Bash input the data from a text file and process it - linux

Question

I want to create a bash script which will take entries from a text file and process it based on the requirement.
I am trying to find out the last login time for each of the mail accounts. I have a text file email.txt contains all the email addresses line by line and need to check the last login time for each of these account using the below command :
cat /var/log/maillog | grep 'mail1#address.com' | grep 'Login' | tail -1 > result.txt
So the result would be inside result.txt
Thanks,

Answer 1

I think you are looking for something like this.
while read email_id ; do grep "$email_id" /var/log/maillog | grep 'Login' >> result.txt ; done < email.txt

Answer 2

To get all the email addresses in parallel (rather than repeatedly scan what is probably a very large log file).
grep -f file_with_email_addrs /var/log/maillog
Weather this is worthwile depends on the size of the log file and the number of e-mail addresses you are getting. A small log file or a small number of users will probably render this mute. But a big log file and a lot of users will make this a good first step. But will then require some extra processing.
I am unsure about the format of the log lines or the relative distribution of Logins in the file, so what happens next depends on more information.
# If logins are rare?
grep Login /var/log/maillog | grep -f file_with_email_addrs | sort -u -k <column of email>
# If user count is small
grep -f file_with_email_addrs /var/log/maillog | awk '/Login/ {S[$<column of email>] = $0;} END {for (loop in S) {printf("%s\n",S[loop]);}'

Answer 3

Your question is unclear, do you want to look for the last login for 'mail_address1' which is read from a file?
If so:
#/bin/bash
while read mail_address1;
do;
# Your command line here
cat /var/log/maillog | grep $mail_address1 | grep 'Login' | tail -1 >> result.txt
done; < file_with_email_adddrs
PS: I've changed the answer to reflect the suggestions (extremely valid ones) in the comments that advise against using a for i in $(cat file) style loop to loop through a file.