Which of the following commands will not modify or delete files?

The command that will not modify or delete files is updatedb. This command is primarily used to update the database utilized by the locate command, which helps in quickly searching for files within the filesystem. It does this by scanning the specified directories for files and updating its records, but it does not change or delete any of the files found during this process.

In contrast, the other options perform functions that can modify or delete files. For instance, the tr command is used for translating or deleting characters from input data, potentially altering the contents of files when redirected. The rmdir command is specifically designed to delete empty directories. The paste command merges lines of files together, which can involve creating new files or modifying existing ones based on its usage.

Thus, updatedb stands out as it strictly performs the function of database maintenance without altering or deleting files themselves.