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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, please also note that there is a double quote beyond the single quote, which makes it even more complicated.
Thanks a lot. I am on CentOS 6.
877
If the backslash ( \ ) is used before the single quote then the value of the variable will be printed with single quote.
Single quotes(') and backslash(\) are used to escape double quotes in bash shell script. We all know that inside single quotes, all special characters are ignored by the shell, so you can use double quotes inside it. You can also use a backslash to escape double quotes.
If you're referring to what happens when you echo something, the single quotes will literally echo what you have between them, while the double quotes will evaluate variables between them and output the value of the variable.
Note that the sed pattern I used, 's/[()&]/\\&/g' , only escapes parentheses and ampersands; if your filenames contain any other shell metacharacters, be sure to add them to the character list in [] .
${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
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'
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