Monthly Archives: February 2016

Backgrounding in Linux

Ctrl-Z + bg

I used to use the trick of hitting Ctrl-Z when a long running process in running in the terminal and then entering bg to background it.


But, I have a new favorite way to background using nohup. Lets say I am running a program like one that calculates pi called pi, for example. I can enter…

nohup pi &

…and it keeps the process running even if I kill the parent process, as in  the shell that I launched it from, by closing it. All of the output from the program winds up in the directory in a file called nohup.out.

Stopping the backgrounded program

If I really want to stop the program, I can do it via kill.

First find the process ID (PID) using…

ps aux | grep pi

…what I get is this…

erick 2315 89.5 0.6 10884 8116 pts/1 R+ 18:24 0:10 pi 10000000

The PID is the first number, in this case 2315. So I will use…

kill 2315

…and this process will be killed gracefully by sending a SIGTERM to ask the process to stop, just like a Ctrl-C would do from the command prompt.

Forcing a backgrounded program to stop

If a process won’t stop when you issue a plain kill (SIGTERM) you an send a sigspec, specifying the type of kill set to KILL to force it to quit…

kill -s KILL 2315

…will shut the program down immediately without saving to disc or whatever the program might do to clean up ( might not free up memory that was MALLOC’d for instance ) when it shuts down.

You can see the difference if you run a process in one terminal and use kill in another.

kill 2315 gives you…

erick@erick-laptop:~/pi$ pi 100000000


kill -s KILL 2315 gives you…

erick@erick-laptop:~/pi$ pi 100000000
Screenshot - 02162016 - 09:32:30 PM

Users, Groups and Sudo

One thing that I did once I got my Raspberry Pi up and running is to add a user account, other than the pi account that is there by default. I have an erick account on my machines, so why not have one on the pi.


So under the default pi prompt I used the useradd command to add erick as a user. I figured that I would not login as pi and gave pi a strong password.

useradd -m erick

This will prompt for a password and make a user directory under homes by default. It also fills the directory with files and directories based on the /etc/skel directory.

Lots of Options for useradd

Usage: useradd [options] LOGIN

-b, –base-dir BASE_DIR       base directory for the home directory of the
new account
-c, –comment COMMENT         GECOS field of the new account
-d, –home-dir HOME_DIR       home directory of the new account
-D, –defaults                print or change default useradd configuration
-e, –expiredate EXPIRE_DATE  expiration date of the new account
-f, –inactive INACTIVE       password inactivity period of the new account
-g, –gid GROUP               name or ID of the primary group of the new
-G, –groups GROUPS           list of supplementary groups of the new
-h, –help                    display this help message and exit
-k, –skel SKEL_DIR           use this alternative skeleton directory
-K, –key KEY=VALUE           override /etc/login.defs defaults
-l, –no-log-init             do not add the user to the lastlog and
faillog databases
-m, –create-home             create the user’s home directory
-M, –no-create-home          do not create the user’s home directory
-N, –no-user-group           do not create a group with the same name as
the user
-o, –non-unique              allow to create users with duplicate
(non-unique) UID
-p, –password PASSWORD       encrypted password of the new account
-r, –system                  create a system account
-s, –shell SHELL             login shell of the new account
-u, –uid UID                 user ID of the new account
-U, –user-group              create a group with the same name as the user
-Z, –selinux-user SEUSER     use a specific SEUSER for the SELinux user mapping


Once I was able to log in under my new account, I tried setting up fswebcam to collect some timelapse video and then I had my first hitch. I needed to be part of the video group to run fswebcam.


The id command ran from the command line…

id username

…will list not only the user and group ID of the user UID and GID. But all of the groups that he user belongs to. It has the options -u, -g, -G. -u lists the UID alone, -g is the users GID alone and -G lists all the group ID’s that the user belongs to.


I was not part of the video group so I would have to add myself, but I was not part of the admin group either so I was not able to even run sudo.

So with a quick logout to the pi user, I was able to add myself to the admin group.

sudo usermod -a -G admin erick

The sudoers file

If you run sudo visudo, it will open the /etc/sudoers.tmp file. At the bottom of this file there is a line that explains that accounts added to the admin group are allowed to run sudo.

# Members of the admin group may gain root privileges
 %admin ALL=(ALL) ALL

Now that I can sudo from my own account, I can login back in as erick and run…

sudo usermod -a -G video erick

…to add myself to the video group. Now I was off and running with using fswebcam under my account.

NFS and Users

With users there is the notion of the name and then there is the numerical UID. NFS uses the numerical UID to map across machines. If you plan on using NFS on multiple machines, it pays to keep the UID’s lined up between them. For example, if you set up 2 Linux machines from scratch, there will be a user at UID 1000, that would be you, whatever you called it by name. The first user is at 1000. If you use NFS to mount a directory from one machine to another, no problem it all lines up. The user at UID 1000 is the same on both machines, permissions work out, files can be moved back and forth, no problems.

But if like with the example of the Raspberry Pi above. User pi is created on the NOOBS Disk when you load the Raspberian option. It is at UID 1000 and GID 1000. So if you add and other user for yourself, guess what it appears at UID 1001. Something to keep in mind when using NFS. You can use NFS in a way that will get around this using the methods laid out in the NFS post.

But it is much easier to try to keep all of the name and UID’s lined up from the beginning and not have to worry about the trickiness business. Even it means adding a user to the Raspberry Pi and then moving the UID of the pi user to some other UID and yourself to UID 1000, GID 1000 if that will line it up with your other machines on the network.