Transcript Processes

Agenda
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The Bourne Shell – Part I
Redirection ( >, >>, < , <<)
Pipes
Controlling Processes
Creating Subshells, ps, ps -l, $!, $$
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Managing Processes (Jobs)
Command Grouping, Running jobs in background (bg),
bringing jobs to foreground (fg), job status (jobs),
Suspending jobs (CTRL Z), Restarting jobs in
background, Displaying/Killing processes or jobs
The Bourne Shell
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The Bourne Shell was the first shell
developed for the UNIX operating system.
Since many other “new and improved” shells
incorporate the Bourne shell, we will be
learning about this shell first.
When you login to PHOBOS, you can access
the Bourne shell by issuing the command: sh
(although you can use Bourne shell
commands in the default shell for PHOBOS –
which is the Korn Shell)
There is no Bourne Shell in Linux…
Standard Input, Standard
Output
and Standard Error
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Three terms are used to indicate the direction
of information when issuing UNIX / Linux
commands:
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Standard Input (stdin) – The default is the
keyboard
Standard Output (stdout) – The default is the
screen
Standard Error (stderr) – The default is the screen
Redirection
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Since everything in UNIX / Linux operating
system is a file, standard input, standard
output or the standard error can be directed
from/to different files:
1>, >
Redirects standard output to a file.
The 1> indicates standard output is
redirected, but “>” is considered “1>” by
default. This will overwrite previous contents
of a file unless you use “>>”
Redirection
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2>
Redirects the standard error to a
file. This can be used to view error messages
in a file for later reference. If you want to
“throw-away” error messages, redirect to
/dev/null commonly referred to as the “bitbucket” or “trash can”
Example:
cat a b c 2> /dev/null
Redirection
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<
Command takes input for a
command from a file. Example:
grep pattern < file 1
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Can You explain the following redirections?
cat < file1 > file2
grep msaul < file3 > file1 2> file7
grep dward < file4 > file 5 2>/dev/null
Pipes ( | )
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Pipes are used to send the standard output of
one command into another command as the
standard input
Pipes have the advantage of modifying
standard output to achieve a task (possibly
without having to create temporary files
which need to be removed later)
Controlling Processes
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A process in UNIX simply represents the
running of a command
The process structure in UNIX is similar to the
hierarchical structure of directories:
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The beginning process is root
Parent processes can create child processes
Processes can spawn other processes
When a child process is finished, execution returns
to its parent process
Controlling Processes
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It is important to learn how UNIX processes
work, since administering a UNIX system
requires knowledge of how to start, stop and
monitor processes.
The UNIX system is built on a series of
processes and sub-processes. If you don’t
understand processes, you could accidentally
shut down the UNIX system.
(not a very smart or popular thing to do!)
Processes
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For your account, the basic process is the
shell (although for the entire system - from
root downwards there are many processes).
When you type in a UNIX command and
press ENTER, a child process is created to
allow execution of command. During this time
the parent process “sleeps” and resumes
when child process is completed. There is
even a command called sleep to suspend the
process for a specific number of seconds!
Process Identification (PID)
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Process Identification Numbers (PIDs) are
used to keep track of separate processes
To view a compact listing of process ID
numbers, you can enter ps
To view a detailed listing of process ID
numbers (relating child (PID)& parent
processes(PPID)) enter ps -l
To view listing of processes for the entire
system, enter ps -ef
Read-only Variables for PIDs
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The shell can store the PID number as a
read-only shell variable
echo $$ displays the PID number that is
currently executing
echo $! displays the PID of the last process
that was run in the background
The next slide shows a practical example of
using these read-only variables
Creating Filenames using $$
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The PID number can be used to create
unique filenames (that need to be different
everytime they are run)
To create temporary files within a script:
cp file_name file_name.$$
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This will create a temporary file with the PID
as the extension. This is useful when
removing all temp files (eg rm *.$$)
Managing Processes (Jobs)
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Command Grouping
Running jobs in background (bg)
brings jobs to foreground (fg)
Displaying job status (jobs command)
Suspending jobs (CTRL Z)
Killing processes or jobs (kill)
Command Grouping
You can run a set of commands on the
same command line by typing such
commands as:
ls ; who ; date <ENTER>
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You can also, combine commands or
files to be run in both the foreground or
background (eg ls ; who or a & b ; c)
Command Grouping
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The UNIX OS has the ability to run programs
in the foreground and in the background.
This feature allows users of the UNIX system
to run lengthy processes without interfering
with their other tasks.
To run a command or script in the
background, type the symbol & after the
command or filename (eg. who &). Note that
spaces are allowed...
Command Grouping
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It is important to note that although these
commands appear to be in a sequence, the
shell may execute these in a different order
may schedule jobs in a order
For example, you may want to run both
commands a & b in the background, and run
command c in the foreground. Here is the
command:
(a;b)&c
Bringing processes
to foreground
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When processes are run in the
foreground, the shell will wait for the
command to finish before allowing the
user to enter another command (thus
background processes are useful)
If a process or processes are in the
background (or suspended), you can
bring it to the foreground by typing fg
Displaying job status (jobs
command)
You can display the status of jobs that are
running in the background by typing the
UNIX command jobs
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jobs commands give a listing of jobs running in the
background:
jobs with a plus sign “+” indicates default job to bring to
foreground by entering fg
Can bring numbered job to foreground by entering fg
%(job#) - Note space between fg & %
Suspending jobs (CTRL Z)
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You can suspend or stop the execution of
processes while they are running in the
“foreground” by pressing <CTRL><Z>
When you press these keys, the process is
suspended and placed into the background in
order to free up the shell for other
operations.
You can resume process by bringing it to the
foreground or have it run in background
Restarting Suspended
Processes
in the Background
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A process may take such a long time that it is
better to suspend the job (i.e. send it to the
background, and then have it run in the
background). This will leave you to operate in
the foreground
If a process or processes are suspended in
the background, you can have it continue
running in the background by entering bg
(rules such as bg %(job#) also apply)
Terminating processes or jobs
(kill)
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Depending on your permissions, you may be
able to abort or “kill” a process.
Only root is allowed to kill other user’s
processes, but as a user, you can processes
that you have created.
Killing a process is also useful to halt
processes that are running in the background
since <CTRL><C> or DELETE keys in
foreground won’t work
Procedure to “Kill” a Process
Procedure:
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We are assuming that you are killing one of
your own process (you are logged in…)
View process by typing ps -l
Carefully locate PID to kill
Type kill %(job#) to kill that job number
(can also use kill PID or kill -9 PID to kill
process. The kill command by itself kills
more recent stopped job