Bash - here doc read input value from another user

Question

I have a here document inside a bash script. I want to read a value from it like that :

su myUser<<SESSION set -x echo -n "Enter your name and press [ENTER]: " read name echo "the name is $name" SESSION 

But when I launch this script from another user, bash does not stop himself to wait for an input and ignore the read command.

Any ideas ?

Answer 1

As L. Scott Johnson correctly found, the read reads from standard input. The standard input in the shell that su runs is connected to the here-document, so the read reads the literal string echo "the name is " (note that since the here-document is unquoted, the $name has already been expanded to an empty string, or to whatever value it had in the calling shell).

Here is the same thing, but with a quoted here-document, and an extra line outputting $name again:

su username <<'SESSION' set -x echo -n "Enter your name and press [ENTER]: " read name echo "the name is $name" echo "What I read was $name" SESSION 

The output would be

Password: + echo -n Enter your name and press [ENTER]: Enter your name and press [ENTER]: + read name + echo What I read was echo "the name is $name" What I read was echo "the name is $name" 

To correctly do this, you can't have read reading from standard input. Instead, open a new file descriptor as a copy of standard input, and get read to read from that:

su username 3<&0 <<'SESSION' set -x echo -n "Enter your name and press [ENTER]: " read name <&3 echo "the name is $name" SESSION 

If the shell of the other user is bash or ksh, then read name <&3 may be replaced by read -u3 name.

Note however that you can't expect the name variable to be set in the shell calling su as a child shell can't modify the environment (variables etc.) of a parent shell.

Answer 2

It doesn't ignore the read command; it executes it and reads stdin. Then the line echo "the name is $name is read (from stdin) and assigned to $name.

su myUser<<SESSION set -x echo -n \"Enter your name and press [ENTER]: \" read name someUser echo \"the name is \$name\" SESSION 

output:

+ echo -n '"Enter' your name and press '[ENTER]:' '"' -n "Enter your name and press [ENTER]: " + read name + echo '"the' name is 'someUser"' "the name is someUser"