使用包含空格 (\\s) 和 sed 的正则表达式的变量

Question

Im trying to create a sort script using literal string variables and Regex and a sort using sed in bash. I cannot seem to find the liternal strings with spaces when using variables, although can find them when using the regex directly. So :

#!/bin/bash
group1="IRISHFHD"
group2="REGIONAL FHD"

sed -i '/group-title="'${group1}/',+1d' JWLINE.m3u
sed -i '/group-title="'${group2}/',+1d' JWLINE.m3u

Ive tried adding \\s into the group variable but it doesnt work.

John

Answer 1

The problem has nothing to do with regex, it's all down to how the shell treats variables' values. When a variable is expanded without double-quotes around it (ie ${group2} ), the shell will split it into "words" based on whitespace. It'll also try to expand any words that contain shell wildcards into lists of matching files, and several regex metacharacters look like shell wildcards, which can cause serious chaos.

In this example:

sed -i '/group-title="'${group2}/',+1d' JWLINE.m3u

It's a little more complicated, because the variable reference is in between two single-quoted sections. In this case, the part before the variable reference gets attached to the first "word" in the variable, and the part after gets attached to the last word. Essentially, it expands into the equivalent of this:

sed -i '/group-title="REGIONAL' 'FHD/,+1d' JWLINE.m3u
                               ^ That's a space between arguments

Anyway, since it gets split on the whitespace, sed gets two partial arguments instead of one whole one, and it doesn't work at all.

Solution: as in almost all situations, you should have double-quotes around the variable reference to prevent weird effects like this. There are a few options for this. You could just add double-quotes around the variable part:

sed -i '/group-title="'"${group2}"/',+1d' JWLINE.m3u

...but IMO this is confusing; some of those quotes are syntactic (ie parsed by the shell), and one is literal (passed to sed as part of the regex), and it's not obvious which are which. I'd prefer to just use double-quotes around the whole thing, and escape the double-quote that's supposed to be literal:

sed -i "/group-title=\"${group2}/,+1d" JWLINE.m3u
                     ^^ Escape makes this " a literal part of the argument.

(In double-quotes, you'd also need to escape any dollar signs, backslashes, or backticks that were supposed to be literal parts of the argument. But in this case, there aren't any of those.)