Shell Scripting Tutorial for Beginners 27 - Signals and Traps Video Lecture | Complete Linux Course: Become a Linux Professional - Software Development

hey guys welcome to the next video and
shell scripting tutorial for beginners
in this video we will see how to use
trap statement in shell scripting now
the trap statement is relatively tough
to explain so I'm going to take an
example and then I will show you how we
can use trap and why we should use stop
okay so I have an example script here
and the first line of this script is to
print the PID of the script itself
okay so dollar dollar symbol you can use
to print the PID of the script itself
right now the other code here is a while
loop and basically what it's doing is
it's just looping around and I have
provided the 10-second delay in every
loop so that you know after every 10
second it's going to loop around and
print the value of count here until and
unless the value is less than 10 and if
the script is run successfully after for
example 100 seconds we will just exit
out of the script with the exit signal 0
right so let me run the script and let's
see what happens so I am going to run
the script and you can see the PID of
this script is printed here right and
then this script goes into this loop and
after every 10 second is going to print
the value of count which is for example
one and then two and three and then four
and five right now while running your
script some scenarios can happen so
while running your script user may press
for example ctrl C keys in order to you
know come out of the script so when you
press control C to terminate the script
is going to be terminated and our script
is in the middle of doing something but
we have provided this control-c signal
and then the script is ended
right now this control-c signal is
called the interrupt signal and in
signal terms it's called sig IND okay so
we have sent the sig int command to the
script and then the script is terminated
right once again I am going to run the
script and we can use some other command
soon right now the different PID is
provided to this script while it's
running right before it was this PID and
now this script is running with this PID
right and we can press control read also
so control Z and it's also going to stop
the script and this control Z signal is
called suspend signal okay and it's in
signal terms it's called sig t s T P
okay so you can see when I press control
Z the script is stopped and now we are
out of the script but this is also a
interrupt signal so we have interrupted
our script because our script was in the
middle of doing something and we have
given some signal to stop it right now
now once again I'm going to run the
script for example so this time the PID
is this one and once again it's running
and I can kill this process by a kill
command also so I know the PID of this
process for this script so I can just
use another terminal so this is the
different terminals than this right so I
can use the another terminal and I can
just give a kill and then minus 9 and
this minus 9 is also a signal and
it's called cig kill signal right so
this is a cig this is a kill come on
and this nine stand for six kill signal
and then I can give the name of the PID
which is three seven eight four in this
case you can see this is the PID right
now right and when I give this kill
command press Enter you can see our
script is killed once again it was in
the middle of doing something but we
have given a signal to it in this case
we have given a sig kill signal and then
the script is stormed right so there are
some scenarios in which signal as our
script is interrupted in the middle of
doing something and it's not yet
finished but some signal can come or
some unexpected behavior can come which
can interrupt the execution of our
script and that's why we use trap
command now trap command provides the
script to capture and interrupt and then
clean it up within the script okay
so now when we use the trout come on our
tab command in the script will be able
to you know catch this kind of for
example kill command or control C or
control said so it's going to trap or
catch this kind of signals and then we
will be able to do some kind of stuff
before exiting out right so that's why
we use the trap command now we will take
an example for the trap but let me show
you something so I'm going to just open
the different terminal for example and
this is a big terminal and here I'm
going to write man seven and then signal
write signal and then press Enter
and when I see the main page of the
signal then I will be able to see more
you know explanation about these signals
right now when I scroll a little bit
down I will be able to see this kind of
table I was talking about right so
control see signal I have given in my
case right so control see signal was
this one interrupt from keyboard I have
given interrupt from keyboard which is
control C and this is the signal for
that which is sig IND
and every signal has its value so SIGINT
has value to right and then we have
provided control Z also right which was
sig T STP you can see this one was the
signal right and this stands for stop
tied at comma at terminal right it has
the value 18 20 and 24 okay and then we
have provided this fig kill command and
it has the value 9 so we have used kill
minus 9 so this was the value of this
signal we have we could have used sig
kill also and the value of this is 9
right so we have given kill 9 so it was
killed
okay and these are some of the signals
you can use to terminate your you know
process right so if you want to read
more about baby signals then just do man
7 and signal okay man 7 and the key
words signal I'm going to just close
this terminal and let's take an example
now okay so now what I want to do is I
want to remove everything here and then
what I need to do is I will just use a
trap come on just write crap
and then I will echo something so for
example I can just take oh that some
exit command is detected right so I can
just write a great fund is detected okay
and then I am expecting the signal zero
from it
okay so exact zero is a success
signal okay so you have shown you the
signals have their value and they are
always greater than zero right so if the
signal sent here is zero then it's a
success right so now what I am going to
is I will just echo for example hello
world okay and I just want to exit out
of the script side I can just do exit
zero exits zero is a success signal
right so zero is a success signal and
let me clear the terminal here also
clear and let me run the script once
again and when I screw on the script I
can see here that the script is starting
so what what is the execution sequence
here the script started it has seen that
there is a trap for some signal and trap
means whenever we receive a signal 0
then we need to execute this kind of
command whatever command is here it will
be executed whatever come on you right
after the trap signal ok so the command
