Linux

System

Description	Command
Boot into CLI mode.	sudo systemctl set-default multi-user.target
Boot into desktop mode.	sudo systemctl set-default graphical.target

SSH

Description	Command
Create private/public SSH key file using 2048 bit encryption and with a comment. The command creates files under ~/.ssh folder.	ssh-keygen -b 2048 -C user@host.domain
Push public SSH key to a server	ssh-copy-id -p 41111 -i ~/.ssh/id_rsa.pub mhan@remote.server.com
Reset (SELinux) permissions related to SSH. This may be needed if all else fails to correct the issue of system not accepting public keys.	restorecon -R -v ~/.ssh

Disk

Description	Command
Check if GRUB is installed on a partition (e.g. /dev/sda)	sudo dd bs=512 count=1 if=/dev/sda 2>/dev/null | strings

Search

• xargs - to be processed - Things you (probably) didn’t know about xargs

LVM

Physical volume

Description	Command
Initialize for use as LVM physical volume.	pvcreate /dev/sdb1
Scanning for block devices.	lvmdiskscan
Display the physical volumes.	pvdisplay
Show all physical devices.	pvscan
Prevent allocation of physical extents on the free space of one or more physical volumes.	pvchange -x n /dev/sdb1
Resize.	pvresize
Remove LVM label.	pvremove

Volume group

Description	Command
Create a volume group.	vgcreate vg1 /dev/sdb1 /dev/sdc1
Add a physical volume to a volume group.	vgextend vg1 /dev/sdd1
Display volume group.	vgdisplay new_vg
Scan disks for volume groups.	vgscan
Remove a physical volume from a volume group.	vgreduce group_name /dev/sdc1
Change the maximum number of logical volumes of volume group vg00 to 128.	vgchange -l 128 /dev/vg00
Activate or deactivate a volume group.	vgchange -a n my_volume_group
Remove a volume group.	vgremove officevg
Split the physical volumes of a volume group and create a new volume group.	$ vgsplit bigvg smallvg /dev/ram15 Volume group "smallvg" successfully split from "bigvg"
Merge the inactive volume group my_vg into the active or inactive volume group databases giving verbose runtime information.	vgmerge -v databases my_vg
Change the name of a volume group.	vgrename vg02 my_volume_group

Users

UID or GID

Description	Command
Check UID.	id -u username
Check GID.	id -g username
Check all of the groups a user belongs to.	id -G username
Check UID and all of the groups a user belongs to.	id username

File

• Processing Text with Linux Shell - Part 1
• Processing Text with Linux Shell - Part 2

Permissions

Letter	Number
r	4
w	2
x	1
rw-	6
r-x	5
rwx	7

Audio

ALSA (Advanced Linux Sound Architecture)

Setting default output device

Ref: StackExchange/superuser: How to make Alsa pick a preferred sound device automatically?

List all audio cards you have:

$ cat /proc/asound/cards
 0 [PCH            ]: HDA-Intel - HDA Intel PCH                                                                                                                
                      HDA Intel PCH at 0xf7530000 irq 41                                                                                                       
 1 [NVidia         ]: HDA-Intel - HDA NVidia                                                                                                                   
                      HDA NVidia at 0xf7080000 irq 17                                                                                                          
 2 [SoundBar       ]: USB-Audio - Dell AC511 USB SoundBar                                                                                                      
                      Dell Dell AC511 USB SoundBar at usb-0000:00:14.0-4, full speed

Get ALSA identifiers of those:

$ aplay -l | awk -F \: '/,/{print $2}' | awk '{print $1}' | uniq                                                                                 
PCH                                                                                                                                                            
NVidia                                                                                                                                                         
SoundBar

Update /etc/asound.conf:

pcm.!default {
        type hw
        card SoundBar
}

ctl.!default {
        type hw
        card SoundBar
}

I found that the numeric identifier for these cards and change on reboot. That may be related to how the hardware detection works on Linux, and the order changes time to time. So I've used the string identifier instead -- seems to stick better than a numeric value.

PulseAudio

Restart pulseaudio. Do not sudo for this.

$ pulseaudio -k && pulseaudio -D

List sinks.

$ pacmd list-sinks
1 sink(s) available.
  * index: 0
        name: <alsa_output.usb-Dell_Dell_AC511_USB_SoundBar-00-SoundBar.analog-stereo>
        driver: <module-alsa-card.c>
        flags: HARDWARE HW_MUTE_CTRL HW_VOLUME_CTRL DECIBEL_VOLUME LATENCY DYNAMIC_LATENCY
        state: SUSPENDED
        suspend cause: IDLE 
        priority: 9049
        volume: front-left: 26202 /  40% / -23.89 dB,   front-right: 26202 /  40% / -23.89 dB
                balance 0.00
        base volume: 107931 / 165% / 13.00 dB
        volume steps: 65537
        muted: no
        current latency: 0.00 ms
        max request: 0 KiB
        max rewind: 0 KiB
        monitor source: 0
        sample spec: s16le 2ch 44100Hz
        channel map: front-left,front-right
                     Stereo
        used by: 0
        linked by: 0
        configured latency: 0.00 ms; range is 0.50 .. 2000.00 ms
        card: 1 <alsa_card.usb-Dell_Dell_AC511_USB_SoundBar-00-SoundBar>
        module: 7
        properties:
                alsa.resolution_bits = "16"
                device.api = "alsa"
                device.class = "sound"
                alsa.class = "generic"
                alsa.subclass = "generic-mix"
                alsa.name = "USB Audio"
                alsa.id = "USB Audio"
                alsa.subdevice = "0"
                alsa.subdevice_name = "subdevice #0"
                alsa.device = "0"
                alsa.card = "0"
                alsa.card_name = "Dell AC511 USB SoundBar"
                alsa.long_card_name = "Dell Dell AC511 USB SoundBar at usb-0000:00:14.0-4, full speed"
                alsa.driver_name = "snd_usb_audio"
                device.bus_path = "pci-0000:00:14.0-usb-0:4:1.0"
                sysfs.path = "/devices/pci0000:00/0000:00:14.0/usb3/3-4/3-4:1.0/sound/card0"
                udev.id = "usb-Dell_Dell_AC511_USB_SoundBar-00-SoundBar"
                device.bus = "usb"
                device.vendor.id = "413c"
                device.vendor.name = "Dell Computer Corp."
                device.product.id = "a503"
                device.product.name = "Dell AC511 USB SoundBar"
                device.serial = "Dell_Dell_AC511_USB_SoundBar"
                device.string = "front:0"
                device.buffering.buffer_size = "352800"
                device.buffering.fragment_size = "176400"
                device.access_mode = "mmap+timer"
                device.profile.name = "analog-stereo"
                device.profile.description = "Analog Stereo"
                device.description = "Dell AC511 USB SoundBar Analog Stereo"
                alsa.mixer_name = "USB Mixer"
                alsa.components = "USB413c:a503"
                module-udev-detect.discovered = "1"
                device.icon_name = "audio-card-usb"
        ports:
                analog-output: Analog Output (priority 9900, latency offset 0 usec, available: unknown)
                        properties:

        active port: <analog-output>

Networking

How To Use Nmap to Scan for Open Ports on your VPS

Security

• GnuPG - GNU's implementation of PGP, aka GPG
• EncFS - encrypt files and folders

Miscellaneous

• X11 - X Window System
• i3wm - i3 window manager
• Running Steam on CentOS 7 with NVIDIA driver