CT320 Bash Lab II

Group Project

You may work in pairs, if space demands.                 

Purpose

The purpose of this assignment is to get more experience writing Linux scripts using bash and other commands. You will create and edit a bash script called pwval that uses many of the features of shell scripts described in the lecture.                 

Background

The following commands might be useful for this lab:

• echo
• cat
• cut
• grep

Also, remember what $# means in a shell script.                 

Password file entry

Lines in /etc/passwd looks like this:                 

    root:x:0:0:root:/root:/bin/bash
    applin:$6$30CrCh90r7Dl5Q4o$uwOkdKHjNkdBLIpreE090Cxt5z0WB765SLXry1QFE8n9fuwYrikKjY37kkPasFwf5My/w2HKgD6kdPsLT0hYV.:2464:1555:Jack Applin:/s/parsons/d/fac/applin:/bin/bash

The fields, separated by colons, are:                 

1. username (e.g., “jsmith42”)
2. encrypted password
3. numeric user ID
4. numeric group ID
5. user name (e.g., “John Smith”)
6. home directory
7. shell program

Program

You will write a bash script called pwval that ensures that /etc/passwd is good. It will:                 

1. verify permissions on /etc/passwd
2. no duplicate usernames
3. no duplicate userids
4. for every entry, verify that:
   • the password is non-empty
   • the home directory exists
   • the shell exists and is executable
   • the group exists in /etc/group

Argument

If an argument is given, then it is a file to be used instead of /etc/passwd. This is useful for testing.                 

Messages

Think of this program as something to be executed every day, from cron. Therefore, it should be relatively quiet. It shouldn’t say anything, unless bad things are detected.                 

However, when the script does emit an error message, it must contain sufficient information to track down the problem. For example, an error message that simply says “duplicate user name” is insufficient—the message must specify, at least, what the duplicate user name is. Line numbers would be even better.                 

All error messages must:

• include the name of the file (not always /etc/passwd) that is being processed
• include $0, so that it’s clear where the messages are coming from
• go to standard error, not standard out
  • Example: echo "this is going to standard error" >&2

Temporary Files

To get a temporary file, use mktemp to generate a unique name:

#! /bin/bash
tmpfile=$(mktemp)
echo "My temp file is $tmpfile"
date >$tmpfile
cat $tmpfile
rm $tmpfile

My temp file is /tmp/tmp.F0nO8q30Ar
Sat Nov 23 05:09:53 MST 2024

Get Credit

When you have tested your script, and it works, show your work to the TA.