Bash: Variable Expansion in Single Quote / Double Quote

Question

I want to add a variable ${date} in the following bash script:

ffmpeg -i in.flv -vf drawtext="fontfile=Sans.ttf:text='Today is ${date}':fontsize=6" out.flv

Obviously, ${date} won't expand in single quote, , which makes it even more complicated. 后面 ，这使得它更加复杂。

Thanks a lot. I am on CentOS 6.

Answer 1

${date} is expanded because it is between double quotes (the single quotes inside the double quotes are just characters)

Test it with:

$ export date=SOMEVALUE
$ echo ffmpeg -i in.flv -vf drawtext="fontfile=/usr/share/fonts/dejavu/DejaVuLGCSans.ttf:text='Today is ${date}':fontsize=6" out.flv
ffmpeg -i in.flv -vf drawtext=fontfile=/usr/share/fonts/dejavu/DejaVuLGCSans.ttf:text='Today is SOMEVALUE':fontsize=6 out.flv

Answer 2

Your ${date} WILL expand correctly. As you said yourself, you surround the whole string with double quotes, and bash will expand variables into double quotes.

The fact that there are inner single quotes does not matter at all:

fg@erwin ~ $ ritchie="Goodbye world"
fg@erwin ~ $ echo "When Dennis passed away, he said '$ritchie'"
When Dennis passed away, he said 'Goodbye world'