How to get a password from a shell script without

Question

I have a script that automates a process that needs access to a password protected system. The system is accessed via a command-line program that accepts the user password as an argument.

I would like to prompt the user to type in their password, assign it to a shell variable, and then use that variable to construct the command line of the accessing program (which will of course produce stream output that I will process).

I am a reasonably competent shell programmer in Bourne/Bash, but I don't know how to accept the user input without having it echo to the terminal (or maybe having it echoed using '*' characters).

Can anyone help with this?

Answer 1

You can also prompt for a password without setting a variable in the current shell by doing something like this:

$(read -s;echo $REPLY)

For instance:

my-command --set password=$(read -sp "Password: ";echo $REPLY)

You can add several of these prompted values with line break, doing this:

my-command --set user=$(read -sp "`echo $'\n '`User: ";echo $REPLY) --set password=$(read -sp "`echo $'\n '`Password: ";echo $REPLY)

Answer 2

The -s option of read is not defined in the POSIX standard. See http://pubs.opengroup.org/onlinepubs/9699919799/utilities/read.html. I wanted something that would work for any POSIX shell, so I wrote a little function that uses stty to disable echo.

#!/bin/sh

# Read secret string
read_secret()
{
    # Disable echo.
    stty -echo

    # Set up trap to ensure echo is enabled before exiting if the script
    # is terminated while echo is disabled.
    trap 'stty echo' EXIT

    # Read secret.
    read "$@"

    # Enable echo.
    stty echo
    trap - EXIT

    # Print a newline because the newline entered by the user after
    # entering the passcode is not echoed. This ensures that the
    # next line of output begins at a new line.
    echo
}

This function behaves quite similar to the read command. Here is a simple usage of read followed by similar usage of read_secret. The input to read_secret appears empty because it was not echoed to the terminal.

[susam@cube ~]$ read a b c
foo \bar baz \qux
[susam@cube ~]$ echo a=$a b=$b c=$c
a=foo b=bar c=baz qux
[susam@cube ~]$ unset a b c
[susam@cube ~]$ read_secret a b c

[susam@cube ~]$ echo a=$a b=$b c=$c
a=foo b=bar c=baz qux
[susam@cube ~]$ unset a b c

Here is another that uses the -r option to preserve the backslashes in the input. This works because the read_secret function defined above passes all arguments it receives to the read command.

[susam@cube ~]$ read -r a b c
foo \bar baz \qux
[susam@cube ~]$ echo a=$a b=$b c=$c
a=foo b=\bar c=baz \qux
[susam@cube ~]$ unset a b c
[susam@cube ~]$ read_secret -r a b c

[susam@cube ~]$ echo a=$a b=$b c=$c
a=foo b=\bar c=baz \qux
[susam@cube ~]$ unset a b c

Finally, here is an example that shows how to use the read_secret function to read a password in a POSIX compliant manner.

printf "Password: "
read_secret password
# Do something with $password here ...

Answer 3

First of all, if anyone is going to store any password in a file, I would make sure it's hashed. It's not the best security, but at least it will not be in plain text.

1. First, create the password and hash it:

   echo "password123" | md5sum  | cut -d '-' -f 1 > /tmp/secret
   

2. Now, create your program to use the hash. In this case, this little program receives user input for a password without echoing, and then converts it to hash to be compared with the stored hash. If it matches the stored hash, then access is granted:

   #!/bin/bash
   
   PASSWORD_FILE="/tmp/secret"
   MD5_HASH=$(cat /tmp/secret)
   PASSWORD_WRONG=1
   
   
   while [ $PASSWORD_WRONG -eq 1 ]
    do
       echo "Enter your password:"
       read -s ENTERED_PASSWORD
       if [ "$MD5_HASH" != "$(echo $ENTERED_PASSWORD | md5sum | cut -d '-' -f 1)" ]; then
           echo "Access Deniend: Incorrenct password!. Try again"
       else
           echo "Access Granted"
           PASSWORD_WRONG=0
       fi
   done