Collectd PostgreSQL Plugin

I couldn't find this link when searching with google

https://www.collectd.org/documentation/manpages/collectd.conf.html#plugin-postgresql

Prepend line with line length with Linux command

 awk '{printf "%2d| %s\n",length,$0}' filename

Credit: https://www.baeldung.com/linux/print-longest-lines-in-file#introduction-to-the-problem

How to run packer on Apple Silicon machine to create an image targeting x86 server

Set this environment variable before running packer command. 

DOCKER_DEFAULT_PLATFORM=linux/amd64

How to set memory for Solr on MacOS (installed via homebrew)

Modify this file /opt/homebrew/Cellar/solr/X.XX/homebrew.mxcl.solr.plist to add these lines

<key>EnvironmentVariables</key>

<dict>

  <key>SOLR_JAVA_MEM</key>

  <string>-Xmx2g</string>

</dict>

Save and restart Solr

$ brew services restart solr

How to set Java version for lein

To select specific Java (JDK) to use for leiningen, set environment variable JAVA_CMD

For example,

JAVA_CMD=/path/to/bin/java lein repl

Credit: https://gist.github.com/camsaul/c982019fd915510677236cd4b720a583#gistcomment-3439527 

How to print the longest line in a file?

awk 'length > max_length { max_length = length; longest_line = $0 } END { print longest_line }' ./text

From: https://unix.stackexchange.com/a/24510

Links related to how force macOS to use integrated/discrete graphics

• How to Force Your MacBook Pro to Use Its Discrete Graphics Card When It’s Plugged In
• https://forums.macrumors.com/threads/good-gpu-switching-application-for-mpb-16.2220002/post-28149852

Copied from the above post in case the post is gone

sudo pmset -a gpuswitch 0

-b when on battery

-c when on AC power

-a always

0 iGPU only

1 dGPU only

2 automatic graphics switching

To reset everything back to default:

sudo pmset -a gpuswitch 2

[PostgreSQL] How to check null in jsonb array

 Use jsonb_array_elements  to convert jsonb array to set (queriable similar to normal table) then we can use  where x::text = 'null' to match the null value.

Examples:

=> select x->'id' as id 
from jsonb_array_elements(
'[{"id": 4, "val": [null,1,null]}, 
{"id": 2, "val": [3]}]'::jsonb
) as x, 
jsonb_array_elements(x->'val') as y 
where y::text = 'null';
 id 
----
 4
 4
(2 rows)


=> select x->'id' as id
from jsonb_array_elements(
'[{"id": 4, "val": [null,1,null]}, 
{"id": 2, "val": [3]}]'::jsonb
) as x, 
jsonb_array_elements(x->'val') as y 
group by x->'id' 
having count(x) filter (where y::text  = 'null') > 1;
 id 
----------
 4
(1 row)

Adapted from: https://dba.stackexchange.com/a/257799

[PostgreSQL] How to check if there's a NULL value in array

WHERE array_position(array_column, NULL) IS NOT NULL 

Credit: https://stackoverflow.com/a/47984368/1032449

Some usages of jstat

-gcutil

$ jstat -gcutil

This command shows utilization percentage of memory spaces of a java process.

S0: Survivor space 0
S1: Survivor space 1
E: Eden space
O: Old-gen space

GC activities

YGC: Young GC count
YGCT: Time spent by Young GC 
FGC: Full GC count
FGCT: Time spent by Full GC


-gc

$ jstat -gc

Find the current vm heap size by sum value under S0C, S01, EC, OC


More at https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstat.html

[Linux] Visualize parent - child process relationship

$ ps -axf -o pid,ppid,tty,stat,cmd

Credit: Orphan vs Zombie vs Daemon processes

[Linux] sum memory usage of processes with the same name (or sub process)

Use atop. Press p and press m.

Credit: https://unix.stackexchange.com/a/209915

Compress a file before scp from remote server to local

ssh user@host "tar -zcvf - -C /path/to/dir filename" | tar -zxvf -

Note: only use when it's much faster to transfer a compressed file to your machine than transferring the original one.

Credits:
https://superuser.com/a/1354868
https://stackoverflow.com/a/3035446/1032449

[PostgreSQL] How to create read-only standby replica (follower)

This link and this link explain it really well and concise

Useful commands for observe running JVM process

$ jcmd

List running processes

$ jinfo 

Get details of a process

$ jps -lvm

List running processes with params

Clojure how to get JVM options from inside application

(.getInputArguments (java.lang.management.ManagementFactory/getRuntimeMXBean))

Credit: How to get vm arguments from inside of java application?

Clojure has lesser typing error problems than the others well-known dynamic typing languages

Give these snippets a try.

// JS
function foo () {
  bar(1)
}

# Ruby
def foo
  bar(1)
end

; Clojure
(defn foo [] 
  (bar 1))

As you can see from the images below, Clojure doesn't allow you to refer to an undeclared symbol. I believe this takes away quite a bit of typing error issues.

Config number of file descriptors on Linux

Check file descriptor limit for a process. Vary depending on owner of the process

$ cat /proc/{process-id}/limits

Check current number of file descriptors of a process

$ sudo ls /proc/{process-id}/fd | wc -l

Set file descriptors limit for a user at /etc/security/limits.conf

*    soft nofile 64000
*    hard nofile 64000
root soft nofile 64000
root hard nofile 64000

Check system file descriptor limit

$ cat /proc/sys/fs/file-max

Check current number of system file descriptors

$ cat /proc/sys/fs/file-nr 

Credits:

• Raising the Maximum Number of File Descriptors (Open Files) on Ubuntu 14.04 Trusty
• https://unix.stackexchange.com/a/104405

Increase video volume

$ ffmpeg -i input -vcodec copy -af "volume=20dB" output

Credit: How to increase volume in a video without re-encoding the video 

Convert .ts to .mp4

$ ffmpeg -i input.ts -acodec copy -vcodec copy out.mp4

Credit: http://stackoverflow.com/a/17916418/1032449