Which method allows you to merge lines of files horizontally?

The method that allows you to merge lines of files horizontally is the command 'paste'. This command is specifically designed to concatenate corresponding lines from multiple input files side by side. Each line from the first file is joined with the line at the same line number from the second file, with a tab character by default separating the contents.

For example, if you have two files, one containing:

line1a
line2a

and another containing:

line1b
line2b

Using the 'paste' command on these two files would yield:

line1a   line1b
line2a   line2b

This horizontal merging is effective for viewing data in a tabular format and is frequently used in shell scripting for tasks that require combining output from multiple sources.

The other commands listed serve different purposes: 'tr' is used for translating or deleting characters, 'rmdir' is for removing empty directories, and 'updatedb' is used to update the database used by the 'locate' command for searching files. Thus, they do not perform the task of merging lines from files side by side.