Actions

Linux

From Han Wiki


System

Description Command
Boot into CLI mode sudo systemctl set-default multi-user.target
Boot into desktop mode sudo systemctl set-default graphical.target
Detach a job from the current session sudo rsync templates/* whatever/ &
jobs
disown -h %1
Change the shape of the cursor

echo -e -n "\x1b[\x30 q" # changes to blinking block
echo -e -n "\x1b[\x31 q" # changes to blinking block also
echo -e -n "\x1b[\x32 q" # changes to steady block
echo -e -n "\x1b[\x33 q" # changes to blinking underline
echo -e -n "\x1b[\x34 q" # changes to steady underline
echo -e -n "\x1b[\x35 q" # changes to blinking bar
echo -e -n "\x1b[\x36 q" # changes to steady bar

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

SSH


Tunneling

Say I'm trying to access 10.101.1.10 server via SSH which is behind a restrictive firewall through 64.106.170.61. The first command will map the port 50010 of the local machine to port 22 of 10.101.1.10 that is accessible from 64.106.170.61.

Description Command Distro
Create a SSH tunnel & connect via port 50010 $ ssh -L 50010:10.101.1.10:22 mhan@64.106.170.61
$ ssh -p 50010 127.0.0.1

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

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

Deleting volumes and then increasing the size of another volume

  • Last tested on RHEL 7.8 (2020-10-21)

These commands remove volumes named laravelhome and laravelhtml, and then increase lv_var volume with the remaining space.

[root@it153coai01lnx ~]# lsblk
NAME                 MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda                    8:0    0   40G  0 disk
├─sda1                 8:1    0  476M  0 part /boot
└─sda2                 8:2    0 39.5G  0 part
  ├─vg00-swap        253:0    0  1.9G  0 lvm  [SWAP]
  ├─vg00-lv_root     253:1    0  9.3G  0 lvm  /
  ├─vg00-lv_tmp      253:2    0  1.9G  0 lvm  /tmp
  ├─vg00-lv_var      253:3    0  3.7G  0 lvm  /var
  ├─vg00-lv_varlog   253:4    0  3.7G  0 lvm  /var/log
  ├─vg00-lv_home     253:5    0    5G  0 lvm  /home
  └─vg00-laravelhome 253:6    0   19G  0 lvm
sdb                    8:16   0    5G  0 disk
└─vg00-laravelhome   253:6    0   19G  0 lvm
sdc                    8:32   0   35G  0 disk
└─vg00-laravelhtml   253:7    0   35G  0 lvm
sr0                   11:0    1 1024M  0 rom
[root@it153coai01lnx ~]# vgdisplay
  --- Volume group ---
  VG Name               vg00
  System ID
  Format                lvm2
  Metadata Areas        3
  Metadata Sequence No  12
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                8
  Open LV               6
  Max PV                0
  Cur PV                3
  Act PV                3
  VG Size               79.52 GiB
  PE Size               4.00 MiB
  Total PE              20358
  Alloc PE / Size       20358 / 79.52 GiB
  Free  PE / Size       0 / 0
  VG UUID               k3r0vo-ZcxE-3aQt-Bfjp-0KHf-zrbw-jD1oaA

