My Tech Notes

openssl: request new/renew ssl certificate

2020-12-09T22:14:00.003-08:00

Generate certificate request

Genreate certificate request and private key:

openssl req -new -config csr.conf -keyout private.key -out server.csr

Generate certificate request using existing private key:

openssl req -new -config csr.conf -key private.key -out server.csr

Submit the cerficate request file to be signed by CA

openssl: check certificate expiry

2020-12-09T22:06:00.002-08:00

openssl x509 -enddate -noout -in server.crt

Create AES256 encrypted zip file on Mac OS

2020-12-09T04:40:00.003-08:00

Install p7zip
```
brew install p7zip
```

Compress a directory

7za a -tzip '-pPASSWORD' -mem=AES256 data.zip data/

Find files and set their permissions

2020-12-08T22:22:00.002-08:00

find files and pipe them to chmod to set permissions:


find . -type d -exec chmod 0755 {} \;
find . -type f -exec chmod 0644 {} \;

MacOS show hidden files and directories

2020-12-08T14:16:00.000-08:00

Show hidden files


defaults write com.apple.finder AppleShowAllFiles True; killall Finder

Make hidden files invisible again


defaults write com.apple.finder AppleShowAllFiles False; killall Finder

python 3 ldap search example

2020-11-23T16:43:00.005-08:00

grep: include lines before and after the matching line

2020-11-03T17:09:00.002-08:00

Include 2 lines before the matching line:

grep -B 2 'keyword' file1

Include 2 more lines after the matching line:

grep -A 2 'keyword' file1

Use netcat (nc) to check if remote port is open

2020-10-28T20:08:00.002-07:00


nc -vz -G 3 a-remote-host 443

-G specifies the connection timeout, 3 seconds in the above example.

Firefox: improve page loading speed

2020-10-09T20:39:00.006-07:00

property	value
network.http.max-persistent-connections-per-server	10
network.http.max-connections	256
browser.tab.animate	false
browser.panorama.animate_zoom	false
network.dns.disablePrefetch	true
network.prefetch-next	false
network.http.speculative-parallel-limit	0

Windows Batch Script: pass multiple line string

2020-10-09T02:26:00.009-07:00



set PS=^
Write-Host Line1;^
Write-Host Line2;^
Write-Host Line3

powershell -NoProfile -Command %PS%

PowerShell: create directories recursively

2020-10-09T02:23:00.004-07:00


New-Item -Path “C:\AAA\BBB\CCC” -Type Directory -Force

similar to mkdir -p on Linux/Mac OS.

PowerShell: write text to file

2020-10-09T02:21:00.002-07:00


Out-File -FilePath .\text.txt -InputObject 'Hello' -Encoding utf8

PowerShell: single quote vs. double quote

2020-10-09T02:17:00.004-07:00

Single quotes:


$i = 5
'The value of $i is $i.'

Output:


The value of $i is $i.

Double quotes:


$i = 5
"The value of $i is $i."

Output:


The value of 5 is 5.

Windows Batch Script: PAUSE with custom message

2020-10-09T02:04:00.002-07:00


@echo off

echo Custom Message
pause>NUL

log4j2: conditional appender using routes

2020-10-07T04:39:00.005-07:00

<?xml version="1.0"?>
<Configuration status="INFO">
  <Appenders>
    <Console name="SYS_OUT" target="SYSTEM_OUT" />
    <Console name="SYS_ERR" target="SYSTEM_ERR" />

    <Routing name="Router">
      <Routes pattern="$${env:STREAM_TO:-OUT}">
        <Route ref="SYS_OUT" key="OUT" />
        <Route ref="SYS_ERR" key="ERR" />
      </Routes>
    </Routing>
  </Appenders>

  <Loggers>
    <Root level="INFO">
      <AppenderRef ref="Router" />
    </Root>
  </Loggers>
</Configuration>

Maven: conditionally execute plugins using profiles

2020-10-07T04:29:00.005-07:00

<project>
<properties>...</properties>
<repositories>...</repositories>
<dependencies>...</dependencies>
<build>
  <plugins>...</plugins>
</build>
<profiles>
  <profile>
    <id>windows</id>
    <activation>
      <os>
        <name>Windows XP</name>
        <family>Windows</family>
        <arch>x86</arch>
        <version>5.1.2600</version>
      </os>
    </activation>
    <properties>...</properties>
    <repositories>...</repositories>
    <dependencies>...</dependencies>
    <build>
      <plugins>...</plugins>
    </build>
  </profile>
</profiles>
</project>

To activate the profile manually:

mvn package -p PROFILE_ID

Email From, Reply-To and Return-Path

2020-10-06T17:16:00.000-07:00

From and Reply-To are originator fields of the email.
Return-Path is created by the receiver NOT the sender. It is derived from the SMTP MAIL FROM command used to deliver the message. This address need not to have anything in common with the From address header within the message, and designates where the message should be sent to in the event of a delivery failure, i.e. exactly what you want the VERP address for.

According to RFC5321 section 4.4:

A message-originating SMTP system SHOULD NOT send a message that already contains a Return-path header field.

