According to https://unix.stackexchange.com/a/489913/674

cron jobs can run as any user, without that user being logged in.

root doesn’t need to log in to start the init process, thankfully (imagine handling a fleet of thousands of servers and millions of VMs otherwise);

If I want to run a process, with me as its owner, without logging in, how can I do that at both system/library call level and utility level?

If root wants to do that, how can it do it?

How can a service user which can't log in start a process as its owner or become its owner later?

Is the only way to call setuid() or seteuid() in the program run by the process?


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    I do not understand the point of the question. Where is the complication or novelty of a user setting up his own crontab? – Rui F Ribeiro Dec 19 '18 at 13:49
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    crontab -u user -e – RubberStamp Dec 19 '18 at 13:53
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    A process doesn't have an owner until it's started, so that part of the question should be reworded, I think. – Jeff Schaller Dec 19 '18 at 13:53
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    Possible duplicate of su options - running command as another user – muru Dec 19 '18 at 13:56
  • >If I want to run a process ~as~ and become its owner without logging in .... The question is now a completely different topic. – RubberStamp Dec 19 '18 at 13:59

There are 3 ways to change user of a process in Unix.

2 system level ways to change user of a process

  • if the process has capability CAP_SETUID, traditionally root has this capability (and all other capabilities), then it can use setuid, setreuid, setresuid, setfsuid, system calls, to change to any other user. Any other user can shuffle uids: A process has 3 uids, it can move them around, at will: it can swap them, or remove them until it is down to one. It can not add uids, unless it has capability CAP_SETUID. In general a process can only loose privileges or move them around, using these system calls. These calls allow the program to continue.
  • exec a suid executable: If an executable file has its suid bit set, and if it is of a valid type (not a scripts, not java, not …), then when it is run, its effective user id is changed to that of the files owner. (same can be done for group with sgid bit). This is the only way to gain privileges. The current program ends when exec is called, it is replaced with the new program, but it is the same process, it also inherits open files (e.g. stdin, stdout, stderr).

fork dose not change user. A forked process is an exact duplicate of its parent, with a few exceptions (see man fork). In particular the uid, gid, and capabilities are not changed.

Utility methods

These programs use the 2 system methods described above.

  • use sudo or su:
    • su will ask for the password of the other user.
    • sudo will ask for your password, but will only work if you are registered in the sudoers file.

sudo, su, login, cron etc use the 2 system methods. (And will create a new process. The other system methods do not create a new process.)

What does sudo, su do?

#↳ ll /usr/bin/sudo
-rwsr-xr-x 1 root root 155K Sep  9  2017 /usr/bin/sudo*

As use can see the sudo executable is owned by root, and has the suid bit set (the s, where you would expect to see the first x).

When sudo is run, it runs as root (don't try this, unless you know what you are doing). It then does security checks. Then it uses set??uid to become the required user, it then execs (and maybe a fork) the required program.

Running a process, without logging in

Use some timed start service.

  • cron
  • at

Send a network message, e.g. a web-server may run a task in response to a web request.

Use automated login: use ssh to launch a process, via a script on another machine.

  • Thanks. Would like to know more about how "Send a network message, e.g. a web-server may run a task in response to a web request" starts a new process with a different user? – Tim Dec 20 '18 at 4:05
  • I believe root process can drop this capability, I know it's nit-picking, but I think it would be better without mentioning "root has this", after all, capability set in indepent from process UIDs, right? – 炸鱼薯条德里克 Dec 20 '18 at 4:41
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    @Tim Your (later) edits to the answer are confusing. First of all, a process can never be "not running". Secondly, the whole Running a process, without logging in, means changing the user of a running process, without invoking the login program. (?) reads like a question that should have been a comment. – Kusalananda Dec 20 '18 at 7:28
  • @Tim additionally, login isn’t often involved when logging in; for example, logging in using a display manager, or connecting with SSH, doesn’t involve login. – Stephen Kitt Dec 20 '18 at 8:41
  • I reverted @Tims edits, and then incorporated the bits that were useful: changed layout of answer, so it maps better to the question. (there were many bits that were incorrect or misleading). – ctrl-alt-delor Dec 20 '18 at 19:47

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