There is a command created specifically for that case: yes

$ yes | ./script

What this does is connect the output of yes to the input of ./script. So when ./script asks for user input it will instead get the output of yes. The output of yes is an endless stream of y followed by enter. So basically as if the user is entering y for every question of ./script.

If you want to say no (n) instead of yes (y) you can do it like this:

$ yes n | ./script

Note that some tools have an option to always asume yes as answer. See here for example: Bypass the yes/no prompt in 'apt-get upgrade'

Other methods to enter input:

If you know exactly how many y your script is expecting you can do it like this:

$ printf 'y

y

y

' | ./script

The newlines (
) are the enter keys.

Using printf instead of yes you have more fine grained control of input:

$ printf 'yes

no

maybe

' | ./script

Note that in some rare cases the command does not require the user to press enter after the character. in that case leave the newlines out:

$ printf 'yyy' | ./script

For sake of completeness you can also use a here document:

$ ./script << EOF

y

y

y

EOF

Or if your shell supports it a here string:

$ ./script <<< "y

y

y

"

Or you can create a file with one input per line:

$ ./script < inputfile

If the command is sufficiently complex and the methods above no longer suffice then you can use expect.

Here is an example of a super simple expect script:

spawn ./script

expect "are you sure?"

send "yes

"

expect "are you really sure?"

send "YES!

"

expect eof

Technical nitpick:

The hypothetical command invocation you gave in your question does not work:

$ ./script < echo 'yyyyyyyyyyyyyy'

bash: echo: No such file or directory

This is because the shell grammar allows a redirect operator anywhere in the command line. As far as the shell is concerned your hypothetical command line is the same as this line:

$ ./script 'yyyyyyyyyyyyyy' < echo

bash: echo: No such file or directory

That means ./script will be called with the argument 'yyyyyyyyyyyyyy' and the stdin will get input from a file named echo. And bash complains since the file does not exists.