02. Viewing Processes ps
We can easily view processes, using ps (process status). This only displays the processes that run from the current terminal, so are limited.
$ ps
PID TTY TIME CMD
2696 pts/0 0:00:00 bash
2745 pts/0 0:00:00 psAt the most basic level, there are only four columns.
- PID
- Process identifier. Necessary for sending signals.
- TTY
- Teletype. Refers to the terminal that is controlling the process. X programs and daemons don't have this, but text-mode programs do (eg. console, xterm or remote login session).
- TIME
- Amount of CPU time consumed by the process. The higher it is, the more resources it's taking from the computer.
- CMD
- The command that started the process.
Options with ps
As we'll see soon, the ps command comes with a variety of options. Depending on where these options originated, you may or may not need dashes preceding the option letter.
- BSD
- These come from an earlier Berkeley Software Distribution platform and require no dashes at all.
- Unix98
- These options are preceded by one dash, and are probably what you're most familiar with.
- GNU long
- As you're most likely familiar with, any GNU long options are preceded by two dashes (--).
More details with -ef
The -f option prints full format listing, meaning it includes additional parameters. The -e or -A options displays all processes (including daemon).
$ ps -efUID PID PPID C STIME TTY TIME CMD root 1 0 3 11:14 ? 00:00:00 /sbin/init root 2 0 0 11:14 ? 00:00:00 [kthreadd] root 3 2 0 11:14 ? 00:00:00 [ksoftirqd/0] root 4 2 0 11:14 ? 00:00:00 [kworker/0:0] root 5 2 0 11:14 ? 00:00:00 [kworker/0:0H] root 6 2 1 11:14 ? 00:00:00 [kworker/u2:0] root 7 2 0 11:14 ? 00:00:00 [rcu_sched] root 8 2 0 11:14 ? 00:00:00 [rcuos/0] root 9 2 0 11:14 ? 00:00:00 [rcu_bh]
You'll notice that some processes have a TTY of ?. This means that they have no controlling terminals. Many of these are daemon processes.
Here are some more parameters listed, adding onto the list above.
- UID
- User ID
- PPID
- Parent process identifier.
- C
- CPU utilization.
- STIME
- Time the process started.
Viewing user processes
To view all processes belonging to a user, use the -u option, followed by the username. If you want to view more than one username, separate the names with a comma.
$ ps -u JohnDoePC
# List all processes started by JohnDoePCMore details with aux
By attaching an aux option (no hyphen), we can see even more details. The a shows processes for all users, the u displays the process's owner, and the x shows processors not attached to a terminal.
Additionally, the w option shows the full command name, not just what fits on one line.
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.4 0.4 33772 2012 ? Ss 22:53 0:01 /sbin/init
root 2 0.0 0.0 0 0 ? S 22:53 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? S 22:53 0:00 [ksoftirqd/0]
root 4 0.0 0.0 0 0 ? S 22:53 0:00 [kworker/0:0]
root 5 0.0 0.0 0 0 ? S< 22:53 0:00 [kworker/0:0H]
root 6 0.0 0.0 0 0 ? S 22:53 0:00 [kworker/u2:0]
root 7 0.2 0.0 0 0 ? S 22:53 0:00 [rcu_sched]
root 8 0.0 0.0 0 0 ? S 22:53 0:00 [rcuos/0]
root 9 0.0 0.0 0 0 ? S 22:53 0:00 [rcu_bh]Here are some more variables that we see.
- USER
- User ID, or the owner of process.
- %CPU
- CPU usage as a percentage.
- %MEM
- Memory usage.
- VSZ
- Virtual memory size.
- RSS
- Resident set size - amount of physical memory (RAM) the process is using (kb).
- STAT
- Short for state. Reveals the current status of the process. See below for explanation.
- START
- Time when processor started.
- TIME
- Amount of CPU time process has acquired so far. Helpful for pinpointing runaway processes.
State Variables
Here are some variables that go along with the STAT column.
- D
- Uninterruptible sleep - waiting for I/O device, such as a drive.
- L
- Pages are locked into memory.
- R
- Running, or ready to run.
- S
- Sleeping, or waiting for user input.
- T
- Stopped
- Z
- Defunct, or a zombie process, aka a child process that stopped, but has not been cleaned up by parent process
- <
- High priority. Low niceness, meaning it takes up more resources.
- +
- In the foreground process group.
- N
- Low priority process. A nice process
- l
- Multi-threaded.
- s
- A session leader.
Inspecting current shell processes
To access the current shell's PID, use the shortcut $$. This evaluates to the current shell's PID.
$ ps u $$
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
JohnDoe 13030 0.0 0.0 27112 4092 pts/8 Ss 00:39 0:00 bashDisplaying process hierarchy
If you want to see where each process spawned from, we can use the -H, -f or --forest option. You'll see that each child process branches out with tabs, signifying its origin
$ ps -AH
...
1 ? 00:00:01 init
299 ? 00:00:00 upstart-udev-br
304 ? 00:00:00 systemd-udevd
517 ? 00:00:00 upstart-socket-
774 ? 00:00:00 upstart-file-br
783 ? 00:00:00 rsyslogd
792 ? 00:00:03 dbus-daemon
805 ? 00:00:00 ModemManager
816 ? 00:00:00 systemd-logind
818 ? 00:00:00 bluetoothd
850 ? 00:00:01 avahi-daemon
852 ? 00:00:00 avahi-daemon
...One drawback with using ps is that it's static - we cannot see the changes while they occur. For a more dynamic view of currently running processes, we can use the top command.
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