Originator fields of email: Sender, From, Reply-To

2020-10-06T16:57:00.005-07:00

From: The "From:" field specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message.
Sender:The "Sender:" field specifies the mailbox of the agent responsible for the actual transmission of the message. For example, if a secretary were to send a message for another person, the mailbox of the secretary would appear in the "Sender:" field and the mailbox of the actual author would appear in the "From:" field. If the originator of the message can be indicated by a single mailbox and the author and transmitter are identical, the "Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD appear.
Reply-To: The originator fields also provide the information required when replying to a message. When the "Reply-To:" field is present, it indicates the address(es) to which the author of the message suggests that replies be sent. In the absence of the "Reply-To:" field, replies SHOULD by default be sent to the mailbox(es) specified in the "From:" field unless otherwise specified by the person composing the reply.
Return-Path

Java: try with AutoCloseable resources

2020-10-04T23:14:00.005-07:00

1. Multiple AutoCloseable resources


  try (InputStream in = getInputStream(); OutputStream out = getOutputStream()) {
      ... ...
  }

The resources are closed in the reverse order of declaration.

2. Resource leaking caused by nested constructors

There is possible resource leaking issue for the code below:


  try(InputStream in = new MyFilterInputStream(new BufferedInputStream(new FileInputStream("/tmp/test.file")))) {
      ... ...
  }

Because any checked exception from the chain of the constructors can cause resource failing to close. The code below with multiple declared AutoCloseables should be used:


  try(FileInputStream fis = new FileInputStream("/tmp/test.file");
      BufferedInputStream bis = new BufferedInputStream(fis);
      MyFilterInputStream mfis = new MyFilterInputStream(bis)) {
      ... ...
  }

3. What will happen if the AutoCloseable is null

A resource is closed only if it initialized to a non-null value. So the code works without having to worry about NPE when in is null.


  try(InputStream in = createNew==true? createInputStream(): null) {
      if(in==null) {
          ... ...
      }
  }

Count the number of symbolic links within a directory

2020-09-29T20:52:00.000-07:00


find folder/ -type l | wc -l

Powershell: compare two directories including their descendants

2020-09-29T18:20:00.004-07:00

create catalog for the source directory:


New-FileCatalog -Path source -CatalogFilePath source.cat -CatalogVersion 2.0

test the destination directory against the source catalog:


Test-FileCatalog -Path destination -CatalogFilePath source.cat

Both New-FileCatalog and Test-FileCatalog cmdlets only work in PowerShell 5.1 and later, if you are on Windows 7 or 8.1, follow the link to upgrade your PowerShell.

Bash: check if two paths refer to the same file

2020-09-28T05:03:00.001-07:00

file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.


if [[ "$HOME/Downloads/file1" -ef "Downloads/file1" ]]; then
    echo "Same file"
fi

PowerShell: execute a .msi installer

2020-09-25T00:21:00.005-07:00


$MSI=AppInst.msi
$LOG=AppInst.log
$MSIArguments = @(
    "/i"
    $MSI
    "/qn"
    "/norestart"
    "/L*v"
    $LOG
)

Start-Process "msiexec.exe" -ArgumentList $MSIArguments -Wait -NoNewWindow

Microsoft OneDrive: get direct download link

2020-09-23T21:51:00.002-07:00

Simply append &download=1 to the link.

PowerShell: show pop up dialog

2020-09-23T19:13:00.005-07:00


Add-Type -AssemblyName PresentationCore,PresentationFramework
$ButtonType = [System.Windows.MessageBoxButton]::YesNo
$MessageIcon = [System.Windows.MessageBoxImage]::Question
$MessageBody = "Are you sure you want to proceed?"
$MessageTitle = "Confirm"

$Result = [System.Windows.MessageBox]::Show($MessageBody,$MessageTitle,$ButtonType,$MessageIcon)

Write-Host "Your choice is $Result"

My Tech Notes

openssl: request new/renew ssl certificate

openssl: check certificate expiry

Create AES256 encrypted zip file on Mac OS

Find files and set their permissions

MacOS show hidden files and directories

python 3 ldap search example

grep: include lines before and after the matching line

Use netcat (nc) to check if remote port is open

see also

Firefox: improve page loading speed

see also

Windows Batch Script: pass multiple line string

PowerShell: create directories recursively

PowerShell: write text to file

see also

PowerShell: single quote vs. double quote

Windows Batch Script: PAUSE with custom message

see also

log4j2: conditional appender using routes

see also

Maven: conditionally execute plugins using profiles

see also

Email From, Reply-To and Return-Path

see also

Originator fields of email: Sender, From, Reply-To

see also

Java: try with AutoCloseable resources

1. Multiple AutoCloseable resources

try (InputStream in = getInputStream(); OutputStream out = getOutputStream()) { ... ... } The resources are closed in the reverse order of declaration.

2. Resource leaking caused by nested constructors

3. What will happen if the AutoCloseable is null

see also

Count the number of symbolic links within a directory

Powershell: compare two directories including their descendants

see also

Bash: check if two paths refer to the same file

see also

PowerShell: execute a .msi installer

see also

Microsoft OneDrive: get direct download link

see also

PowerShell: show pop up dialog

try (InputStream in = getInputStream(); OutputStream out = getOutputStream()) {
... ...
}

The resources are closed in the reverse order of declaration.