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The problem happened on one of my local machines with CentOS 6. I ssh to it from my MacBook trying to test a script, which is in /usr/local/bin. However, whenever I tried to execute the command, it is immediately suspended, displaying "[1]+ Stopped". I had to use fg to make it the foreground. Since it's 3 a.m. in the morning, I was wondering if it is related to nice. But I tried some command with nice and renice, it didn't work. As the script calls an I/O-bound Perl script, I also tried ionice but it didn't work either.

It seems difficult to reproduce the problem, since after a reboot the problem is gone. Does anyone know the reason?

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Sounds like the script you're running has a line in it that's getting backgrounded. Look for a line that has an ampersand at the end of it, &. – slm Mar 12 '14 at 8:22
No, I didn't use any ampersands in my script. My script simply calls the Perl script. When I executed the Perl script, or even wget, the same thing happened. – moleculea Mar 12 '14 at 19:07
Please share your script, if possible, also I'd take a look at the settings for your shell, specifically the shopt command to see if you've accidentally enabled a unusual mode. – slm Mar 12 '14 at 19:18
@slm This is the script yaanyap, shopt output link and set -o output. link. Because after the reboot the problem was gone , the options may not be exactly the same as the issue occurred, though I remembered I didn't accidentally enable any mode. – moleculea Mar 13 '14 at 3:27
@slm I just use URLs from YouTube. – moleculea Mar 15 '14 at 22:38

The process is asking for something from the standard input. When a background task needs to read standard input, the task is stopped. Let's take a simple example:

cat | awk '{print ++linenum " " $0}'

Now, let's call this linenum.sh and run it in the background of a Bash script:

sh ./linenum.sh &

What happens?

[1] Stopped sh ./linenum.sh

Why is it stopped? Because it needs standard input to continue.

So, how do we stop the annoying Stopped message? By giving it something in the standard input!

cat /dev/null | sh ./linenum.sh &

This particular script will immediately terminate (since /dev/null is a 0-length file with read)

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My script does not ask for stdin. Neither does the Perl script. I think the problem is something related to the system, since before the reboot, the Perl script or even wget was suspended. – moleculea Mar 12 '14 at 19:11

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