Useful Commands

Imagemagick

Splitting PDF files

convert -density 600 {INPUT.PDF} -crop 50x100% +repage {OUT.PDF}

-density: adjusts the quality of the resulting pdf. Higher values look better, but take longer to process
-crop 50x100%: this splits the pdf into "left" and "right". 100x50% would split into "top" and "bottom"

Ghostscript

Merge multiple PDF files

Ghostscript is preinstalled on a lot of Linux systems and can quite easily be used to merge and optimize multiple PDF files into one.

gs -q -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -o merged.pdf .pdf .pdf

Merge / convert multiple PDF files into one PDF

Find

Change filtered permissions

Using find with its exec switch one can set different permissions based on the usual find filters.
One example would be only changing file or directory permissions.

find (directory) -type f -exec chmod 744 {} +

Replacing -type f with -type d would execute the chmod for directories instead.

Arch Linux

Detect AUR package rebuilds

Install the package extra/rebuild-detector
It will create a pacman hook to check which packages need to be rebuild, but can also be executed with the following command.

checkrebuild

The packages might be rebuilt automatically a lot of the time, but sometimes it is necessary to for a rebuild using the AUR helper.
This is an example using paru

paru -S --rebuild=yes 

Various

Unlock user after too many failed sudo attempts

Use faillock to check for failed sudo attempts

faillock --user 

Unlock a locked account

faillock --user  --reset

Unlock user after too many failed sudo attempts by Josh Sherman

Overwrite disk with pseudorandom data

Using openssl on CPUs with AES acceleration one can create pseudorandom data with high speeds.
Much faster than /dev/urandom at least

openssl enc -aes-128-ctr -md sha512 -pbkdf2 -nosalt -pass file:/dev/urandom < /dev/zero | pv > {TARGET DISK}

Around 2GiB/s on my Ryzen 7 1700x if output to /dev/null

Formatted csv in terminal

From Pretty CSV viewing on the Command Line

column -t -s, < {FILE.CSV}

Download directory from webdav

Using wget, it's possible to download directories recursively from WebDAV.

wget -r -nH -np --cut-dirs=1 --user={USERNAME} --password={PASSWORD} https://WEBDAVHOST/DIR/DIR

Find USB speed in use

First, the USB device you want to check has to be selected

lsusb
(out)Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
(out)Bus 003 Device 003: ID 0c45:652f Microdia Backlit Gaming Keyboard
(out)Bus 003 Device 002: ID 046d:c084 Logitech, Inc. G203 Gaming Mouse
(out)Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
(out)Bus 002 Device 002: ID 0781:55a3 SanDisk Corp.  SanDisk 3.2Gen1
(out)Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
(out)Bus 001 Device 002: ID 2972:0047 FiiO Electronics Technology FiiO BTR5
(out)Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Now, using the ID, the USB class used can be determined

lsusb -d 0781:55a3 -v | grep bcdUSB
(out)bcdUSB               3.20

Change file modify time

Using touch it is possible to change the timestamps on a file.
Note: The file has to be owned by the user executing the command

Example changing the access and modify timestamps:

touch -a -m -t 202307291506.07 

-a: accessed time
-m: modified time
-t: timestamp - [[CC]YY]MMDDhhmm[.ss] time format

Alternatively to -t it is also possible to use -d for a looser format.
ISO8061 obviously works as well.

touch -a -m -d "2 hours ago" 
touch -a -m -d "2023-07-29T00:23" 

Linux - modify file modify/access/change time
How can I change the date modified/created of a file?

Create a random temporary directory / file

Using mktemp, a randomly named file or directory will be created in /tmp

Create a file:

mktemp

Create a directory:

mktemp -d

Save the output into an environment variable for future referencing

sh / Bash:

export TMPDIR=$(mktemp -d)

Fish:

set TMPDIR (mktemp -d)