Let's circle back to our testfile. It has the following permissions: -rw-rw----. Read/writable by user and group, readable by others. While these permissions might be fine for most files, they are definitely not a great fit for all files. What about private files? You would not want those to be readable by everyone, perhaps not even by group members.
The Linux command to change permissions on a file or directory is chmod, which we like to read as change file mode. chmod has two operating modes: symbolic mode and numeric/octal mode. We will begin by explaining symbolic mode (which is easier to understand), before we move to octal mode (which is faster to use).