Back-End Programming Exam > Back-End Programming Videos > Shell Scripting: Discovering to Automate Command-Line Tasks > Shell Scripting Tutorial-56: Know when users log in Part One
Shell Scripting Tutorial-56 Know when users log in Part One Video Lecture
Text Transcript from Video
hello guys my name is mother and welcome
to my channel this is the 56th tutorial
in the series of tutorials on UNIX shell
scripting in this tutorial we are going
to do something interesting for a change
and we are going to build a script that
will check every minute whether a user
has logged in or not and it will also
report whenever the user logs in along
with the time that the user was laid in
logging in right so we have to cover a
lot of ground so I won't waste me time
and I'll bring up my editor and I've
saved this file in the home folder it's
called check and has the extension SH so
the first thing that we'd have to do in
the script is we have to ask the user of
the script to provide us the user name
of the user for which the script is
supposed to wait right so then I'm going
to do by typing echo on line 1 and the
message that we will display is enter
user name right and on the next line
I'll type in read and store the input in
a variable called log name and next I'm
going to declare a variable or time and
I'm going to give it the value 0 and
this I'm doing because I want the script
to start with the assumption that the
user has already logged in right so if
that's the case then obviously we can't
say that the user you know is late by
any time at all and even if we are going
to display a message in such a case we
are just going to say that the user was
zero minutes late and logging in right
so that's why I've initialized it you
know with the value 0 and next I'm going
to type in two keywords while and then
through now this is different you know
in this course whenever we used the
while loop we always put a test
condition after the keyword while but
this time I have typed in the keyword
true here and that's because I want
while to do whatever is going to be
there in its body for as long as you
know it does not encounter either the
exit statement or the break statement
right so this you do when you are sure
that you are going to use some sort of
termination statement in the body of the
loop and if you do not use exit or break
and if you you know replace a task and
with the keyword true then while we'll
just keep on executing and you know that
sort of loop is known as infinite loop
and you know that's something that we
discussed probably later but in order to
get inside the body of the while loop
I'll have to type in the keyword do and
that goes on line seven and next I'm
going to type in who because who is
going to tell me the users that have
already logged in in the system right so
the output of who will become the input
of grep because we are going to use grep
to check whether the username sorry
whether the user whose username is
stored in the variable log name is a to
be found in the output of who or not
right so grep space dollar logname and
since this is going to be a string I'm
gonna put in put it within double quotes
and since I do not want grep to show
unnecessary lines of text on screen if
the pattern was found I'm going to
redirect the output of you know the grep
operation to a file in the de Vie
directory and the file is called null
right so this will avoid the unnecessary
printing of text on-screen now I'm going
to use if you check the exit status of
the operation on line a and if the exits
RS is zero that means a grep could find
the user in the output of who and if the
exercise s is one then that means that
you know the user hasn't logged in yet
so we are going to check the exit status
by typing in dollar question mark
- EQ zero and that's going to be the
test condition for the if block and or
to get inside the body of the if block I
type in the keyword then and I'm gonna
use echo to display a message taller log
name as logged in alright and we will
also have to display the time that the
user took in logging in and that we
should do only if the time is not equal
to zero right because it doesn't really
make much sense if we you know display a
message like the user was zero minutes
late and logging in only if the time has
some other value like two minutes or
three minutes or let's say
you know 70 minutes it makes sense to
display you know the time so I'm going
to check for the value of time by typing
in another if statement and the test
condition for this one is going to be
while sorry if the euler time is not
equal to zero then we you know display a
message using echo the taller log name
was dollar time minutes late in logging
in all right and well we terminate the
inner if block by typing in the keyword
fi and after we've done this that is
after we've displayed the message that
the user has logged in and after we've
displayed the time we obviously do not
want the script to do anything else so
that's why I'm going to type in the
keyword exit and this exit statement is
going to be a part of the outer if and
well we are done with the you know if
LOC of the outer if block now we have to
type in the statements for the else
block and the first thing that we are
going to do in else is we are going to
increment the time variable and this is
because if this entire thing does not
execute that is if the edge status of
this command on line eight is 1 then the
loop will then the else block will
execute and you know this is where we
have to increment the time variable
because for the next iteration we do not
want the value of time to be zero we
wanted to be 1 right and I hope you guys
are getting the point that I'm trying to
make here and of course I'm going to use
expr to increment the time variable so
tour time plus 1 and I'm going to put
this expression within reverse codes and
then since I want the loop to check for
the user 60 seconds later or a minute
later I'm going to type in the keyword
sleep and then 60 right so the next
thing that the loop is going to do that
is it's going to check again whether
long name could be found in the output
of whoo
you know that's going to be the next
thing that the loop is going
do and it's going to do that off for 60
seconds right so that's it and now we
can move out of the outer if block by
typing in the keyword fi and well next I
just have to type in the keyword done
