Modify the content of variable using sed (or something similar)

Question

I wrote a BASH file that features multiple embedded loops of the form

for P in {'0.10','0.20', [...] '0.90','1.00'}; do 
   for Q in {'0.10','0.20', [...] ,'0.90','1.00'}; do
    [...] 

I use these variables both as parameters for a command line application, and to create file names directly in BASH. I would like to create duplicates, say $P_REP=0_10 that replaces the dot by an underscore without writting a explicit switch statement for every case, or some hardcoded equivalent. The (non-elegant way) I found to go about it is to

1. dump the content of P,Q to a temporary file.
2. replace the dot by an underscore using sed 's/./_/ -i.
3. read the file again and load its content to the new variable.

Hence, I was wondering if it is possible to run a sed like command directly on the content of a variable?

Solution 1

You can do pattern substitution directly in bash:

P_REP=${P/./_}
Q_REP=${Q/./_}

From the bash(1) man page:

Paramter Expansion

${parameter/pattern/string}

Pattern substitution. The pattern is expanded to produce a pattern just as in pathname expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. If pattern begins with /, all matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with #, it must match at the beginning of the expanded value of parameter. If pattern begins with %, it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pattern may be omitted. If parameter is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.

Solution 2

John Kugelman's answer is fine for your example, but if you need to process the content of a variable with the actual sed program (or some other arbitrary command), you can do it like this:

P_REP=$(sed 's/\./_/' <<< "$P")

Comments

[jgyou]

Oh wow thank you. I'm not a bash expert and I did not know that there was such a thing as a man page for bash :-/. I will look there first next time!

[John Kugelman]

@JGab Check out {parameter#word} and ${parameter%word} as well, both under the same "Parameter Expansion" section of the man page. Bash can do a lot of cool things, but man oh man, its man page is dense.

[OrangeDog]

This is the answer if you require captures in the substitution.

[mgouin]

That is awesome. I've been doing echo + pipe for nothing :)