Process Groups vs Job Control – Key Differences

job-controlprocessprocess-groups

What's the difference between a process group and a job? If I type pr * | lpr then is it both a process group as well a job?
What exactly is the difference between a process group ID and a job ID?

Edit: I know it appears similar to What is the difference between a job and a process?, but it is slightly different. Also, I didn't understand this concept from this thread.

Best Answer

A process group is a unix kernel concept. It doesn't come up very often. You can send a signal to all the processes in a group, by calling the kill system call or utility with a negative argument.

When a process is created (with fork), it remains in the same process group as its parent. A process can move into another group by calling setpgid or setpgrp. This is normally performed by the shell when it starts an external process, before it executes execve to load the external program.

The main use for process groups is that when you press Ctrl+C, Ctrl+Z or Ctrl+\ to kill or suspend programs in a terminal, the terminal sends a signal to a whole process group, the foreground process group. The details are fairly complex and mostly of interest to shell or kernel implementers; the General Terminal Interface chapter of the POSIX standard is a good presentation (you do need some unix programming background).

Jobs are an internal concept to the shell. In the simple cases, each job in a shell corresponds to a process group in the kernel.

Related Solutions

Process Terminology – Difference Between a Job and a Process

A process is any running program with its own address space.

A job is a concept used by the shell - any program you interactively start that doesn't detach (ie, not a daemon) is a job. If you're running an interactive program, you can press CtrlZ to suspend it. Then you can start it back in the foreground (using fg) or in the background (using bg).

While the program is suspended or running in the background, you can start another program - you would then have two jobs running. You can also start a program running in the background by appending an "&" like this: program &. That program would become a background job. To list all the jobs you are running, you can use jobs.

For more information on jobs, see this section of the bash man page.

Files – Difference Between Owner/Root and RUID/EUID

Here are the answers:

root has always full access to files and directories. The owner of the file usually has them too, but this is not always true. For example:
```
-r-xr----- 1 user1 users 199 Oct 14 18:42 otherfile.bin
```
user1 is the owner; however they can only read and execute, but root still has full access (rwx) to the file.
RUID is the Real User ID and it never (almost) changes. If user2 logs in to the system, the shell is then launched with its real ID set to user2. All processes they start from the shell will inherit the real ID user2 as their real ID.

EUID is the Effective User ID, it changes for processes (not for the user) that the user executes that have set the setuid bit.

If user2 executes file.bin, the RUID will be user2 and the EUID of the process started will be user1.

Let's use the case of passwd:

-rwsr-xr-x 1 root root 45396 may 25  2012 /usr/bin/passwd

When user2 wants to change their password, they execute /usr/bin/passwd.
The RUID will be user2 but the EUID of that process will be root.
user2 can use passwd to change only their own password because internally passwd checks the RUID and, if it is not root, its actions will be limited to real user's password.
It's neccesary that the EUID becomes root in the case of passwd because the process needs to write to /etc/passwd and/or /etc/shadow.

Best Answer

Related Solutions

Process Terminology – Difference Between a Job and a Process

Files – Difference Between Owner/Root and RUID/EUID

Related Question