mjhjmtu

I want to know if there's a way to put the ampersand in a variable and still use it to send a process to the background.

This works:

BCKGRND=yes
if [ "$BCKGRND" = "yes" ]; then
 sleep 5 &
else
 sleep 5
fi

But wouldn't it be cool to accomplish those five lines with only one? Like so:

BCKGRND='&'
sleep 5 $BCKGRND

But that doesn't work. If BCKGRND isn't set it works - but when it is set it's interpreted it as a literal '&' and errors out.

It's not possible to use a variable to background the call because variable expansion happens after the command-line is parsed for control operators (such as && and &).

Yet another option would be to wrap the calls in a function:

mayberunbg() 
 if [ "$BCKGRND" = "yes" ]; then
 "$@" &
 else
 "$@"
 fi

... and then set the variable as needed:

$ BCKGRND=yes mayberunbg sleep 3
[1] 14137
$
[1]+ Done "$@"
# or
$ BCKGRND=yes
$ mayberunbg sleep 3
[1] 14203
$
[1]+ Done "$@"
$ BCKGRND=no mayberunbg sleep 3
# 3 seconds later
$

You can flip things and variabilise “foregrounding”:

FOREGROUND=fg
sleep 5 & $FOREGROUND

Set FOREGROUND to true or empty to run the process in the background. (Setting FOREGROUND to true to run in the background is admittedly confusing! Appropriate variable names are left as an exercise for the reader.)

You would probably have to use eval:

eval "sleep 5" "$BCKGRND"

eval causes the shell to re-evaluate the arguments given. A literal & would therefore be interpreted as & at the end of a command and not as an argument to the command, putting the command in the background.