why bash script adds single quotes on variable expansion? - linux

Question

I am trying to use bash script to add resolution through xrandr and i keep getting error, here is my script:
#!/bin/bash
out=`cvt 1500 800`
out=`echo $out | sed 's/\(.*\)MHz\(.*\)/\2/g'`
input=`echo $out | sed 's/Modeline//g'`
#echo $input
xrandr --newmode $input
input2=`echo $out | cut -d\" -f2`
#echo $input2
xrandr --addmode VNC-0 $input2
running with bash -x
input=' "1504x800_60.00" 98.00 1504 1584 1736 1968 800 803 813 831 -hsync +vsync'
+ xrandr --newmode '"1504x800_60.00"' 98.00 1504 1584 1736 1968 800 803 813 831 -hsync +vsync
if you look at the last line, it adds for some reason single quote ' at the start (before ") and after ", why ??

Answer 1

Singe quotes are added by bash -x when printing debug output.
It won't affect your actual variables value:
out=`cvt 1500 800`
echo $out
# 1504x800 59.92 Hz (CVT) hsync: 49.80 kHz; pclk: 98.00 MHz Modeline "1504x800_60.00" 98.00 1504 1584 1736 1968 800 803 813 831 -hsync +vsync
echo $input
"1504x800_60.00" 98.00 1504 1584 1736 1968 800 803 813 831 -hsync +vsync 98.00 1504 1584 1736 1968 800 803 813 831 -hsync +vsync
What's actually happens, that quotes inside a variable's value aren't parsed when the variable is substituted.
The best way to do this sort of thing is using an array instead of a simple text variable:
xrandr_opts=() # declaring array
input=`echo $out | sed 's/Modeline//g'`
read -a xrandr_opts <<< $input # splitting $input to array
xrandr --newmode "${xrandr_opts[#]}"
As for your specific case, following change will do the trick:
#!/bin/bash
out=`cvt 1500 800`
out=`echo $out | sed 's/\(.*\)MHz\(.*\)/\2/g'`
input=`echo $out | sed 's/Modeline//g'`
#echo $input
#xrandr --verbose --newmode $input
xrandr_opts=() # declaring array
input=`echo $input | sed 's/\"//g'`
read -a xrandr_opts <<< $input # splitting $input to array
opts_size=`echo ${#xrandr_opts[#]}`
xrandr --newmode `printf \'\"%s\"\' ${xrandr_opts[0]}`
${xrandr_opts[#]:1:$opts_size}
input2=`echo $out | cut -d\" -f2`
#echo $input2
xrandr --verbose --addmode VNC-0 $input2
Looks like xrandr --newmode won't accept double quotes. I can't say exactly what's the reason, but at least the script works :)