---
title: 'Useful Commands'
---

[toc]
## 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"
```

## 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.  

## Various
### 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`