shell script if statement trouble

Question

I'm trying to set up a hardware mute button for my notebook running chrunchbang linux, I've got the key event handling working and pointing to a script like this :

curvol=$(amixer get Master | grep 'off')
if ["$curvol" != ""]
then
amixer set Master unmute 
else
amixer set Master mute
fi

what happens is on pressing the button assigned, it will unmute if muted; but it won't mute if it isn't already muted.

I think the problem is in the if statement where I check for output from the command; it seems to be always doing the unmute, regardless of whether the if returns true or not.

Any help would be greatly appreciated! Thanks in advance.

Include spaces around the [ command/operator and the ] token — user166390
– user166390, Commented Jan 9, 2013 at 1:21
I added an echo, it seems as if it's not using the grep, and is just returning the full output of amixer get Master — fredricton1
– fredricton1, Commented Jan 9, 2013 at 1:23
adding spaces seems to have worked, I now feel rather silly. Thank you! — fredricton1
– fredricton1, Commented Jan 9, 2013 at 1:24

Carl Norum · Accepted Answer · 2013-01-09 01:25:47Z

3

[ is the name of a command (or shell builtin, sometimes). You need a space after it for it to work:

if [ "$curvol" != "" ]

answered Jan 9, 2013 at 1:25

Carl Norum

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1 Comment

Carl Norum Over a year ago

Don't feel silly - it's a common problem brought on by the fact that [ used to be a synonym for the test command.

perreal · Accepted Answer · 2013-01-09 01:25:32Z

2

You can use the return value of grep:

amixer get Master | grep 'off' &> /dev/null
if [ $? -eq 0 ] 
then
  amixer set Master unmute 
else
  amixer set Master mute
fi

answered Jan 9, 2013 at 1:25

perreal

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1 Comment

Gordon Davisson Over a year ago

Even simpler: use if amixer get Master | grep -q 'off'; then

William Pursell · Accepted Answer · 2013-01-09 03:34:37Z

2

Seems like it would be a lot simpler to just write:

amixer set Master ${curvol:+un}mute

which is equivalent to:

if test -n "$curvol"; then
  amixer set Master unmute
else
  amixer set Master mute
fi

but much less wordy. Also, note that by using test instead of [, the syntax error becomes much more difficult to make.

edited Jan 9, 2013 at 3:34

answered Jan 9, 2013 at 3:12

William Pursell

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Comments

AAAfarmclub · Accepted Answer · 2013-09-05 18:07:16Z

You can do it in Python. I added a BASH function to toggle the mute state. Stick it in ~/.bashrc

I'm currently using a laptop, so, I don't have multiple sound cards.
I'm not doing any error checking.

See /usr/share/doc/python-alsaaudio/examples/mixertest.py for more sample code.

# toggle Master mute                                            
function tm(){
python -c "                                                     
import alsaaudio                                                

mixerObj = alsaaudio.Mixer()                                    
currentMute = mixerObj.getmute()[0]                             
newMute = not currentMute                                       
mixerObj.setmute(newMute)                                       
"
}

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