I use php to convert pdf to jpeg but sometimes it’s very slow. So, the solution is to use poppler and use pdftoppm directly to shell.

https://stackoverflow.com/questions/43085889/how-to-convert-a-pdf-into-jpg-with-command-line-in-linux

If you are using blanks in filenames or any other special character then you need to use escapeshellarg.

https://stackoverflow.com/questions/43513228/php-shell-exec-failed-due-to-some-specific-character

Also, exec and exec_shell are returning status and string of the last command executed (read docs) but it’s somewhat hard to understand when it’s true or false. I didn’t manage to use === operator to check logical expressions returned from the ran command.

https://stackoverflow.com/questions/41212053/php-shell-exec-and-exec-returning-false-when-ran

Also,for pdf merging use:

https://stackoverflow.com/questions/2507766/merge-convert-multiple-pdf-files-into-one-pdf

rsync -av --include '*.md' --include '*/' --exclude '*' --prune-empty-dirs src/ dst/

https://unix.stackexchange.com/questions/741904/rsync-to-exclude-everything-except-specific-files

If you need to convert pdf to jpg I was working with php but it might be slow, obviously. But if you are using linux shell then it’s easier and faster.

72 dpi (web) = 595 X 842 pixels 
300 dpi (print) = 2480 X 3508 pixels 
600 dpi (print) = 4960 X 7016 pixels

https://stackoverflow.com/questions/43085889/how-to-convert-a-pdf-into-jpg-with-command-line-in-linux

Poppler has one command to merge all pdfs from folder:

pdfunite *.pdf out.pdf

https://stackoverflow.com/questions/2507766/merge-convert-multiple-pdf-files-into-one-pdf

If you need to find what is the biggest folders in current path in linux then this is the command:

du -sh * 2>/dev/null | sort -h

It needs couple of seconds to calculate (if you have slow io vps or hdd) and this is what you get:

root@vps3345:/home/nick/web# du -sh * 2>/dev/null | sort -h
713M    alle.ch
1.1G    moji.com
1.6G    rab.de
1.8G    zoek.nl
2.5G    al.at
5.4G    mojia.si
7.6G    a.co.uk
11G     sle.cz
13G     akceer.com

About linux screen program. Really nice to have it but usually when I find some article about using some linux software, there are always some missing pieces and then I have to search google again and again and that’s not good.

Type:

screen -> it opens up new screen window and you are in it

ctrl +a is for switching windows

ctrl+a c is for creating new screen window

Now to close it (detach) use:

ctrl+a d

To list all current screens:

screen -ls

To reattach to it use:

screen -r (number)

To close it use:

screen -XS (number) quit

Here are references:

https://linuxize.com/post/how-to-use-linux-screen/

https://askubuntu.com/questions/356006/kill-a-screen-session