[root@it153coai01lnx ~]# lvs
  LV          VG   Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  laravelhome vg00 -wi-a----- <19.04g
  laravelhtml vg00 -wi-a----- <35.00g
  lv_home     vg00 -wi-ao----   5.00g
  lv_root     vg00 -wi-ao----   9.31g
  lv_tmp      vg00 -wi-ao----   1.86g
  lv_var      vg00 -wi-ao----  <3.73g
  lv_varlog   vg00 -wi-ao----  <3.73g
  swap        vg00 -wi-ao----   1.86g
# remove entries from /etc/fstab
[root@it153coai01lnx ~]# vim /etc/fstab
[root@it153coai01lnx ~]# lvchange -an /dev/vg00/laravelhome
[root@it153coai01lnx ~]# lvchange -an /dev/vg00/laravelhtml
[root@it153coai01lnx ~]# lvremove /dev/vg00/laravelhome
  Logical volume "laravelhome" successfully removed
[root@it153coai01lnx ~]# lvremove /dev/vg00/laravelhtml
  Logical volume "laravelhtml" successfully removed
[root@it153coai01lnx ~]# lvs
  LV        VG   Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  lv_home   vg00 -wi-ao----  5.00g
  lv_root   vg00 -wi-ao----  9.31g
  lv_tmp    vg00 -wi-ao----  1.86g
  lv_var    vg00 -wi-ao---- <3.73g
  lv_varlog vg00 -wi-ao---- <3.73g
  swap      vg00 -wi-ao----  1.86g
[root@it153coai01lnx ~]# lvextend -L+2G /dev/vg00/lv_var
  Size of logical volume vg00/lv_var changed from <3.73 GiB (954 extents) to <5.73 GiB (1466 extents).
[root@it153coai01lnx ~]# df -h
Filesystem                  Size  Used Avail Use% Mounted on
devtmpfs                    899M     0  899M   0% /dev
tmpfs                       910M  232K  910M   1% /dev/shm
tmpfs                       910M   98M  813M  11% /run
tmpfs                       910M     0  910M   0% /sys/fs/cgroup
/dev/mapper/vg00-lv_root    9.4G  6.0G  3.4G  65% /
/dev/sda1                   473M  181M  293M  39% /boot
/dev/mapper/vg00-lv_home    5.0G  1.2G  3.9G  23% /home
/dev/mapper/vg00-lv_tmp     1.9G   33M  1.9G   2% /tmp
/dev/mapper/vg00-lv_var     3.8G  3.7G   62M  99% /var
/dev/mapper/vg00-lv_varlog  3.8G  103M  3.7G   3% /var/log
tmpfs                       182M     0  182M   0% /run/user/4323
# for ext3 or ext4, it'd be: resize2fs /dev/vg00/lv_var; this is for an xfs fs
[root@it153coai01lnx ~]# xfs_growfs /dev/vg00/lv_var
meta-data=/dev/mapper/vg00-lv_var isize=256    agcount=4, agsize=244224 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=0        finobt=0 spinodes=0
data     =                       bsize=4096   blocks=976896, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal               bsize=4096   blocks=2560, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data blocks changed from 976896 to 1501184
[root@it153coai01lnx ~]# df -h
Filesystem                  Size  Used Avail Use% Mounted on
devtmpfs                    899M     0  899M   0% /dev
tmpfs                       910M  232K  910M   1% /dev/shm
tmpfs                       910M   98M  813M  11% /run
tmpfs                       910M     0  910M   0% /sys/fs/cgroup
/dev/mapper/vg00-lv_root    9.4G  6.0G  3.4G  65% /
/dev/sda1                   473M  181M  293M  39% /boot
/dev/mapper/vg00-lv_home    5.0G  1.2G  3.9G  23% /home
/dev/mapper/vg00-lv_tmp     1.9G   33M  1.9G   2% /tmp
/dev/mapper/vg00-lv_var     5.8G  3.7G  2.1G  64% /var
/dev/mapper/vg00-lv_varlog  3.8G  103M  3.7G   3% /var/log
tmpfs                       182M     0  182M   0% /run/user/4323


Adding a new disk to a LVM

  • Last tested on CentOS 8.1.1911 (2020-07-06)
[root@chara mhan]# lsblk
NAME                  MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                     8:0    0 238.5G  0 disk
└─sda1                  8:1    0 238.5G  0 part
  ├─centos_chara-root 253:0    0     2T  0 lvm  /
  └─centos_chara-home 253:2    0   200G  0 lvm  /home
sdb                     8:16   0 119.2G  0 disk
├─sdb1                  8:17   0   200M  0 part /boot/efi
├─sdb2                  8:18   0     1G  0 part /boot
└─sdb3                  8:19   0   118G  0 part
  ├─centos_chara-root 253:0    0     2T  0 lvm  /
  └─centos_chara-swap 253:1    0   7.9G  0 lvm  [SWAP]
sdc                     8:32   0   1.8T  0 disk
└─sdc1                  8:33   0   1.8T  0 part
  └─centos_chara-root 253:0    0     2T  0 lvm  /
sdd                     8:48   0 931.5G  0 disk
└─sdd1                  8:49   0 931.5G  0 part
sr0                    11:0    1  1024M  0 rom

[root@chara mhan]# fdisk /dev/sdd

Welcome to fdisk (util-linux 2.32.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x8d7dc33f.

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1):
First sector (2048-1953525167, default 2048):
Last sector, +sectors or +size{K,M,G,T,P} (2048-1953525167, default 1953525167):

Created a new partition 1 of type 'Linux' and of size 931.5 GiB.

