operating systems, security is enforced through file permissions. Each file has permission bits grouped into three sets of owners: Owner (user), Group, and Others (all other users). Each set can have Read (r), Write (w), and Execute (x) privileges.

For a shell script (e.g., a Bash script) to run successfully, two conditions must be met:

  1. Execute Permission (x): The operating system must be allowed to execute the file as a program.
  2. Read Permission (r): The shell interpreter (e.g., /bin/bash) must be able to open and read the commands written inside the script to execute them.

A common system administration pitfall is granting only execute permissions (chmod +x) while leaving read permissions restricted. This leads to a Permission denied error when standard users attempt to run the script, because the interpreter cannot read the command sequence inside the file. To make a script universally executable, it must be given both read and execute permissions for all users (r-x).

This guide details the steps to modify file permissions on /tmp/xfusioncorp.sh using both symbolic and numeric (octal) notation, and verify successful script execution.

Infrastructure & Configuration Requirements

  • Target Host: Nautilus App Server (e.g., stapp01, stapp02, stapp03)
  • SSH User: Standard administrative user (e.g., tony, steve, banner)
  • Script Path: /tmp/xfusioncorp.sh
  • Target Permissions: Read and execute (r-x) for all users (Owner, Group, and Others)

Step-by-Step Implementation

Step 1: Connect to the Target Host

SSH into the assigned application server from the Jump Host:

# Example for App Server 1
ssh tony@stapp01

Step 2: Inspect Current Script Permissions

Locate the script and inspect its current owner, group, and permissions using the long listing command:

ls -l /tmp/xfusioncorp.sh

Example starting output showing restricted permissions:

-r-------- 1 root root 128 Jun 27 11:30 /tmp/xfusioncorp.sh

(In this example, only the root owner can read the file; no other users have read or execute access.)

Step 3: Grant Read and Execute Permissions

To allow all users to read and execute the script, use the chmod utility. You can apply this using either symbolic or numeric mode:

Add read (r) and execute (x) permissions to all (a) users:

sudo chmod a+rx /tmp/xfusioncorp.sh

Alternatively, you can target specific categories: sudo chmod u+rx,g+rx,o+rx /tmp/xfusioncorp.sh.

Option B: Numeric (Octal) Mode (Sets explicit absolute permissions)

Assign 7 (read, write, execute) to the owner, and 5 (read, execute) to the group and others:

sudo chmod 755 /tmp/xfusioncorp.sh

Step 4: Verify the Updated Permissions

Verify that the permission bits have been updated correctly:

ls -l /tmp/xfusioncorp.sh

Expected output structure:

-rwxr-xr-x 1 root root 128 Jun 27 11:30 /tmp/xfusioncorp.sh

The permission string rwxr-xr-x confirms:

  • User (owner): rwx (read, write, execute)
  • Group: r-x (read, execute)
  • Others: r-x (read, execute)

Post-Deployment Verification

1. Test Script Execution

Attempt to execute the script directly from the shell path:

/tmp/xfusioncorp.sh

Or navigate to /tmp and run it:

cd /tmp
./xfusioncorp.sh

If the permissions are correct, the script should execute successfully and output its designated text or side-effect without throwing a Permission denied error.

Log out of the Application Server to return to the Jump Host:

exit

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