Evaluating Variables in a Flat File

Based on the logic in the script, the proper message will be sent to the user or users.. A more advanced implementation of this technique provides a template that refers to environment v

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C H A P T E R 2 6

Evaluating Variables

in a Flat File

One common scripting technique is to create a flat file that is one of a potential

number of canned messages for users to receive—messages such as notifications of

downtimes, changes in the environment, or use of system quota (I consider a flat file

simply a file that contains text.) Based on the logic in the script, the proper message

will be sent to the user or users A more advanced implementation of this technique

provides a template that refers to environment variables and contains customizable

elements so the message can be tailored specifically to the recipient

In Chapter 36, I present a script that checks every morning for user passwords that are

aging and therefore should be changed In that script, when a password has reached the

predetermined cut-off age, the script sends an e-mail to the account owner to state that the

account will be locked if the owner doesn’t update her password The script annoys the user

every day for a couple of weeks, after which the account is locked After that, if the user

wants to use the account she would have to call and explain why she didn’t heed the friendly

e-mail warnings

The canned template file looks like this:

$ENVIRONMENT account password for \\"$USERID\\" expires in $REMAINING day\\(s\\)

==============================================================

++ ACTION NEEDS TO BE TAKEN OR YOUR ACCOUNT WILL BE LOCKED ++

++ IN $REMAINING day\\(s\\) ++

==============================================================

If the password isnt changed within $REMAINING day\\(s\\), account will be locked

Instructions for changing passwords are located at:

\\<a href=\\"http://server.company.com/chg_passwd.html\\"\\>

http://server.company.com/chg_passwd.html\\</a\\>

If you are unable to change your password, please call the Help Desk

182 C H A P T E R 2 6 ■ E V A L U A T I N G V A R I A B L E S I N A F L A T F I L E

If you no longer need this account, please let us know so we can remove it

System Administration

Note in particular the shell variables (ENVIRONMENT, USERID, and REMAINING) and their escape sequences When the script executes, the account expiration date for each possi-ble USERID is checked, and the script determines how many days REMAINING there are before the account is frozen The ENVIRONMENT variable specifies the environment from which the message is being sent The occurrences of these variables must be replaced with their values before the script is run and the message is sent Likewise, each escape sequence \\ evaluates to a single backslash that causes the following special character to

be escaped so that it is treated as plain text when the script executes

For each user with a soon-to-expire password, the template file is evaluated at runtime

to replace the included variables with their contents The code to perform the replace-ments in each case is fairly simple It consists of one small loop that looks at each line

in the flat file and replaces any variables with their assigned values You can easily modify the code for other purposes—for example, to display the modified file

cat $flat_file | while read a_line

do

place_holder=`eval echo $a_line`

echo $place_holder

done | $MAIL -s "$ENVIRONMENT UNIX Account Notification" $RECIPIENT

The loop processes each line of the file using the eval statement, which causes the variables to be replaced with their string values The expanded line is assigned to the variable place_holder so that it can be echoed to standard output Once all lines have been processed, the complete output is sent via e-mail to the specified RECIPIENT by the e-mail command in the final element of the command pipeline The specific e-mail command is up to you; a couple of common ones are mailx and mail, depending on your operating system

This technique has many potential uses The example script here is much like a tradi-tional mail merge where you customize the message based on specific users You could also use this technique for creating and maintaining configuration for an application, or custom files for individual users patterned after a default file