What permissions are needed for a file to run in Linux? How does the permission check mechanism work before execution?

The following permissions are required to run a file in Linux:

1. The file must have the execute bit set for the user, group, or others, depending on who is running the file.
2. The user running the file must have the appropriate permissions on the file (execute) and access to the directory where it is located.

Mechanism of permission checking during execution:

• When a user attempts to run a file, the kernel checks if the user has execute permission for the file.
• If the execute bit is set for the relevant category (user, group, or others), and the user belongs to that category, execution is permitted.
• If the file is a script, the kernel runs the interpreter specified in the shebang (the first line of the file, e.g., #!/bin/bash).
• If the file is a binary executable, the kernel loads it into memory and begins execution.

Example of permission check:

ls -l myscript.sh
# -rwxr-xr-- 1 user group 1234 Jun 1 12:00 myscript.sh

Here, the owner (user) and group (group) have execute rights, others do not.

If the execute bit is not set, attempting to run the file will result in a "Permission denied" error.