Arithmetic in shell scripts

Kenny_Strawn · 11-19-2010, 09:27 PM

Is it possible to do arithmetic in shell scripts? If so, just how is it done?

Here is the code so far:

Code:

#!/bin/bash

time=$(date +%H%M)

for time in $time+1; do
    echo $time
done

What I am trying to do here really is create a shell script that automatically prints the time every time it changes.

I am wondering whether to use a condition or a loop here. I am settling on a condition, but then there's the issue of a lack of a '!=' or equivalent operator.

neonsignal · 11-19-2010, 10:24 PM

Quote:

Originally Posted by Kenny_Strawn

then there's the issue of a lack of a '!=' or equivalent operator.

http://tldp.org/HOWTO/Bash-Prog-Intr...11.html#ss11.3

catkin · 11-19-2010, 10:37 PM

For date arithmetic the easiest solution is to use the date command, for example:

Code:

c@CW8:~$ some_time=$(date)
c@CW8:~$ date -d "$some_time + 1 hour"
Sat Nov 20 11:05:19 IST 2010

Kenny_Strawn · 11-19-2010, 11:02 PM

Yes, but I want this script to execute indefinitely, as though it were a clock, printing the time in the %H%M format every minute. Any idea how to do that?

propofol · 11-19-2010, 11:21 PM

How about:

Code:

some_time=$(date)
c@CW8:~$some_time=$(date -d "$some_time + 1 hour")

Your script:

Code:

for time in $time+1; ...

does not make sense. Look at this.

Use $(( ... )) notation for shell arithmetic:

Code:

t=2
t=$(( t + 5 ))

Regards,
Stefan

catkin · 11-19-2010, 11:59 PM

Quote:

Originally Posted by Kenny_Strawn

Yes, but I want this script to execute indefinitely, as though it were a clock, printing the time in the %H%M format every minute. Any idea how to do that?

Then you don't need to do any arithmetic; you just need an infinite delayed loop

Code:

#!/bin/bash

last_time=$(date +%H%M)

while true
do
    sleep 1
    this_time=$(date +%H%M)
    if [[ $this_time != $last_time ]]; then
        echo "$this time"
        last_time=$this_time
    fi
done

Dark_Helmet · 11-20-2010, 12:10 AM

You're posts indicate that you want two very different things:

Quote:

What I am trying to do here really is create a shell script that automatically prints the time every time it changes.

Quote:

I want this script to execute indefinitely, as though it were a clock, printing the time in the %H%M format every minute

The first quote implies that you want the script to wait until the system clock changes before printing. In other words, you want to "poll" the system clock.

The second quote implies that you want to recreate the system clock.

I'll give an example of each (though, it won't be a "true" polling script for the first quote):

For quote #1:

Code:

#!/bin/bash

old_time="junk"
while true ; do
  current_time=$( date +%H%M )
  if [ "${current_time}" != "${old_time}" ] ; then
    echo ${current_time}
    old_time="${current_time}"
  fi
  sleep 1
done

For quote #2:

Code:

#!/bin/bash

while true ; do
  date +%H%M
  sleep 60
done

As I was writing this, catkin posted his response which, funny enough, is almost exactly the same for quote #1.

I should point out that the version for quote #2 may eventually "get off." By that I mean that you're not guaranteed that your loop will execute precisely at 60 seconds. You're likely to get output that seemingly "skips" a minute as the fractional delays add up.

MTK358 · 11-20-2010, 08:53 AM

Quote:

Originally Posted by Dark_Helmet

You're posts indicate that you want two very different things

I think they don't.

He wants to make a clock that prints the time every time the minute changes.

Dark_Helmet · 11-20-2010, 12:53 PM

That's ok, we can have a difference of opinion

While the end result may look very similar, the requests imply two different methods of accomplishing the result.

I look at it from a triggering perspective.

In the first quote, the printing is triggered by the system time changing. In catkin's script and the first one I gave, we both reference the system clock indirectly--through the date command. Once the output of the date command changes, the time is printed. We're polling the system clock.

In the second quote, the printing is triggered by an internal mechanism in the script--where the internal mechanism mimics a clock. In my second script, the date command is not used to check the time, but only to format it for proper display. The internal mechanism is the sleep command. The script has recreated a clock.

As a final example, let me change up the OPs quoted statements a little. Let's assume that, instead of a clock, the OP wanted an email notification.

Quote:

What I am trying to do here really is create a shell script that automatically notifies me when I receive an email.

versus

Quote:

I want this script to execute indefinitely, as though it were an email server, and notify me when I receive an email every minute

The first quote implies integration with the email server, to give a notification precisely when the email arrives. The second quote can avoid the integration by simply checking the contents of /var/mail, ~/Mail, ~/mbox, etc. once per minute.

Anyway, that's how I look at it.

AnanthaP · 11-20-2010, 06:12 PM

As to arithmetic in shell scripts, integer arithmetic is possible using expr.

a=10
a=`expr $a + 10`
echo $a

archtoad6 · 01-02-2011, 01:40 AM

Or w/ $((...)):

Code:

echo $((1+1))

Only works w/ integers, e.g.:

Code:

$ echo $((14/3))
4

There also is bc (bc arbitrary precision calculator language), which must be installed.

vonbiber · 01-02-2011, 07:03 AM

Code:

$ echo $((14/3))
4

If you have bc available (most linux distros have it):

Code:

$ echo 'scale=2; 14/3' | bc
4.66
$ echo 'scale=4; 14/3' | bc
4.6666

wje_lq · 01-02-2011, 09:44 AM

Quote:

Originally Posted by archtoad6

There also is bc (bc arbitrary precision calculator language), which must be installed.

Real programmers use dc. ;)

archtoad6 · 01-02-2011, 01:09 PM

Thanks.

I had an (original) HP-35, so dc looks like a briar patch to me.

Edit: briar patch is a link, follow it.

wje_lq · 01-02-2011, 09:18 PM

Quote:

Originally Posted by archtoad6

I had an (original) HP-35, so dc looks like a briar patch to me.

Um, dc should appeal to you if you had an HP-35. Are you saying you don't like RPN? I'm confused.

(I have an HP-16c, and it's still my calculator of choice. I hope to have it entirely emulated by the time it stops working, which I hope is never.)