This is a Reference Page: Reference pages are collections of short bits of information that are useful to look up but not long or interesting enough to justify having their own dedicated post.

This reference page contains operations-related mini-guides and minor posts.

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Table of Contents

• Find which active-passive bond member is active
• Convert p7b certificate to pem format
• Journalctl commands
• Get real RAM use of processes
  • of ceph
  • of VMs
• Bypass Chrome's self-signed cert warning
• Create a virtual loopback volume on Ubuntu
  • Use Systemd to re-map loopback on reboot
• Test MTU from Ubuntu
• Find big files on Mac OS
• Renew NTP Lease Ubuntu
• Create AWS ECR Registry If Not Found
• Passwordless sudo on Ubuntu Server
• VIM Tasks
  • Find & Replace
  • Spell-Check commands
• Use an Insecure Docker Registry
• Ansible ignore errors
• How to tail -f on dmesg
• Ubuntu Downgrade a package
• Make Ansible run Python 3 on target nodes
• Bash: Print all but first line
• Bash: Run code whenever a script exits

Find which active-passive bond member is active

cat /proc/net/bonding/bond0

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Convert p7b certificate to pem format

openssl pkcs7 -print_certs -in certnew.p7b -out cert.pem

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Journalctl commands

Watch the logs as they come in:

journalctl -f

When journalctl logs too much and you need to pair it down:

# to 30 days
journalctl --vacuum-time=30d

# to 500MB
journalctl --vacuum-size=500M

Show for a specific service

journalctl ceph*

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Get real RAM use of processes

You can always use top or htop or whatever. I use this to specifically get the info about those processes.

of ceph

ps aux | grep ceph | grep -v -e grep -e qemu -e "\[" | while read line; do bin=$(echo $line | awk '{print $11}'); pid=$(echo $line | awk '{print $2}'); id=$(echo $line | awk '{print $16}'); size=$(grep 'VmSize' /proc/$pid/status); echo -e "pid: $pid, \tbin: $bin, \tid: $id,    \tRAM: ($size)"; done

of VMs

for KVM

ps aux | grep qemu | grep -v -e grep| while read line; do bin=$(echo $line | awk '{print $11}'); pid=$(echo $line | awk '{print $2}'); guest=$(echo $line | awk '{print $13}'); size=$(grep 'VmSize' /proc/$pid/status); echo -e "pid: $pid, \tbin: $bin, \tguest: $guest,   \t\tRAM: ($size)" ; done

Bypass Chrome's self-signed cert warning

As of Mac Catalina, developing a site using self-signed certs is a bigger pain than before. Yeah you can go get the key and add it, but that's a nuisance. It looks like the Chrome team built something they call an "interstitial bypass keyword" to deal with that. That's one cool feature name.

Obvious disclaimer If you're not a developer doing this to access your own site, don't do this.

To bypass the warning, type

thisisunsafe

Apparently it used to be badidea but they rotated it since it got too well known. They'll probably do it again someday. I suspect people are using this on sites they "trust" without really knowing any better and getting hurt by it.

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Create a virtual loopback volume on Ubuntu

Useful for simulating a real drive when building a test environment. I use this to test OpenStack in a VM as a Ceph or LVM back-end.

# create empty file of all zeros
sudo dd if=/dev/zero of=/root/virtual-disk bs=1M count=512
# show list of loopback devices. Note its not there
losetup --list
# create the loopback device
losetup /dev/loop0 /root/virtual-disk
# confirm it worked
losetup --list

You can now treat it like a normal volume. Mount it, put a filesystem on it, whatever. Example:

pvcreate /dev/loop0
vgcreate cinder-volumes /dev/loop0

Use Systemd to re-map loopback on reboot

I found this useful systemd unit example that didn't quite work for me, but was close. Here's mine:

vi /etc/systemd/system/loops-setup.service

[Unit]
Description=Setup loopback devices

DefaultDependencies=no
Conflicts=umount.target

Requires=lvm2-lvmetad.service mnt-host.mount
Before=local-fs.target umount.target
After=lvm2-lvmetad.service mnt-host.mount

[Service]
ExecStart=/sbin/losetup /dev/loop0 /root/virtual-disk
ExecStop=/sbin/losetup -d /dev/loop0

RemainAfterExit=yes
Type=oneshot

[Install]
WantedBy=local-fs-pre.target

Then enable it

systemctl enable loops-setup

Reboot, then check vgdisplay (if that's your use case). It'll be there.

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Test MTU from Ubuntu

You would not believe the strange problems that an MTU issue can be the root cause of.

ping -M do -s 1472 <remote server>
ping -M do -s 8900 <remote server>

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Find big files on Mac OS

My SSD is always almost full. Here are useful commands for cleaning up:

# Find all the files over 1G
sudo find / -type f -size +1G -exec ls -lh {} \; | awk '{ print $9 ": " $5 }' 2>/dev/null

# List files in Downloads directory by size
ls -lhS ~/Downloads

Renew NTP Lease Ubuntu

service ntp stop
ntpd -gq
service ntp start

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Create AWS ECR Registry If Not Found

aws ecr describe-repositories --repository-names $repo_name \
    || aws ecr create-repository --repository-name $repo_name

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Passwordless sudo on Ubuntu Server

Run visudo as root.

# as root
export EDITOR=vim
visudo

Add the user (replace exampleUser) at the end.

exampleUser ALL=(ALL) NOPASSWD: ALL

For those who don't use vim, a quick reminder: i enter to insert mode , [esc] to exit insert mode, :wq to save and quit.

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VIM Tasks

Find & Replace

# replace findThis with ReplaceWithThis
:%s/findThis/ReplaceWithThis/g

Spell-Check commands

# enable/disable
:set spell spelllang=en_ca
:setlocal spell spelllang=en_ca
:set nospell

# next/last
]s
[s

# suggest change
z=

# add/remove word from dictionary
zg
zw`

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Use an Insecure Docker Registry

To use a registry with no HTTP cert:

vi /etc/docker/daemon.json

{
    "insecure-registries" : ["myregistrydomain.com:5000"]
}

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Ansible ignore errors

The command is ignore_errors: yes

Just throw this into the task that's failing. It will still fail, but the play won't stop.

Example:

- name: Ensure the docker service is running
  script: ../tools/validate-docker-execute.sh
    ignore_errors: yes

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How to tail -f on dmesg

dmesg -wH

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Ubuntu Downgrade a package

# Find the available versions under Versions:
apt-cache showpkg <package>

# Install one of those
apt-get install <package>=<version>

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Make Ansible run Python 3 on target nodes

Useful for OS that ship with python3 but not python2. Edit the inventory, find a group of the groups and apply this var.

[baremetal:vars]
ansible_python_interpreter=/usr/bin/python3

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Bash: Print all but first line

Use tail -n +2. This was unintuitive to me because +1 seems like it should do it.

# Example
docker images | awk '{print $3}' | tail -n +2

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Bash: Run code whenever a script exits

Use trap.

• This is a nice post covering lots of examples.
• Here's where I first saw it used

temp_file=$(mktemp)

cleanup() {
  rm "$temp_file"
}

trap cleanup EXIT