From 7e8edd71d4e3a4c689099b283eb1b40c895cba5f Mon Sep 17 00:00:00 2001
From: Michele Cereda <cereda.michele@gmail.com>
Date: Tue, 27 Sep 2022 23:07:50 +0200
Subject: [PATCH] Added howto to mount files as virtual file systems

---
 knowledge base/dd.md                          | 36 +++++++++++++++++
 .../mount files as virtual file systems.md    | 36 +++++++++++++++++
 knowledge base/truncate.md                    | 39 +++++++++++++++++++
 3 files changed, 111 insertions(+)
 create mode 100644 knowledge base/dd.md
 create mode 100644 knowledge base/mount files as virtual file systems.md
 create mode 100644 knowledge base/truncate.md

diff --git a/knowledge base/dd.md b/knowledge base/dd.md
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index 0000000..30e97da
--- /dev/null
+++ b/knowledge base/dd.md	
@@ -0,0 +1,36 @@
+# dd
+
+Convert and copy a file.
+
+## TL;DR
+
+```sh
+# Read 512 random Bytes for each iteration and save them .
+dd if='/dev/urandom' of='output/file' count=2 bs=512
+
+# Read 1000 Bytes for each iteration and save them while watching the progress.
+dd if='/input/file' of='output/file' count=4M bs=1k status='progress'
+
+# Create a 1GiB file with nothing but zeros, ready to mkswap(8) it.
+dd if='/dev/zero' of='/swapfile' count=1048576 bs=1024
+
+# Make a bootable USB drive from an isohybrid file.
+dd if=file.iso of=/dev/usb_drive status=progress
+
+# Clone a drive to another drive with 4 MiB block.
+# Ignore any error and show the progress.
+dd if=/dev/source_drive of=/dev/dest_drive bs=4M conv=noerror status=progress
+
+# Generate a system backup into an IMG file and show the progress:
+dd if=/dev/drive_device of=path/to/file.img status=progress
+
+# Restore a drive from an IMG file and show the progress:
+dd if=path/to/file.img of=/dev/drive_device status=progress
+```
+
+## Sources
+
+- [cheat.sh]
+
+<!-- -->
+[cheat.sh]: https://cheat.sh/dd
diff --git a/knowledge base/mount files as virtual file systems.md b/knowledge base/mount files as virtual file systems.md
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index 0000000..2639e8f
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+++ b/knowledge base/mount files as virtual file systems.md	
@@ -0,0 +1,36 @@
+# Mount files as virtual file systems
+
+## TL;DR
+
+```sh
+# Create the file.
+truncate -s '10G' 'path/to/file'
+dd if='/dev/zero' of='path/to/file' bs=4MiB count=250K status='progress'
+
+# Create the file system on such file.
+mkfs.ext4 'path/to/file'
+
+# Create the mount point.
+mkdir 'mount/point'
+
+# Mount the file system.
+sudo mount -t 'ext4' 'path/to/file' 'mount/point'
+```
+
+Prefer `truncate` to `dd` to let the file expand dynamically and be resized (both larger or smaller) without damaging data with `losetup` and `resize2fs`.
+
+## Further readings
+
+- [dd]
+- [truncate]
+
+## Sources
+
+- [How do I create a file and mount it as a filesystem?]
+
+<!-- -->
+[dd]: dd.md
+[truncate]: truncate.md
+
+<!-- -->
+[how do i create a file and mount it as a filesystem?]: https://askubuntu.com/questions/85977/how-do-i-create-a-file-and-mount-it-as-a-filesystem#1402052
diff --git a/knowledge base/truncate.md b/knowledge base/truncate.md
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index 0000000..0102f1b
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+++ b/knowledge base/truncate.md	
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+# truncate
+
+Shrink or extend the size of a file to the specified size.
+
+## TL;DR
+
+```sh
+# Empty the contents of files.
+truncate -s 0 'file'
+
+# Set the size of an existing file.
+# if the file does not exist, create it anew of the specified size.
+truncate -s 100 'file'
+truncate --size 5k 'file'
+truncate --size 10G 'file'
+
+# Extend a file's size by 50 MiB and fill it with holes.
+# Holes read as zero bytes.
+truncate --size +50M 'file'
+
+# Shrink a file by 2 GiB.
+# Removes data from the end of file.
+truncate --size -2G 'file'
+
+# Empty the file's content, but do not create it if existing.
+truncate --no-create --size 0 'file'
+```
+
+## Further readings
+
+- [GNU's documentation]
+
+## Sources
+
+- [cheat.sh]
+
+<!-- -->
+[cheat.sh]: https://cheat.sh/truncate
+[gnu's documentation]: https://www.gnu.org/software/coreutils/truncate