Error attempting to run alsamixer Post: 303029392

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Top Forums UNIX for Beginners Questions & Answers Error attempting to run alsamixer Post 303029392 by Corona688 on Friday 25th of January 2019 04:23:29 PM

01-25-2019

Registered User

So, no init system or anything? Your terminal probably hasn't been set up. This is a surprisingly tricky bit of "magic" done with a special syscall which we're for the most part happy to let some linux distribution do for us.

One workaround for it, funny enough, could be ssh-ing into your device. A virtual terminal should be close enough.

I was sure that busybox had a utility for it, and their faq turns out to have that and a better explanation what it does:

Quote:

"Why do I keep getting "sh: can't access tty; job control turned off" errors? Why doesn't Control-C work within my shell?"

Job control will be turned off since your shell can not obtain a controlling terminal. This typically happens when you run your shell on /dev/console. The kernel will not provide a controlling terminal on the /dev/console device. You should run your shell on a normal tty such as tty1 or ttyS0 and everything will work.

Example: you booted into your machine with init=/bin/sh and got "sh: can't access tty" error because sh has its stdio opened to /dev/console. You want to reopen stdio to, say, /dev/tty1 and thus acquire a controlling tty.

Code:

    # Let's try this:
    exec </dev/tty1 >/dev/tty1 2>&1
    # No, doesn't work: even if opening /dev/tty1 gave sh the ctty,
    # sh wouldn't know it - it checks for ctty just once at startup.

    # Let's try re-execing sh:
    exec </dev/tty1 >/dev/tty1 2>&1
    exec sh
    # Got "sh: can't access tty" again. Why?
    # The reason is somewhat obscure: kernel starts process with PID=1
    # (in this case, shell) with SID=0 and PGID=0, not with SID=1 and PGID=1
    # as you'd expect. IOW: our sh is not a session leader, and therefore
    # cannot acquire ctty by opening /dev/tty1 (or any other tty).

    # Let's try making us a session leader:
    exec setsid sh
    exec </dev/tty1 >/dev/tty1 2>&1
    exec sh
    # Yes, this worked!

    # This can be combined into one command,
    # but need to be careful and perform these operations
    # in the correct order:
    # 1. make ourself session leader,
    # 2. open /dev/tty1 and thus acquire a ctty,
    # 3. re-execute the shell, allowing it to notice that it has a ctty:
    exec setsid sh -c 'exec sh </dev/tty1 >/dev/tty1 2>&1'

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LEARN ABOUT CENTOS

amixer

AMIXER(1)						      General Commands Manual							 AMIXER(1)

NAME

       amixer - command-line mixer for ALSA soundcard driver

SYNOPSIS

       amixer [-option] [cmd]

DESCRIPTION

       amixer allows command-line control of the mixer for the ALSA soundcard driver.  amixer supports multiple soundcards.

       amixer  with no arguments will display the current mixer settings for the default soundcard and device. This is a good way to see a list of
       the simple mixer controls you can use.

COMMANDS

       help   Shows syntax.

       info   Shows the information about a mixer device.

       scontrols
	      Shows a complete list of simple mixer controls.

       scontents
	      Shows a complete list of simple mixer controls with their contents.

       set or sset <SCONTROL> <PARAMETER> ...
	      Sets the simple mixer control contents. The parameter can be the volume either as a percentage from 0% to 100% with % suffix,  a	dB
	      gain  with dB suffix (like -12.5dB), or an exact hardware value.	The dB gain can be used only for the mixer elements with available
	      dB information.  When plus(+) or minus(-) letter is appended after volume value, the volume is incremented or decremented  from  the
	      current value, respectively.

	      The parameters cap, nocap, mute, unmute, toggle are used to change capture (recording) and muting for the group specified.

	      The  optional modifiers can be put as extra parameters to specify the stream direction or channels to apply.  The modifiers playback
	      and capture specify the stream, and the modifiers front, rear, center, woofer are used to specify channels to be changed.

	      A simple mixer control must be specified. Only one device can be controlled at a time.

       get or sget <SCONTROL>
	      Shows the simple mixer control contents.

	      A simple mixer control must be specified. Only one device can be controlled at a time.

       controls
	      Shows a complete list of card controls.

       contents
	      Shows a complete list of card controls with their contents.

       cset <CONTROL> <PARAMETER> ...
	      Sets the card control contents. The identifier has these components: iface, name, index, device, subdevice, numid. The next argument
	      specifies the value of control.

       cget <CONTROL>
	      Shows the card control contents. The identifier has same syntax as for the cset command.

OPTIONS

       -c card

	      Select the card number to control. The device name created from this parameter has syntax 'hw:N' where N is specified card number.

       -D device

	      Select the device name to control. The default control name is 'default'.

       -s | --stdin

	      Read from stdin and execute the command on each line sequentially.  When this option is given, the command in command-line arguments
	      is ignored.

	      Only sset and cset are accepted.	Other commands are ignored.  The commands to unmatched ids are ignored without errors too.

       -h     Help: show syntax.

       -q     Quiet mode. Do not show results of changes.

       -R     Use the raw value for evaluating the percentage representation.  This is the default mode.

       -M     Use the mapped volume for evaluating the percentage representation like alsamixer, to be more natural for human ear.

EXAMPLES

       amixer -c 1 sset Line,0 80%,40% unmute cap
	      will set the second soundcard's left line input volume to 80% and right line input to 40%, unmute it, and select it as a source  for
	      capture (recording).

       amixer -c 1 -- sset Master playback -20dB
	      will  set  the  master  volume  of  the second card to -20dB.  If the master has multiple channels, all channels are set to the same
	      value.

       amixer -c 1 set PCM 2dB+
	      will increase the PCM volume of the second card with 2dB.  When both playback and capture volumes exist, this  is  applied  to  both
	      volumes.

       amixer -c 2 cset iface=MIXER,name='Line Playback Volume",index=1 40%
	      will set the third soundcard's second line playback volume(s) to 40%

       amixer -c 2 cset numid=34 40%
	      will set the 34th soundcard element to 40%

SEE ALSO

	alsamixer(1)

BUGS

       None known.

AUTHOR

       amixer is by Jaroslav Kysela <perex@perex.cz>.  This document is by Paul Winkler <zarmzarm@erols.com> and Jaroslav Kysela <perex@perex.cz>.

								    11 Aug 2000 							 AMIXER(1)