Linux with Operating System Concepts

 keyname
, you would specify
–new –newkey 
keyinformation
–nodes –keyout 
keyname
where 
keyinformation
includes the algorithm used to generate the key and the number of bits 
of the key, for instance, rsa:1024, and 
keyname
is the name by which the key will be stored.
The openssl program calls upon numerous lesser Linux programs. These include genrsa 
and gendsa to generate private keys, rsa and dsa to generate public keys, enc to encrypt 
and decrypt messages given a key, and x509 to generate, test, and verify certificates. It also 
calls upon the program ca to sign certificates. Other Linux encryption tools are available, 
including gpg that is also open source and can generate keys, certificates and digital sig-
natures, and can be used to encrypt and decrypt messages and files. The choice between 
openssl and gpg is commonly one of preference and efficiency. Both have their strengths 
and weaknesses.
5.6 EMAIL PROGRAMS
We briefly look at the standard email program of Linux in this section. Email is a com-
plex and challenging topic. We make the assumption that email in your system is only 
between users on the local machine and not across the Internet. The email program we 

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Linux Applications
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look at here is 
mailx
although numerous other email programs exist for Linux including 
both text-based programs such as elm and Pine, and GUI-based programs such as Mozilla 
Thunderbird and Balsa. All of these programs are 
email clients
, not servers. This means 
that you would use the given program to read your email messages and send out email 
messages or responses.
The 
email server
has the responsibility of packaging the email up into a message that 
can be transported across the network and delivered to another email server. An email 
message waits on the server until the user accesses the new email via the client. While a 
computer can run both an email client and an email server, the two are usually separate 
and it is the client’s responsibility to make contact with the server to access new emails or 
to send out new emails.
The first Unix mail program was called 
mail
and was eventually in competition against 
a different mail program called 
Mail
. Today, the most commonly used program is called 
mailx
. To avoid confusion, both mail and Mail are symbolically linked to mailx so that 
the user could use any of these to start the mailx program. We will refer to the mailx pro-
gram as mail in this section.
5.6.1 Sending Email Messages
To send an email message using mail, the command is 
mail 
username(s)
. This invokes 
the interactive mailx program that first asks you to input a Subject. Then, you are dropped 
into a buffer to enter the body of your email. The buffer is not like an editor in that you 
are only able to use the backspace key and the enter key when editing your messages. You 
enter your text ending with control
+
d to exit the buffer and send your email. As we are 
assuming that your mail program will only permit emails to local users, the usernames are 
simply the account names of the users in the system (i.e., there is no extension such as 
@someplace.com).
A session may look like the following:
mail zappaf 
<
enter
>
Subject: missing document 
<
enter
>
We discovered a missing document in the file you gave us
this morning. It’s the receipt from the sheet music you
said you purchased. Do you know where it is? 
<
control
+
d
>
The word Subject: appears after you press the enter key allowing you to enter a subject. 
You can press the 
<
enter
>
key if you want to omit a subject for this email. You would 
receive a message indicating that the email had no subject but it would still be sent.
The mail program allows for a number of options to provide additional functionality 
to the email message you want to send. For instance, to attach a file to the email, use 
–a