Is a comment line allowed in /etc/passwd

Question

I know, in RHEL 7.8, that you can have blank lines in /etc/passwd

But can you have comment lines, and if so what would the line start with?

Answer 1

The musl C library routines for processing the account database, getpwent() et al., do not permit any sort of commentary in the /etc/passwd or /etc/shadow files. The library code simply does not treat any character as a comment introducer. (It also does not recognize a + line as a YP flag or permit any special markers at all.)

If you have any programs built against the musl C library, do not attempt adding comments to these files.

The GNU C library and FreeBSD C library, however, both do permit commentary.

Or, rather:

The GNU C library code providing the files sources of all NSS databases (see the nsswitch.conf manual) permits line comments with a # as the first non-whitespace character. So more than just /etc/passwd and /etc/shadow can contain comments.

On the BSDs, the /etc/passwd file is not the account database. On the BSDs the account database is not a flat file database, but is a Berkeley DB file (two, in fact, /etc/pwd.db and /etc/spwd.db), which by the nature of Berkeley DB files does not permit commentary. /etc/master.passwd is compiled into the account database(s) by the pwd_mkdb tool.

The /etc/passwd file on the BSDs is a dummy backwards-compatibility file emitted by pwd_mkdb, not actually used for lookups by C library routines and there only for people with shell scripts using direct awk /etc/passwd and the like, rather than the better getent passwd command (which wraps the C library for shell scripts).

On FreeBSD and its derivatives, /etc/master.passwd can contain comments and the pwd_mkdb tool copies commentary through from /etc/master.passwd to /etc/passwd, without putting it into the main Berkeley DB files. The # can optionally be preceded by exactly only TAB and SPC characters, which is not quite the same as the GNU C library (which allows any whitespace character that satisfies isspace() in the locale of the process using the GNU C library routines, which is probably a very obscure it-is-sometimes-a-comment-sometimes-a-secret-superuser-record bug waiting to happen).

OpenBSD's and NetBSD's pwd_mkdb commands, in contrast, do not allow for commentary in /etc/master.passwd and do not insert comments into the dummy /etc/passwd.

So you can use # comments preceded by TAB and SPC in /etc/master.passwd on FreeBSD; and # comments precended by the current locale's isspace() characters in /etc/passwd, /etc/shadow, and a whole bunch of other NSS sources on Linux-based operating systems as long as all (relevant) programs are linked against the GNU C library.

Answer 2

Only user entries are allowed in /etc/passwd, anything else is considered malformed and causes some tools that depend on this file to fail.

TLDR: No

Answer 3

Each record in /etc/passwd describes one user account and comment lines are not supported.

See passwd(5) (man 5 passwd) for the file format.