Unix / Linux Fundamentals

1. Process Management

2. Overview
• Each task in Unix starts a process.
• Each process is assigned a unique process identification
number (PID).
• Some processes are started by the system and are used
to control activities, these are usually called daemons
• Processes started by the user from the shell inherit the
shell’s terminal, and interact through it
• Processes started in the GUI don’t usually have a
terminal associated with them

3. Processes and PIDs
• Use the ‘ps’ command to list the processes currently
running on the system.
• The command is:
ps [options]
• Commonest options are -ef: (in some UNIXs aux)
– –e Every process on the system
– –f Full listing
• Use pgrep command to search for a specific process.
This is equivalent to ps –ef | grep
• Without options, ps will just print all processes that share
the terminal with it.

4. Processes and PIDs
• ‘ps’ can also print out resource usage information for all
processes on the system.
Example
$ ps aux | head
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.0 10348 732 ? Ss 2010 0:04 init [3]
apache 6019 0.0 0.8 241120 9184 ? S Jun26 0:01 /usr/sbin/httpd
root 6108 0.0 0.0 5908 620 ? Ss Mar30 0:16 syslogd -m 0

5. Process Life Line
• There are three stages in the process life cycle:
– Fork – the parent process creates an exact image of
itself in the memory and gets a new PID.
– Exec – the new process rewrites its image in the
memory and executes. The parent (i.e. shell) either
waits for the process to finish (foreground) or
continues to run (background)
– Termination – the child process returns a signal to the
parent, which contains an exit code. The parent
process deletes the child from the process table.

6. Signals
• UNIX systems use signals to communicate different
scenarios between processes
• The kernel manages signal transfers
• When a process receives a signal it stops its current
activity and moves to a signal handling routine
• Different signals have different default routines, which
can be overridden by the programmer
• Among other things signals terminate, suspend and
continue process.
• From a regular user perspective, most signals are
irrelevant – they are used by the kernel

7. Signaling with kill
• Use the command kill to send a signal to a process.
$ kill [-signal] %job-id|process-id
• The name is derived from the fact the default signal kill
sends is the SIGTERM (15) signal, which by default
terminates the process
• For a foreground process, ^C achieves the same result,
but it is actually the terminal sending a SIGINT (2) to the
process
• Both these signals can be blocked by the process (the
BASH, for example, blocks them)
• The signal SIGKILL (9) cannot be blocked, so if all else
fails, this is the way to destroy a stuck process
• ^Z sends the signal SIGTSTP, while SIGSTOP
is a non blocked equivalent

8. Cron
• The cron daemon continually checks the users crontab
files and execute periodic tasks
• Edit the user’s crontab with the command crontab –e
• An editor (usually vi) is opened with the crontab. The
table has six fields:
minute,hour,day,month,weekday,command
59,23,*,*,*, echo Hello > hello.$(date +%s)
• You can list your crontab with crontab –l
• In the system level, there is an additional mechanism of
crontabs for once-a-day, once-a-week, etc.

9. Monitoring processes by top
• ‘top’ - Displays the top processes on the system and
periodically updates the information.
• The first few lines of the display show general
information about the system’s state:
– Load averages in the last 1, 5, and 15 minutes.
– Existing processes no. and the no. of processes in
each state (sleeping, waiting, running, starting,
zombie, and stopped).
– Percentage of time spent in each of the processor
states (user, nice, system, idle, interrupt and
swapper) per processor on the system.
– Average value for each of the processor states (only
on multi-processor systems).

10. Monitoring processes by top – cont.
CPU Processor number on which the process is executing (only on multi-processor systems).
TTY Terminal interface used by the process
PID Process ID number
USERNAME Name of the owner of the process
PRI Current priority of the process
NI Nice value ranging from -20 to +20
SIZE Total size of the process in kilobytes.
RES Resident size of the process in kilobytes
STATE Current state of the process. The various states are sleep, wait, run, idle, zomb, or stop
TIME Number of system and CPU seconds the process has consumed
%WCPU Weighted CPU (central processing unit) percentage
%CPU Raw CPU percentage. This field is used to sort the top processes
COMMAND Name of the command the process is currently running