uchg

uchg is a user-immutable file flag used in BSD-derived operating systems, including macOS. It is one of several file flags that can be set or cleared with the chflags system call. The flag indicates that the file is immutable from the perspective of ordinary users.

When the uchg flag is set on a file, its contents and most metadata cannot be modified, renamed, moved, or deleted by non-privileged processes. In practice, only privileged users, typically the superuser, can clear the flag and make changes to the file again. This provides protection against accidental edits or deletions.

Setting and clearing the flag is done with the chflags command. To set the flag on a file, use: chflags uchg filename. To clear it, use: chflags nouchg filename. The flag can be applied recursively to a directory with chflags -R uchg directory, and removed with chflags -R nouchg directory. The exact syntax may vary slightly by system, so consulting the local man page for chflags is recommended.

Viewing the current flags can be done with a long listing that shows file flags, for example using ls with the appropriate options (such as ls -lO on many BSD-derived systems). If uchg is present, the listing will indicate it.

Related concepts include the system-immutable flag schg, which is a stronger protection at the system level. The uchg flag is not universal on Linux, which uses different attributes (such as chattr +i). Overall, uchg serves as a practical mechanism to prevent unintended modification of important files on BSD-like systems.