What command changes file permissions in Linux?

The Linux command that changes file permissions is `chmod`, short for “change mode.” It modifies a file or directory’s permission bits and certain special mode flags, controlling access for the owner, the owning group, and other users.

Permissions can be written symbolically or numerically. For example, `chmod u+x script.sh` adds execute permission for the owner, while `chmod 644 notes.txt` gives the owner read/write access and gives group and others read access. The familiar read, write, and execute values are represented numerically as 4, 2, and 1, respectively.

A frequent mistake is confusing permissions with ownership. `chown` changes the owner, and `chgrp` changes the owning group; neither is the general command for changing permission bits. `umask` is different again: it limits default permissions when new files are created. On Unix-like systems, changing a file’s permissions generally requires being its owner or the superuser. Directory permissions also have special behavior: execute means users may traverse the directory, so granting read access alone does not provide full access to its contents.