Checks are made to verify that each entry has:
The checks for correct number of fields and unique group name are fatal. If the entry has the wrong number of fields, the user will be prompted to delete the entire line. If the user does not answer affirmatively, all further checks are bypassed. An entry with a duplicated group name is prompted for deletion, but the remaining checks will still be made. All other errors are warnings and the user is encouraged to run the groupmod command to correct the error.
The commands which operate on the /etc/group file are not able to alter corrupted or duplicated entries. grpck should be used in those circumstances to remove the offending entry.
By default, grpck operates on the files /etc/group and /etc/gshadow. The user may select alternate files with the group and shadow parameters. Additionally, the user may execute the command in read-only mode by specifying the -r flag. This causes all questions regarding changes to be answered no without user intervention. grpck can also sort entries in /etc/group and /etc/gshadow by GID. To run it in sort mode pass it -s flag. No checks are performed then, it just sorts.
group(5), passwd(5), shadow(5), groupmod(8).
The grpck command exits with the following values: