30 Sep 2021

Some useful commands under Ubuntu

In this post we record several useful commands/softwares under Ubuntu.

convert is a very powerful command for post-processing images. For example, the following command is used to reduce the size of a jpg file.
```
	convert -resize 30% input.jpg output.jpg
```
pdftk can be used to process pdf files. Below are two examples.
1. To merge 1.pdf and 2.pdf files into one pdf file named 1-and-2.pdf, use
```
	pdftk 1.pdf 2.pdf cat output 1-and-2.pdf
```
1. Extract the first, the third and the fourth pages of the file input.pdf to output file out.pdf
```
	pdftk input.pdf cat 1 3-4 output out.pdf
```
pdftoppm can convert a pdf file to JPEG format. Suppose the input pdf file contains two pages, then the command
```
  pdftoppm -jpeg -r 300 input.pdf output
```
will produce two JPEG files, named as output-1.jpg and output-2.jpg. In the above, ‘'-r 300’' specifies the resolution of the output pictures.

The following command uses gs to reduce the size of a pdf file.

	gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

More details can be found here.

img2pdf converts a JPEG file to PDF format. As an example,
```
  img2pdf --pagesize A4 input.jpg -o output.pdf
```