Pass commands as input to another command (su, ssh, sh, etc)

Question

I have a script where I need to start a command, then pass some additional commands as commands to that command. I tried

su
echo I should be root n         


        

Answer 1

If you want a generic solution which will work for any kind of program, you can use the expect command.

Extract from the manual page:

Expect is a program that "talks" to other interactive programs according to a script. Following the script, Expect knows what can be expected from a program and what the correct response should be. An interpreted language provides branching and high-level control structures to direct the dialogue. In addition, the user can take control and interact directly when desired, afterward returning control to the script.

Here is a working example using expect:

set timeout 60

spawn sudo su -

expect "*?assword" { send "*secretpassword*\r" }
send_user "I should be root now:"

expect "#" { send "whoami\r" }
expect "#" { send "exit\r" }
send_user "Done.\n"
exit

The script can then be launched with a simple command:

$ expect -f custom.script

You can view a full example in the following page: http://www.journaldev.com/1405/expect-script-example-for-ssh-and-su-login-and-running-commands

Note: The answer proposed by @tripleee would only work if standard input could be read once at the start of the command, or if a tty had been allocated, and won't work for any interactive program.

Example of errors if you use a pipe

echo "su whoami" |ssh remotehost
--> su: must be run from a terminal

echo "sudo whoami" |ssh remotehost
--> sudo: no tty present and no askpass program specified

In SSH, you might force a TTY allocation with multiple -t parameters, but when sudo will ask for the password, it will fail.

Without the use of a program like expect any call to a function/program which might get information from stdin will make the next command fail:

ssh use@host <<'____HERE'
  echo "Enter your name:"
  read name
  echo "ok."
____HERE
--> The `echo "ok."` string will be passed to the "read" command