Why can’t I use job control in a bash script?

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What he meant is that job control is by default turned off in non-interactive mode (i.e. in a script.)

From the bash man page:

JOB CONTROL
       Job  control refers to the ability to selectively stop (suspend)
       the execution of processes and continue (resume) their execution at a
       later point.
       A user typically employs this facility via an interactive interface
       supplied jointly by the system’s terminal driver and bash.

and

   set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
      ...
      -m      Monitor mode.  Job control is enabled.  This option is on by
              default for interactive shells on systems that support it (see
              JOB CONTROL above).  Background processes run in a separate
              process group and a line containing their exit status  is
              printed  upon  their completion.

When he said “is stupid” he meant that not only:

  1. is job control meant mostly for facilitating interactive control (whereas a script can work directly with the pid’s), but also
  2. I quote his original answer, … relies on the fact that you didn’t start any other jobs previously in the script which is a bad assumption to make. Which is quite correct.

UPDATE

In answer to your comment: yes, nobody will stop you from using job control in your bash script — there is no hard case for forcefully disabling set -m (i.e. yes, job control from the script will work if you want it to.) Remember that in the end, especially in scripting, there always are more than one way to skin a cat, but some ways are more portable, more reliable, make it simpler to handle error cases, parse the output, etc.

You particular circumstances may or may not warrant a way different from what lhunath (and other users) deem “best practices”.

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