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May 27, 2011

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Unix Command 'nohup' Does Not Work. Please Help!

Amazon I am trying to use Unix command 'nohup' to run a process in the background even when I log out. However 'nohup' does NOT work. Why? Read on to find answers!

Introduction
'nohup' stands for 'no hang up' and allows you to run a process continually until it ends during which you can log out and close your terminal. This is because 'nohup' suppresses or ignores HUP (also known as hangup) Unix signal allowing the process to still run even after the user who issued it logs out. This is useful when for example you'd like to start running a big process, shut down your computer, go home. When you get home you'd like to log in and see that the process is still running.

Tutorial
Suppose you have a script called 'shell-script.sh'. You run the following to run the script in the background persistently:

$ nohup shell-script.sh &
In the same directory a file called 'nohup.out' will be created if it hasn't been created yet. The output of running shell-script.sh goes into nohup.out. Therefore you can run the following to see the output of running shell-script.sh as it rolls:

$ tail -f nohup.out
Problem
The problem is sometimes 'nohup' just doesn't work even though I can run the script fine! Recently I wrote a script to crawl my website and I call it spider.sh. When I run './spider.sh' in the directory where spider.sh exists it works perfectly. However when I run 'nohup ./spider.sh &' it doesn't work. Here's the command prompt trace:

$ nohup ./spider.sh &
[1] 21724
$ nohup: ignoring input and appending output to `nohup.out'

[1]+ Exit 2 nohup ./spider.sh
$
$ cat nohup.out
./spider.sh: 1: Syntax error: "(" unexpected
$

I know there's a problem because when I press Enter after I run 'nohup ./spider.sh &' my shell says 'Exit 2' meaning 'nohup' process has ended. Then in nohup.out I see the syntax error. The weird thing is that I can run 'spider.sh' successfully if I simply run it. How come 'nohup' complains that spider.sh has syntax errors? This is because the shell 'nohup' uses to run the process is different from the shell that you account uses to run the process. The syntax of each shell (e.g. C Shell, Bash Shell, Korn Shell) is different but is mostly minor.

Solution
Script spider.sh begins with:

function ff() {
And 'nohup' complains about "(" (however shell does NOT complain about it). When I changed it to:

ff() {
It works for both 'nohup' and shell! This is because the shell 'nohup' uses to run the process is different from the shell that you account uses to run the process. The syntax of each shell (e.g. C Shell, Bash Shell, Korn Shell) is different but should be minor. Fix the syntax errors and 'nohup' will work! Any feedback feel free to share with us!
Please leave a comment here!
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