How to pass the value of a variable to the stdin of a command?

Passing a value to stdin in bash is as simple as:

your-command <<< "$your_variable"

Always make sure you put quotes around variable expressions!

Be cautious, that this will probably work only in bash and will not work in sh.

Something as simple as:

echo "$blah" | my_cmd

Note that the 'echo "$var" | command operations mean that standard input is limited to the line(s) echoed. If you also want the terminal to be connected, then you'll need to be fancier:

{ echo "$var"; cat - ; } | command

( echo "$var"; cat -   ) | command

This means that the first line(s) will be the contents of $var but the rest will come from cat reading its standard input. If the command does not do anything too fancy (try to turn on command line editing, or run like vim does) then it will be fine. Otherwise, you need to get really fancy - I think expect or one of its derivatives is likely to be appropriate.

The command line notations are practically identical - but the second semi-colon is necessary with the braces whereas it is not with parentheses.