is echo exit command is detected so this
this is the command command echo is used
and then this is printed when this
signal is cached which is on exit 0 you
know command right now
here once again I'm going to just cut
this code and I'm going to go back to
our previous code so for example we had
this code right and then we can write
trap here so crap and then we can once
again echo something so this time for
example we are expecting some other
signals so for example we want to detect
that it's a fig in signal we can just
write sig int here or we can even write
its value to okay to is the value of sig
in signal and this second signal is for
you know delay it's for interrupt the
thing is interrupting the sequence right
which is control C so when I run the
script once again the script with it
will start with some PID and then I will
just press control C and then see what
happens so it's going to print this
value so whenever the signal is received
is going to execute whatever command we
right after the trap command so I'm
going to just press control C and you
can see you it's printed a great signal
is detected
once again control C and it says exit
signal is detected right and you can see
right now it's not you know coming out
of the terminal and giving this control
C command so many times and it has
completed its execution and you know
every time when I press control C it's
prints exit signal is detected right let
me clear the terminal once again and now
this time I want to catch the sig kill
come on so you can either write sig kill
here or either write 9 the value of sig
kill okay
so cigs kill come on and then I will
just run the script and I know the PID
of it so I can just use the skin command
with -9 signal and then the PID of it
which is four to nine six four two nine
six right and then press ENTER and you
can see this process is killed and this
time we don't you know see this exit
message you can see it's killed but I
told you that we can trap any command
any signal and we can execute any
command when this signal is received and
I have send the signal and nothing is
printed right so there is some
exceptions to the trap
okay so trap cannot trap or catch sig
kill command and sig stop command okay
so these two you know signals it cannot
catch so whenever you write sig kill or
sig stop then trap is not going to catch
them it's unable to catch them okay so
these are the two exceptions okay so
don't use a sig kill or don't catch say
kill or sig stop from your trap command
now let's take one more example and
let's understand how it works okay so
what I'm going to do is I'm going to
just going to my desktop or directory
and I will do LS there okay so there is
only one script file there which is my X
script itself right so I'm going to
create one more file which is while dot
txt and this file is created I'm not
going to insert any data inside it I
will just LS once again and now we can
see that this file is there okay now I
will do PWD to know in which directory
we are in
right and for example this time what we
want to do is whenever we receive any
signal we want to delete this file
somehow okay so how can we do it so I'm
going to declare a variable file for
example and then I'm going to give some
value to this file so I will just take
the complete path of this directory
desktop directory and the name of the
file right so just copy this and paste
it here okay and this time instead of
echoing something I will just do RM
minus F to force the deletion and then
the file variable name which is file
itself okay
and you can after removing you can exit
out of the script so you can just give
the semicolon and exit command to exit
out of the script right so this
semicolon is used to combine two
commands if you know how they works
right now I want to catch some signals
so for example I want to catch the
sacrifice signal which is zero and then
for example I want to catch the
control-c signal which is two second and
then for example I want to catch the
signal 15 which is sig term signal which
is termination signal or even you can do
nine also for example for sig kill or
any other signal right but nine will not
work here because sig kill is an
exception right so I want to just remove
this file whenever I send either of
these signals I can do this right so I'm
going to run the script and script is
running and now I will just press
control C for example or let me just
give this 15 which is a fig term so
I know the PID so I can just give kill
minus 15 and the PID which is 1 which is
5 6 3 3 right and then plus enter this
signal is received in this script and
then we are exited out of the script and
let's do LS once again and you can see
that this file is removed ok
so you can do that also so if you want
to remove a file or you want to do any
cleanup you can do with the command and
then you can just catch that signal and
then do the cleanup right and you could
have done something like this so if the
file is removed successfully you could
have used for example and here and then
you could have echoed the for example
while deleted right so you can just
write file deleted something like this
also you can do ok to know that this
trap is executed ok so once again I am
going to touch this file and I will do
LS now once again this while a exists
and once again I am going to run the
script and script is running and this
time the PID is 5 6 6 7 so just 2 5 6 so
6 7 and press enter and you can see here
this signal is received here it says
file deleted ok so the signal is trapped
here and then let's see if the file is
deleted or not
yes file is also deleted ok so you can
execute any command using trap when some
signal is received
ok now let's run the script once again
and let me show you one more thing
so script is running and then I just to
trap here and then press enter and I can
see here
that there are some traps or there is a
list of some traps right so this is the
list from these three traps are from
this script and this these three traps
are from some other trap command I have
given in the terminal right so
previously I have given a trap command
to test this trap and this is from the
crack command in the terminal previously
I have given and these three values are
coming from this script okay now the
question is how we can remove traps okay
because we have set some trap and
whenever we use trap c'mon they are
listed right so somehow we want to
remove these traps also so how do we
remove it so we can just remove the trap
by using - and name of the signal so I
want to remove for example these three
traps then I can just do trap - and then
these three signals whatever trap I want
to remove so I do this and once again
trap and then I only see these traps now
okay so these three traps I have removed
right so this is how also you can remove
traps
so this - is for removing okay so this
is how you can use traps in shell
scripting I hope you enjoyed this video
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