mjhjmtu

I have a passing understanding of Parameter Substitution, including substrings like so

foo="Hello World";
greeting=$foo:0:6

But do I do this with commands?

greeting="Hello"
md5greeting=$(echo $greeting | md5sum :0:6)
>>b1946a

Where the output is the first 6 characters of the md5sum of the 'hello'.

How do I achieve this?

Parameter substitution doesn't work like that, at least not in bash. You need a real variable (parameter) and directly operate on it. So for example like that:

var=$(echo abcdefgh | cmd1 | cmd2 | ... )
var2="$var:2:4"

The exception is zsh where you can convert command substitution to parameter "on the fly" and do above in one line:

var2=$ cdm1 

Note that:

echo $greeting | md5sum

Or more reliably:

printf '%sn' "$greeting" | md5sum

Does not give you the MD5 hash of the content of the variable, but of the content of the variable followed by a newline character. For the MD5 hash of the content of the variable:

printf %s "$greeting" | md5sum

For the first 6 bytes of it, with some head implementations:

printf %s "$greeting" | md5sum | head -c 6

or more portably:

printf %s "$greeting" | md5sum | dd bs=1 count=6 2> /dev/null