sed giving invalid arithmetic operator error

Question

sed keeps giving invalid arithmetic operator error. I am trying to assign the output of sed to a variable.

This one works,

var=$(sed  -e 's/"currentGeneration":5010/"currentGeneration":5011/'  <<< $content )

but when I try to do the same with variable instead of 5010 and 5011, it fails with invalid arithmetic operator

var=$((sed  -e 's/"currentGeneration":$currg/"currentGeneration":$nextg/'  <<< $content ))

I tried below also but it won't substitute anything.

var=$(sed  -e 's/"currentGeneration":$currg/"currentGeneration":$nextg/'  <<< $content )

I am pretty sure I am missing something basic. Learning shell scripting on the job :(

$( ... ) is command substitution, $(( ... )) is arithmetic expansion. (If you're running that on an interactive Bash, the error message should begin with bash: ..., implying it's from Bash and not sed.) I can't tell any reason why the parentheses would have been doubled there. The other issue is with double quotes "" vs. single quotes '', see What is the difference between the "...", '...', $'...', and $"..." quotes in the shell? — ilkkachu, Sep 27 '21 at 15:18
Copy/paste that and any other shell scripts you write into http://shellcheck.net and fix the issues that tool can tell you about. — Ed Morton, Sep 28 '21 at 22:20

score 3 · Accepted Answer · answered Sep 27 '21 at 15:34

In you command

var=$(sed  -e 's/"currentGeneration":$currg/"currentGeneration":$nextg/'  <<< $content )

To avoid the problem, you have a couple of options:

Use sed surrounded by double quotes and escape the inner double quotes:

$ var=$(sed  -e "s/\"currentGeneration\":$currg/\"currentGeneration\":$nextg/"  <<< "$content" )

Concatenate single quotes with double quotes:

$ var=$(sed  -e 's/"currentGeneration":'"$currg"'/"currentGeneration":'"$nextg/"  <<< "$content" )

Note that $content should be enclosed in double quotes.

Thank you so much. You helped me learn something as well. – Ram Sep 27 '21 at 15:41 — Ram, Sep 27 '21 at 15:41

1 Answers1