Run one script, but execute commands at the same time

Micro420 · 07-17-2007, 09:14 PM

Strange situation, but how do you execute commands in a single file script to execute at the same time without waiting for the first command to finish? For example:

Code:

#!/bin/sh
cp bigfile.iso ~/
hostname >> computername.txt

So in this case, I would have to wait for the bigfile.iso to finish copying and then it will run the hostname command into a text file. however, I want them to to happen simultaneously. Any way to do this in a single shell script file?

slakmagik · 07-17-2007, 09:27 PM

Code:

#!/bin/sh
cp bigfile.iso ~/ &
hostname >> computername.txt

Not exactly 'simultaneously', but command2 will execute before the backgrounded command1 finishes.

gilead · 07-17-2007, 09:30 PM

You could use the following:

Code:

#!/bin/sh
cp bigfile.iso ~/ &
hostname >> computername.txt
wait

The wait statement is to prevent the script exiting and leaving the cp command as an orphan process. There's some info on wait at http://tldp.org/LDP/abs/html/x8573.html#WAITHANG

Micro420 · 07-17-2007, 09:43 PM

Hmm, adding the & doesn't appear to work, but I should have been more realistic in my example. I'm actually not using cp, but actually running a program called 'R', which does some statistical analysis. Does that make a difference? Basically this 'R' program runs and takes days for it to finish doing its thing. In the meantime, I need it to output the hostname to a text file, but it's not happening. The shell script still sits there and waits for the first line, R, to finish, and then it runs the hostname command.

gilead · 07-17-2007, 10:11 PM

It shouldn't matter what command is being pushed to the background. If you run the script with the wait command in it, the main script will execute all of its commands and then wait until the background process is finished before exiting. If you leave out the wait statement it will exit without waiting for the background process to finish.

To see what is happening, you can run it with -x to see debug output:

Code:

#!/bin/sh -x
cp bigfile.iso ~/ &
hostname >> computername.txt
wait

Micro420 · 07-17-2007, 10:46 PM

I realized what was going on, and I see that the -x helped me see what was going on. Basically the R program is writing to a file, and I wanted the hostname on to begin on this file, but I think it won't work because R keeps writing to the file from scratch, thus overwriting the hostname at the beginning of the file (does that make sense?)

R inputfile outputfile &
hostname >> outputfile

And unfortunately R doesn't work like this

hostname >> outputfile
R inputfile >> outputfile

jschiwal · 07-17-2007, 11:07 PM

You probably want to use "screen" to be able to log out and at a later time or date, log in again and reattach. Another thing you can use is to use "nohup" to detach the process and have it run in the background. Using nohup, you could redirect the output to a file, or let it write to the default nohup.out file. You can't re-attach to a terminal if you use nohup.

gilead · 07-17-2007, 11:23 PM

Instead of hostname >> outputfile you could use this:

Code:

#!/bin/sh
/path/to/R
sed -i -e "1 i\
`hostname`
" computername.txt

It doesn't background the R command, it just waits until it's finished and uses sed to insert the hostname at line 1.

Micro420 · 07-18-2007, 12:05 AM

I just realized that even if the R program crashes, the shell script will still execute the next command, hostname, since it is indepedent of R so I will still be able to grab the hostname information I need.

Good suggestion, gilead. It would be nice to have the hostname at the top of the output file and will use sed to stick it at the top for easy readability. Thanks all!