because we are done with the script I'll
save the file and bring up my terminal
and I execute this script by typing in
SH space check third SH because that's
the name of the file and when I press
the Enter key the script asks me for a
username and if you guys remember in the
last tutorial we created a user account
and the username for that account was
new user two so that's the username with
which I'm going to check the script and
when I press the Enter key you see that
the cursor blinks on the next line and I
you know don't have my prompt anymore
and let's wait for a couple of minutes
because I you know want you guys to see
ah you know some value for the time
variable and meanwhile we can walk
through the stripped again the first
thing that we did is on line one we used
echo to ask the user of the strip to
provide us a username and then we store
the input in the variable long name and
then we declared and initialized a
variable called time and we give the
value 0 and then on line 6 we have two
keywords while and true and we have true
here instead of a test condition because
we want everything that's there between
the do keyword and the done keyword to
you know keep on executing as long as
you know the exit statement is not
encountered right and online aid I have
who and who is going to tell me the
people who have already logged in and if
the user for which we are waiting if you
know the user could be found in the
output food then we are going to execute
the if block and if he could not be
found then we are just going to
increment our time variable and we are
going to delay the execution of the next
iteration of the loop by 60 seconds
right and inside this if block we are
first using echo to display a message
like the user has logged in and then we
are checking for the value of the time
variable if the timer is not equal to
zero only in that case we are displaying
a message
you know the user was a daughter time in
its logging and that's you know it's
just going to display the value of the
time variable the current value so now
what I'm going to do is I'm gonna open
up another terminal window and I'm going
to use sudo login login with the new
user to account and when I press the
Enter key I'm asked for my password so I
type in my password and then it asks me
to type in the username of the account
with which I want to log in and that's
new user two when I press the Enter key
it asks me for the password for the
account and I'll type in the password
when I press the Enter key you see that
I get a new prompt and this problem
tells me that you know I've logged in
with new usually when I minimize this
window you see that I get the message
new user to his logged in and new user
two was three minutes late and logging
in right so it took us three minutes to
open a new terminal window and then log
in with the other account and you know
that's basically what I wanted to talk
about in this tutorial in the next
tutorial we are going to improve this
script and you know there's a lot that
can be done with this script and we're
not going to learn any new technique but
you know I'm just going to make this
script you know more efficient so see
you guys in the next tutorial thank you
so much for watching this one and please
subscribe to my
to my channel this is the 56th tutorial
in the series of tutorials on UNIX shell
scripting in this tutorial we are going
to do something interesting for a change
and we are going to build a script that
will check every minute whether a user
has logged in or not and it will also
report whenever the user logs in along
with the time that the user was laid in
logging in right so we have to cover a
lot of ground so I won't waste me time
and I'll bring up my editor and I've
saved this file in the home folder it's
called check and has the extension SH so
the first thing that we'd have to do in
the script is we have to ask the user of
the script to provide us the user name
of the user for which the script is
supposed to wait right so then I'm going
to do by typing echo on line 1 and the
message that we will display is enter
user name right and on the next line
I'll type in read and store the input in
a variable called log name and next I'm
going to declare a variable or time and
I'm going to give it the value 0 and
this I'm doing because I want the script
to start with the assumption that the
user has already logged in right so if
that's the case then obviously we can't
say that the user you know is late by
any time at all and even if we are going
to display a message in such a case we
are just going to say that the user was
zero minutes late and logging in right
so that's why I've initialized it you
know with the value 0 and next I'm going
to type in two keywords while and then
through now this is different you know
in this course whenever we used the
while loop we always put a test
condition after the keyword while but
this time I have typed in the keyword
true here and that's because I want
while to do whatever is going to be
there in its body for as long as you
know it does not encounter either the
exit statement or the break statement
right so this you do when you are sure
that you are going to use some sort of
termination statement in the body of the
loop and if you do not use exit or break
and if you you know replace a task and
with the keyword true then while we'll
just keep on executing and you know that
sort of loop is known as infinite loop
and you know that's something that we
discussed probably later but in order to
get inside the body of the while loop
I'll have to type in the keyword do and
that goes on line seven and next I'm
going to type in who because who is
going to tell me the users that have
already logged in in the system right so
the output of who will become the input
of grep because we are going to use grep
to check whether the username sorry
whether the user whose username is
stored in the variable log name is a to
be found in the output of who or not
right so grep space dollar logname and
since this is going to be a string I'm
gonna put in put it within double quotes
and since I do not want grep to show
unnecessary lines of text on screen if
the pattern was found I'm going to
redirect the output of you know the grep
operation to a file in the de Vie
directory and the file is called null
right so this will avoid the unnecessary
