What are the numbers in /etc/passwd? (and is there a way to find answers to these kind of questions without searching the internet)?

Question

If i run grep user /etc/passwd, i get the string user:x:1021:1021::/home/user:/bin/bash. What does the numbers "1021:1021" mean? And is there a offline way to find what these mean? If i do man passwd i get information about the command, not the file

Answer 1

man 5 passwd:

/etc/passwd contains one line for each user account, with seven fields delimited by colons (“:”). These fields are:

• login name
• optional encrypted password
• numerical user ID
• numerical group ID
• user name or comment field
• user home directory
• optional user command interpreter

Well, that's pretty much it.

_{(man7.org has two versions of that man page, the above is the one I had on Debian. Luckily, there's really no variation in the contents of passwd or shadow, at least on Linuxen, I think. )}

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The meaning of that 5 is described by e.g. the man page for man itself, it tells that the manual "sections" are:

1   Executable programs or shell commands
2   System calls (functions provided by the kernel)
3   Library calls (functions within program libraries)
4   Special files (usually found in /dev)
5   File formats and conventions eg /etc/passwd
6   Games
7   Miscellaneous (including macro packages and conventions),
    e.g. man(7), groff(7)
8   System administration commands (usually only for root)

You're looking for the file passwd, so section 5 it is.

crontab is similar that it's a command (crontab(1)) and a file (crontab(5)). Also, open appears to have man pages for a command (in Linux, alias for openvt(1)), a system call (open(2)), and a Perl pragma (open(3perl)). Section numbers with tailing text like 3perl are also somewhat common.

Use whatis foo to find out if there's more than one match:

$ whatis passwd
passwd (1)           - change user password
passwd (1ssl)        - compute password hashes
passwd (5)           - the password file

_{(oh, right, there's also openssl passwd)}

Then there's apropos, which "[searches] the manual page names and descriptions". Usually giving a lot more hits.

See also: What do the numbers in a man page mean?

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If you have multiple chapters where the same command/config file appears, this is usually mentioned in the man page. E.g. at the bottom of man passwd one will find

SEE ALSO

chpasswd(8), passwd(5), shadow(5), usermod(8).

hinting to a chapter 5 entry for passwd. And in reverse man 5 passwd has

SEE ALSO

crypt(3), getent(1), getpwnam(3), login(1), passwd(1), pwck(8), pwconv(8), pwunconv(8), shadow(5), su(1), sulogin(8).