Command (m for help): t
Selected partition 1
Hex code (type L to list all codes): 8e
Changed type of partition 'Linux' to 'Linux LVM'.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.
[root@chara mhan]# pvcreate /dev/sdd1
  Physical volume "/dev/sdd1" successfully created.
[root@chara mhan]# vgs
  VG           #PV #LV #SN Attr   VSize  VFree
  centos_chara   3   3   0 wz--n- <2.17t    0
[root@chara mhan]# vgextend centos_chara /dev/sdd1
  Volume group "centos_chara" successfully extended
[root@chara mhan]# lvdisplay | grep Path
  LV Path                /dev/centos_chara/swap
  LV Path                /dev/centos_chara/root
  LV Path                /dev/centos_chara/home
[root@chara mhan]# lvresize -l +100%free /dev/centos_chara/root
  Size of logical volume centos_chara/root changed from 1.96 TiB (514983 extents) to 2.87 TiB (753449 extents).
  Logical volume centos_chara/root successfully resized.
[root@chara mhan]# df -h | grep root
/dev/mapper/centos_chara-root  2.0T  1.9T   89G  96% /
[root@chara mhan]# grep root /etc/mtab
/dev/mapper/centos_chara-root / xfs rw,relatime,attr2,inode64,noquota 0 0
[root@chara mhan]# df -h
Filesystem                     Size  Used Avail Use% Mounted on
devtmpfs                       7.8G     0  7.8G   0% /dev
tmpfs                          7.8G  176K  7.8G   1% /dev/shm
tmpfs                          7.8G  9.3M  7.8G   1% /run
tmpfs                          7.8G     0  7.8G   0% /sys/fs/cgroup
/dev/mapper/centos_chara-root  2.0T  1.9T   89G  96% /
/dev/sdb2                     1014M  293M  722M  29% /boot
/dev/mapper/centos_chara-home  200G  103G   98G  52% /home
/dev/sdb1                      200M   11M  190M   6% /boot/efi
tmpfs                          1.6G  4.0K  1.6G   1% /run/user/1000
[root@chara mhan]# xfs_growfs /
meta-data=/dev/mapper/centos_chara-root isize=512    agcount=161, agsize=3276800 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=0, sparse=0, rmapbt=0
         =                       reflink=0
data     =                       bsize=4096   blocks=527342592, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=6400, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data blocks changed from 527342592 to 771531776
[root@chara mhan]# df -h
Filesystem                     Size  Used Avail Use% Mounted on
devtmpfs                       7.8G     0  7.8G   0% /dev
tmpfs                          7.8G  176K  7.8G   1% /dev/shm
tmpfs                          7.8G  9.3M  7.8G   1% /run
tmpfs                          7.8G     0  7.8G   0% /sys/fs/cgroup
/dev/mapper/centos_chara-root  2.9T  1.9T 1020G  66% /
/dev/sdb2                     1014M  293M  722M  29% /boot
/dev/mapper/centos_chara-home  200G  103G   98G  52% /home
/dev/sdb1                      200M   11M  190M   6% /boot/efi
tmpfs                          1.6G  4.0K  1.6G   1% /run/user/1000
[root@chara mhan]#

Links

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

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

Description Command Distro
Scan a network for connected computers $ sudo nmap -sP 129.24.22.0/24

Firewall

Description Command Distro
Open a firewall port $ sudo ufw allow 55301
Open a firewall port $ sudo firewall-cmd --zone=public --add-port=55301/tcp --permanent
$ sudo firewall-cmd --reload
  • Last tested on CentOS 7.7.1908 (2020-04-21)
Close a firewall port $ sudo ufw deny 55301
Close a firewall port $ sudo firewall-cmd --zone=public --remove-port=55301/tcp --permanent
$ sudo firewall-cmd --reload
  • Last tested on CentOS 7.7.1908 (2020-04-21)

Package management

Transaction check errors

  • Last tested on RHEL 7.8 (2020-10-21)
# running a yum update yields the following
  file /etc/os-release from install of redhat-release-server-7.9-3.el7.x86_64 conflicts with file from package oraclelinux-release-7:7.8-1.0.7.el7.x86_64
  file /etc/redhat-release from install of redhat-release-server-7.9-3.el7.x86_64 conflicts with file from package oraclelinux-release-7:7.8-1.0.7.el7.x86_64
  file /etc/rpm/macros.dist from install of redhat-release-server-7.9-3.el7.x86_64 conflicts with file from package oraclelinux-release-7:7.8-1.0.7.el7.x86_64
  file /etc/system-release from install of redhat-release-server-7.9-3.el7.x86_64 conflicts with file from package oraclelinux-release-7:7.8-1.0.7.el7.x86_64
  file /etc/system-release-cpe from install of redhat-release-server-7.9-3.el7.x86_64 conflicts with file from package oraclelinux-release-7:7.8-1.0.7.el7.x86_64

$ sudo yum install --downloadonly redhat-release-server
$ sudo find /var/cache/yum -type f -name "redhat-release-server*" -exec rpm -Uvh --force {} \;
$ sudo yum --disablerepo="*" --enablerepo="rhel*" --disablerepo="oraclelinux*" --disablerepo="rhel-source*" --disablerepo="rhel-beta*" update -y && sudo yum -y upgrade && sudo yum -y autoremove


Security

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

Links

Miscellaneous