Even a simple Linux system is usually doing a lot of things at once.
ps lists processes. If you run it with no options, it'll show you the processes running under your current login.
pi@raspberrypi ~ $ ps PID TTY TIME CMD 30054 pts/2 00:00:00 bash 30116 pts/2 00:00:00 ps
The PID column lists a process ID. Every running program on the system will have one of these. If a process is stuck otherwise bothersome, you can stop it with kill [process ID].
If you want to see more detail about all of the things, try ps aux:
ps takes a gazillion different options in a confusing array of different formats, but memorizing one or two of them is usually good enough to give you a window into what's happening.
If you're looking for a specific program, you might try grepping the output of ps. Let's say I wanted to know about tmux sessions running on my Raspberry Pi. One approach might be ps aux | grep tmux:
Alternatively, you can use pgrep, which is specifically for finding process IDs:
pi@raspberrypi /proc/16514 $ pgrep -l tmux 16513 tmux 22350 tmux 30247 tmux
(The -l is for listing process names along with the PID.)
Under Linux, ps works by reading the files in /proc, which is a virtual filesystem full of info about running processes and suchlike things. If you know the id of a running process, you can inspect all sorts of info about it like so:
pi@raspberrypi ~ $ ps PID TTY TIME CMD 16514 pts/1 00:00:01 bash 30268 pts/1 00:00:00 ps pi@raspberrypi ~ $ cd /proc/16514 pi@raspberrypi /proc/16514 $ ls autogroup exe mountstats sched auxv fd net smaps cgroup fdinfo ns stack clear_refs io oom_adj stat cmdline limits oom_score statm comm maps oom_score_adj status coredump_filter mem pagemap syscall cwd mountinfo personality task environ mounts root wchan
ps is useful for checking on the state of a running system or writing a shell pipeline, but often what you want is something more like an interactive monitor. Enter top:
top is old school, and it's probably available on just about every Unix system in use. Unfortunately, it's also kind of cryptic and difficult to read at a glance. My advice is to install htop instead - it's prettier, friendlier, and has a ton of features for sorting and filtering the process list.
sudo apt-get install htop
Page last edited February 20, 2015
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