printing of text on-screen now I'm going
to use if you check the exit status of
the operation on line a and if the exits
RS is zero that means a grep could find
the user in the output of who and if the
exercise s is one then that means that
you know the user hasn't logged in yet
so we are going to check the exit status
by typing in dollar question mark
- EQ zero and that's going to be the
test condition for the if block and or
to get inside the body of the if block I
type in the keyword then and I'm gonna
use echo to display a message taller log
name as logged in alright and we will
also have to display the time that the
user took in logging in and that we
should do only if the time is not equal
to zero right because it doesn't really
make much sense if we you know display a
message like the user was zero minutes
late and logging in only if the time has
some other value like two minutes or
three minutes or let's say
you know 70 minutes it makes sense to
display you know the time so I'm going
to check for the value of time by typing
in another if statement and the test
condition for this one is going to be
while sorry if the euler time is not
equal to zero then we you know display a
message using echo the taller log name
was dollar time minutes late in logging
in all right and well we terminate the
inner if block by typing in the keyword
fi and after we've done this that is
after we've displayed the message that
the user has logged in and after we've
displayed the time we obviously do not
want the script to do anything else so
that's why I'm going to type in the
keyword exit and this exit statement is
going to be a part of the outer if and
well we are done with the you know if
LOC of the outer if block now we have to
type in the statements for the else
block and the first thing that we are
going to do in else is we are going to
increment the time variable and this is
because if this entire thing does not
execute that is if the edge status of
this command on line eight is 1 then the
loop will then the else block will
execute and you know this is where we
have to increment the time variable
because for the next iteration we do not
want the value of time to be zero we
wanted to be 1 right and I hope you guys
are getting the point that I'm trying to
make here and of course I'm going to use
expr to increment the time variable so
tour time plus 1 and I'm going to put
this expression within reverse codes and
then since I want the loop to check for
the user 60 seconds later or a minute
later I'm going to type in the keyword
sleep and then 60 right so the next
thing that the loop is going to do that
is it's going to check again whether
long name could be found in the output
of whoo
you know that's going to be the next
thing that the loop is going
do and it's going to do that off for 60
seconds right so that's it and now we
can move out of the outer if block by
typing in the keyword fi and well next I
just have to type in the keyword done
because we are done with the script I'll
save the file and bring up my terminal
and I execute this script by typing in
SH space check third SH because that's
the name of the file and when I press
the Enter key the script asks me for a
username and if you guys remember in the
last tutorial we created a user account
and the username for that account was
new user two so that's the username with
which I'm going to check the script and
when I press the Enter key you see that
the cursor blinks on the next line and I
you know don't have my prompt anymore
and let's wait for a couple of minutes
because I you know want you guys to see
ah you know some value for the time
variable and meanwhile we can walk
through the stripped again the first
thing that we did is on line one we used
echo to ask the user of the strip to
provide us a username and then we store
the input in the variable long name and
then we declared and initialized a
variable called time and we give the
value 0 and then on line 6 we have two
keywords while and true and we have true
here instead of a test condition because
we want everything that's there between
the do keyword and the done keyword to
you know keep on executing as long as
you know the exit statement is not
encountered right and online aid I have
who and who is going to tell me the
people who have already logged in and if
the user for which we are waiting if you
know the user could be found in the
output food then we are going to execute
the if block and if he could not be
found then we are just going to
increment our time variable and we are
going to delay the execution of the next
iteration of the loop by 60 seconds
right and inside this if block we are
first using echo to display a message
like the user has logged in and then we
are checking for the value of the time
variable if the timer is not equal to
zero only in that case we are displaying
a message
you know the user was a daughter time in
its logging and that's you know it's
just going to display the value of the
time variable the current value so now
what I'm going to do is I'm gonna open
up another terminal window and I'm going
to use sudo login login with the new
user to account and when I press the
Enter key I'm asked for my password so I
type in my password and then it asks me
to type in the username of the account
with which I want to log in and that's
new user two when I press the Enter key
it asks me for the password for the
account and I'll type in the password
when I press the Enter key you see that
I get a new prompt and this problem
tells me that you know I've logged in
with new usually when I minimize this
window you see that I get the message
new user to his logged in and new user
two was three minutes late and logging
in right so it took us three minutes to
open a new terminal window and then log
in with the other account and you know
that's basically what I wanted to talk
about in this tutorial in the next
tutorial we are going to improve this
script and you know there's a lot that
can be done with this script and we're
not going to learn any new technique but
you know I'm just going to make this
script you know more efficient so see
you guys in the next tutorial thank you
so much for watching this one and please
subscribe to my
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Apr 29, 2026 Last updated
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