I wonder: Why are there these two commands on Debian Linux, with very similar names and functionality? This has always been confusing for me.
useradd is native binary compiled with the system. But, adduser is a perl script which uses useradd binary in back-end.
adduser is more user friendly and interactive than its back-end useradd. There's no difference in features provided.
The main advantage to adduser over useradd is dealing with system level users. With adduser the system level users account when created puts a user directory in home for the system user where useradd does not automatically. If you read the man they are essentially the same except for system level users and associated home directories are automatic. adduser creates a /home/user directory automatically for system level users where there is not a provision in useradd, except if specified by -m option. The other readings I have come across generically state that UID and GUID are assigned by accepted conformal Debian standards in adduser.
In a couple of Redhat instances I checked (4.9 from 2011 and 6.9 from 2017), adduser is simply an alias to useradd, not a Perl script. Here it is in Redhat 6.9:
$ ls -la /usr/sbin/useradd /usr/sbin/adduser
lrwxrwxrwx. 1 root root 7 Nov 2 2016 /usr/sbin/adduser -> useradd
-rwxr-x---. 1 root root 111320 Feb 9 2016 /usr/sbin/useradd
The way I remember that useradd is the 'correct' program is this: adding a user is just one operation in the CRUD spectrum. You also need 'modify' and 'delete' operations ('read' is presumably covered by viewing /etc/passwd). And so, the programs are named user* (useradd, usermod and userdel respectively). I suppose the alternative *user naming convention (adduser, moduser and deluser -- these don't exist) could also work. But it would be slightly more awkward (for a reason I cannot express clearly at the moment :)
One significant difference I ran into (and did not see in an answer here), is that useradd --system seems to imply --shell /bin/bash, while adduser --system implies --shell /usr/sbin/nologin, which is what I wanted.
But I also wanted no home directory (which is the default with useradd but not with adduser), so I could either use useradd like this
useradd --system --shell /usr/sbin/nologin foo
(or /bin/false is also popular for no shell)
Which leads to e.g. the following entry in /etc/passwd
foo:x:994:991::/home/foo:/usr/sbin/nologin
Or use adduser like this
adduser --system --no-create-home foo
Which leads to e.g. the following entry in /etc/passwd
foo:x:110:65534::/home/foo:/usr/sbin/nologin
Note that there are also very different automatic assignment of (G)UIDs. useradd seems to pick them counting downwards from 999, while adduser counts updwards from 100 for the UID and uses 65534 for the GUID (which seems to mean no group). In fact, useradd also creates a group (see /etc/group), while adduser doesn't.
So for me, I will stick with adduser having learnt all those facts.
Source: Tested on Raspberry Pi OS (Debian Buster based).
Within raspian (I would need to check for other OSs), 'adduser USER' starts a dialog (TUI) while 'useradd USER' does NOT.
Apparently, using adduser, once the dialog comes up, the full 'useradd' command is already executed. Even if you CTRL-C your user is there.
As in the new systems they create "personal" groups, to remove the newly created user you would need to:
I assume that because adduser "uses" useradd, the rest is metadata about the user. So technically no changes in "skel", "file masks", or the like, i.e. your system defaults.
On the other side, because useradd does not bring up any TUI, it might be more useful in scripting.
To be honest I tested it only in rasbian, because that was the reason I stepped on the question.
I can imagine that other OSs might implement a different version of adduser.
