unix at command pass variable to shell script?

Question

Hi,

I'm trying to setup a simple timer that gets started from a Rails Application. This timer should wait out its duration and then start a shell script that will start up ./script/runner and complete the initial request. I need script/runner because I need access to ActiveRecord.

Here's my test lines in Rails

output = `at #{(Time.now + 60).strftime("%H:%M")} < #{Rails.root}/lib/parking_timer.sh STRING_VARIABLE`
return render :text => output

Then my parking_timer.sh looks like this

#!/bin/sh               
~/PATH_TO_APP/script/runner -e development ~/PATH_TO_APP/lib/ParkingTimer.rb $1
echo "All Done"    

Finally, ParkingTimer.rb reads the passed variable with

ARGV.each do|a|
   puts "Argument: #{a}"
end

The problem is that the Unix command "at" doesn't seem to like variables and only wants to deal with filenames. I either get one of two errors depending on how I position "s

If I put quotes around the right hand side like so

... "~/PATH_TO_APP/lib/parking_timer.sh STRING_VARIABLE"

I get,

-bash: ~/PATH_TO_APP/lib/parking_timer.sh STRING_VARIABLE: No such file or directory

I I leave the quotes out, I get,

at: garbled time

This is all happening on a Mac OS 10.6 box running Rails 2.3 & Ruby 1.8.6

I've already messed around w/ BackgrounDrb, and decided its a total PITA. I need to be able to cancel the job at any time before it is due.

Answer 1

After playing around with irb a bit, here's what I found.

The backtick operator invokes the shell after ruby has done any interpretation necessary. For my test case, the strace output looked something like this:

execve("/bin/sh", ["sh", "-c", "echo at 12:57 < /etc/fstab"], [/* 67 vars */]) = 0   

Since we know what it's doing, let's take a look at how your command will be executed:

/bin/sh -c "at 12:57 < RAILS_ROOT/lib/parking_timer.sh STRING_VARIABLE"

That looks very odd. Do you really want to pipe parking_timer.sh, the script, as input into the at command?

What you probably ultimately want is something like this:

/bin/sh -c "RAILS_ROOT/lib/parking_timer.sh STRING_VARIABLE | at 12:57"

Thus, the output of the parking_timer.sh command will become the input to the at command.

So, try the following:

 output = `#{Rails.root}/lib/parking_timer.sh STRING_VARIABLE | at #{(Time.now + 60).strftime("%H:%M")}`
return render :text => output

You can always use strace or truss to see what's happening. For example:

strace -o strace.out -f -ff -p $IRB_PID

Then grep '^exec' strace.out* to see where the command is being executed.