copy var partial bytes in shell - shell

Question

The following contents in my shell file :
#!/usr/bin/env bash
api_params_with_timestamp="somethingXXXX"
SECRET_KEY="XXXXXXXXXXXXXX"
signature=$(echo -n "$api_params_with_timestamp" \
| openssl dgst -sha256 -hmac "$SECRET_KEY")
echo "signature"=$signature
execute this shell I got
signature=(stdin)= 3eee8d204c83381dddffgggg
I just need "3eee8d204c83381dddffgggg" part , what can I do
so that (stdin)= part would be skip ?!
Edit:
signature=$(echo -n "$api_params_with_timestamp" \
| openssl dgst -sha256 -hmac "$SECRET_KEY" | cut -c10-1000)
not perfect but works !

Answer 1

The openssl-dgst page doesn't provide a direct option to drop the "(stdin)= " portion of the output, but that can be done trivially with cut, grep or sed. For example:
with cut
signature=$(echo -n "$api_params_with_timestamp" \
| openssl dgst -sha256 -hmac "$SECRET_KEY" \
| cut -d ' ' -f2)
with grep
signature=$(echo -n "$api_params_with_timestamp" \
| openssl dgst -sha256 -hmac "$SECRET_KEY" \
| grep -E -o '[^ ]+$')
with sed
signature=$(echo -n "$api_params_with_timestamp" \
| openssl dgst -sha256 -hmac "$SECRET_KEY" \
| sed 's/^.*[[:space:]]//')
Note: you can also remove the line separator characters "\" and use the pipe for that purpose instead, e.g.
signature=$(echo -n "$api_params_with_timestamp" |
openssl dgst -sha256 -hmac "$SECRET_KEY" |
sed 's/^.*[[:space:]]//')
Either way is fine -- up to you.
Example Use/Output
$ api_params_with_timestamp="somethingXXXX"
> SECRET_KEY="XXXXXXXXXXXXXX"
> signature=$(echo -n "$api_params_with_timestamp" |
> openssl dgst -sha256 -hmac "$SECRET_KEY" |
> sed 's/^.*[[:space:]]//')
> echo $signature
46c86ea27df85b8ab9a8c565a344cadd1230f32ff1d773ba0fa5aa1076116d0b