mjhjmtu

I have written a bash script which calls other process (the ones that ask for password on terminal).

#!/bin/bash
source pc.txt
snp_pw=$export_snapshot
export snapshot_pw=snp_pw
echo_time() 
 date "+%d:%b:%Y:%H:%M:%S"

# Export Snapshot
echo "$(echo_time) :STARTING EXPORT SNAPPSHOT SCRIPT" | tee -a $mail_log
loop_var=3
echo "$(echo_time) :ON FAILURE TRY AT MAX $loop_var ATTEMPTS" | tee -a $mail_log
i=1
while [ "$loop_var" -gt 0 ]; do
expect <(cat << 'EOD'
spawn $::env(Snp_Script_Path)/export_service_instance.sh bootstrap Export_Files/test.bar
expect "Enter RPD Password:"
send -- "$::env(snapshot_pw)r"
expect "Re-enter RPD Password:"
send -- "$::env(snapshot_pw)r"
interact
EOD
) &> $export_snapshot_log_location

Instead of directly hardcoding the password here, how can I read it from within a variable in other file?

It has to be a bash and expect both.
I am currently trying to read the file through source and $pass gives password. But within expect, it is not working.

I don't want to encrypt/encode anything. I just want to keep it simple.

I'm not sure of everything that's happening,
and what is supposed to be happening,
but when you say

export snapshot_pw=snp_pw

you are setting the environment variable snapshot_pw
to the string s n p _ p w. 
You probably want to do

export snapshot_pw="$snp_pw"

to set the environment variable snapshot_pw
to the value of variable snp_pw, i.e., $snp_pw.

Also, you should quote the shell variable "$mail_log"
and any others you use without quoting. 
(You don't need the and .)

A minimal test case shows that both bash and expect do read the same environment variable:

#!/bin/bash
export foo="bar$$"
echo "bash $foo"

expect <(cat <<'EOD'
puts "tcl $env(foo)"
EOD
)

which when run shows the same values in both languages:

-bash-4.2$ bash mixin
bash bar7242
tcl bar7242
-bash-4.2$ bash mixin
bash bar7247
tcl bar7247
-bash-4.2$ 

so it is not clear what you mean by "it is not reading" nor what your overall goal is for this code.

You can set a variable based on the contents of a file with a syntax like

snp_pw="$(cat passwd_file)"

e.g.

$ cat passwd_file 
anewpassword

$ cat x
#!/bin/bash

snp_pw=$(cat passwd_file)

echo Password is $snp_pw

$ ./x
Password is anewpassword