Scripting

1. Write a script which displays only the login, UID and Path of each entry of the /etc/passwd file.

Fields in /etc/passwd

Understanding /etc/passwd File Format:

1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
2. Password: An x character indicates that encrypted password is stored in /etc/shadow file. Please note that you need to use the passwd command to computes the hash of a password typed at the CLI or to store/update the hash of the password in /etc/shadow file.
3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
4. Group ID (GID): The primary group ID (stored in /etc/group file)
5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

How to write bash script

Writing Your First Script And Getting It To Work:

#!/bin/bash
# My first script

echo "Hello World!"

The first line of the script is important. This is a special clue, called a shebang, given to the shell indicating what program is used to interpret the script. In this case, it is /bin/bash. Other scripting languages such as Perl, awk, tcl, Tk, and python also use this mechanism.

The second line is a comment. Everything that appears after a "#" symbol is ignored by bash.

Answer

#!/bin/bash
# Write a script which displays only the login, UID and Path

IFS=':'

while read -r username password uid gid comment home_directory shell
do
    echo "$username $uid $home_directory"
done < /etc/passwd

Alternative answer

#!/bin/bash
# Write a script which displays only the login, UID and Path

awk -F: '{print $1 " " $3 " " $6}' /etc/passwd

2. Write a script which updates all the package sources, then all the packages, and then logs everything in a file named /var/log/update_script.log. Create a scheduled task for this script, once per week at 4 AM.

3. Write a script which displays the list of files from the folder given as parameter, sorted by size.

4. Write a script which monitors the modifications made to the /etc/crontab file and sends an e-mail to root if the file is modified. Create a scheduled task to run this script everyday at midnight.

5. Write a script which displays 42.