Saving grep output to variable changes value

I want to use grep to determine if a string contains a substring. (The plan is to use that result as the test command in a bash if statement.)

Here, I check the length of the output of grep . :

$ echo "abc" | grep "j" | wc -c
       0

Since wc -c shows zero, I know that grep returned an empty string.

But if I save output from grep to a variable before calling wc -c, I get a different value:

$ match=$(echo "abc" | grep "j")
$ echo "$match" | wc -c
       1

The output from grep now is a string with 1 character in it.

I suspect that it's a newline in there:

$ echo $match

$

Why is there now an extra character in $match, and how can I keep that from happening?

match does not contain a newline, but echo "$match" writes a newline (but see note below). In the first case, you are directly passing the output of grep to wc, but in the second case you are passing the output of grep plus a newline to wc.

But don't do this at all. There is no need to introduce wc into the problem. Just test the value returned by grep. eg:

if ... | grep -q "$pattern"; then echo "$pattern was found in the input"; fi

Note, echo "$match" is bad practice. For example, consider if $match expands to the string -e or -n. It is much more robust to use printf '%s' "$match"

Consider the following:

$ echo "$match" | od -c
0000000  \n
0000001
$ echo "$match" | wc -c
1

$ printf "$match" | od -c
0000000
$ printf "$match" | wc -c
0

The only difference in the 2 commands is that echo appends a \n on the end of the ouput.

Of course, we can make the printf generate the same result by explicitly adding a \n on the end:

$ printf "$match\n" | od -c
0000000  \n
0000001

$ printf "$match\n" | wc -c
1