Category: Frequently used

CIDR & Subnet mask

Damien
CIDRSubnet maskWildcard mask# of IP addresses# of usable IP addresses
/32255.255.255.2550.0.0.011
/31255.255.255.2540.0.0.120*
/30255.255.255.2520.0.0.342
/29255.255.255.2480.0.0.786
/28255.255.255.2400.0.0.151614
/27255.255.255.2240.0.0.313230
/26255.255.255.1920.0.0.636462
/25255.255.255.1280.0.0.127128126
/24255.255.255.00.0.0.255256254
/23255.255.254.00.0.1.255512510
/22255.255.252.00.0.3.2551,0241,022
/21255.255.248.00.0.7.2552,0482,046
/20255.255.240.00.0.15.2554,0964,094
/19255.255.224.00.0.31.2558,1928,190
/18255.255.192.00.0.63.25516,38416,382
/17255.255.128.00.0.127.25532,76832,766
/16255.255.0.00.0.255.25565,53665,534
/15255.254.0.00.1.255.255131,072131,070
/14255.252.0.00.3.255.255262,144262,142
/13255.248.0.00.7.255.255524,288524,286
/12255.240.0.00.15.255.2551,048,5761,048,574
/11255.224.0.00.31.255.2552,097,1522,097,150
/10255.192.0.00.63.255.2554,194,3044,194,302
/9255.128.0.00.127.255.2558,388,6088,388,606
/8255.0.0.00.255.255.25516,777,21616,777,214
/7254.0.0.01.255.255.25533,554,43233,554,430
/6252.0.0.03.255.255.25567,108,86467,108,862
/5248.0.0.07.255.255.255134,217,728134,217,726
/4240.0.0.015.255.255.255268,435,456268,435,454
/3224.0.0.031.255.255.255536,870,912536,870,910
/2192.0.0.063.255.255.2551,073,741,8241,073,741,822
/1128.0.0.0127.255.255.2552,147,483,6482,147,483,646
/00.0.0.0255.255.255.2554,294,967,2964,294,967,294

* /31 is a special case detailed in RFC 3021 where networks with this type of subnet mask can assign two IP addresses as a point-to-point link.

Find & delete files

Damien 
find /path/to/dir -args

Delete empty directories

find /path/to/dir -empty -type d -delete

Delete empty files

find /path/to/dir -empty -type f -delete

Count all empty files or directories

Count empty dirs only 

find /path/to/dir/ -empty -type d | wc -l

Count empty files only 

find /path/to/dir/ -empty -type f | wc -l

Where :

  • -empty : Only find empty files and make sure it is a regular file or a directory.
  • -type d : Only match directories.
  • -type f : Only match files.
  • -delete : Delete files. Always put -delete option at the end of find command as find command line is evaluated as an expression, so putting -delete first will make find try to delete everything below the starting points you specified.

This is useful when you need to clean up empty directories and files in a single command.

Cleanup unnecessary files and Desktop OSses clutter

cd "$SOURCEDIR"
find ./ \( -name ".DS_Store" -or -name "._*" -or -name "Thumbs.db" -or -name "*.tmp" -or -name "*.lnk" -or -name "TemporaryItems" -or -name "folders.501" -or -name ".TemporaryItems" -or -name "__MACOSX" \) -ls -delete

Change ownership of files owned by a specific username

find /path/to/dir/ -user username -exec chown newuser:newgroup {} +

Create bootable macOS install media

Damien 
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/UNTITLED

To avoid getting a ‘Installation Failed’ message, you need to change the date before installing : 

date {month}{day}{hour}{minute}{year}

Set the Mac’s date to a date right around the time of release of the OS, e.g.

  • 10.7 : Mac OS X Lion was released on July 20, 2011.
  • 10.8 : OS X Mountain Lion was released on July 25, 2012.
  • 10.9 : OS X Mavericks was released on October 22, 2013.
  • 10.10 : OS X Yosemite was released on October 16, 2014.
  • 10.11 : OS X El Capitan was released on September 30, 2015.
  • 10.12 : macOS Sierra was released on September 20, 2016.
  • 10.13 : macOS High Sierra was released on September 25, 2017.
  • 10.14 : macOS Mojave was released on September 24, 2018.
  • 10.15 : macOS Catalina was released on October 7, 2019.
  • 11 : macOS Big Sur was released November 12, 2020.
  • 12 : macOS Monterey was released on October 25, 2021.

When creating an Intel install media on an ARM-based Mac, the OS expects the createinstallmedia executable to be signed before running, but the Apple-made program isn’t.
You’ll encounter a zsh : killed. error otherwise.
Sign the program with the following : 

codesign -s - -f createinstallmedia