Andrew Hudson - Fedora™ Unleashed, 2008 edition

$ free

total used free shared buffers cached

Mem: 1026320 822112 204208 0 41232 481412

-/+ buffers/cache: 299468 726852

Swap: 2031608 0 2031608

This information constantly changes as the system is used. You can get the same information by using the catcommand to see the contents of the meminfofile:

$ cat /proc/meminfo

MemTotal: 1026320 kB

MemFree: 204200 kB

Buffers: 41252 kB

Cached: 481412 kB

SwapCached: 0 kB

Active: 307232 kB

Inactive: 418224 kB

HighTotal: 122692 kB

HighFree: 244 kB

LowTotal: 903628 kB

LowFree: 203956 kB

SwapTotal: 2031608 kB

SwapFree: 2031608 kB

Dirty: 0 kB

Writeback: 0 kB

AnonPages: 202804 kB

Mapped: 87864 kB

Slab: 21736 kB

SReclaimable: 12484 kB

SUnreclaim: 9252 kB

PageTables: 5060 kB

NFS_Unstable: 0 kB

Bounce: 0 kB

CommitLimit: 2544768 kB

Committed_AS: 712024 kB

VmallocTotal: 114680 kB

VmallocUsed: 6016 kB

VmallocChunk: 108148 kB

HugePages_Total: 0

HugePages_Free: 0

HugePages_Rsvd: 0

Hugepagesize: 4096 kB

The /procdirectory can also be used to dynamically alter the behavior of a running Linux kernel by "echoing" numeric values to specific files under the /proc/sysdirectory. For example, to "turn on" kernel protection against one type of denial-of-service (DoS) attack known as SYN flooding, use the echocommand to send the number 1(one) to the following /procpath:

# echo 1 >/proc/sys/net/ipv4/tcp_syncookies

The Linux kernel has a number of built-in protections, but good system administration security policies and a secure firewall protecting your gateway, router, or Internet-connected system are the best protection you can use. See Chapter 30, "Securing Your Machines," for an overview of firewalling and examples of how to implement network security tools included with Fedora.

Other ways to use the /procdirectory include

► Getting CPU information, such as the family, type, and speed from / proc/cpuinfo.

► Viewing important networking information under /proc/net, such as active interfaces information under /proc/net/dev, routing information in /proc/net/route,and network statistics in /proc/net/netstat.

► Retrieving file system information.

► Reporting media mount point information via USB; for example, the Linux kernel reports what device to use to access files (such as /dev/sda) if a USB camera or hard drive is detected on the system. You can use the dmesgcommand to see this information.

► Getting the kernel version in /proc/version, performance information such as uptime in /proc/uptime,or other statistics such as CPU load, swap file usage, and processes in /proc/stat.

The /usrdirectory (nearly 3GB in size if you do a default install) contains software applications, libraries, and other types of shared data for use by anyone on the system. Many Linux system administrators give /usrits own partition. A number of subdirectories under /usrcontain the X Window System ( /usr/bin), manual pages ( /usr/share/man), software package shared files ( /usr/share/ name_of_package , such as /usr/share/emacs), additional application or software package documentation ( /usr/share/doc), and an entire subdirectory tree of locally built and installed software, /usr/local.

As its name implies, the /tmpdirectory is used for temporary file storage; as you use Linux, various programs create files in this directory. The /tmpdirectory is cleaned of stale files each day by the tmpwatchcommand. (A stale file is any file not used after 10 days.) Settings in your system's scheduling table, /etc/crontab, configure Fedora by default to use tmpwatchto check /tmpeach day.

The /var directory contains subdirectories used by various system services for spooling and logging. Many of these variable data files, such as print spooler queues, are temporary, whereas others, such as system and kernel logs, are renamed and rotated in use. Incoming electronic mail is usually directed to files under /var/spool/mail.

Linux also uses /varfor other important system services. These include the topmost File Transfer Protocol (FTP) directory under /var/ftp(see Chapter 20, "Remote File Serving with FTP"), and the Apache web server's initial home page directory for the system, /var/www/html. (See Chapter 17, "Apache Web Server Management," for more information on using Apache.)

You can access and use a Linux system in a number of ways. One way is at the console with a monitor, keyboard, and mouse attached to the PC. Another way is via a serial console, either by dialup via a modem or a PC running a terminal emulator and connected to the Linux PC via a null modem cable. You can also connect to your system through a wired or wireless network, using the telnetor sshcommands. The information in this section shows you how to access and use the Linux system, using physical and remote text-based logins.

This chapter focuses on text-based logins and use of Linux. Graphical logins and using a graphical desktop are described in Chapter 3.

If you sit down at your PC and log in to a Linux system that has not been booted to a graphical login, you see a prompt similar to this one:

Fedora release 8 (Werewolf)

Kernel 2.6.23-1.217 fc8 on an i686

login:

Your prompt might vary, depending on the version of Fedora you are using. In any event, at this prompt, type in your username and press Enter. When you are prompted for your password, type it in and press Enter.

Note that your password is not echoed back to you, which is a good idea. Why is it a good idea? Well, people are prevented from looking over your shoulder and seeing how many characters are on your screen. It is not difficult to guess that a five-letter pass word might correspond to the user's spouse's first name!

Use the exitor logoutcommands to exit your session. Type the command and press Enter. You are then returned to the login prompt. If you use virtual consoles, remember to exit each console before leaving your PC. (Otherwise, someone could easily sit down and use your account.)

Although you can happily log in on your computer, an act known as a local login, you can also log in to your computer via a network connection from a remote computer. Linux-based operating systems provide a number of remote access commands you can use to log in to other computers on your local area network (LAN), wide area network (WAN) , or the Internet. Note that not only must you have an account on the remote computer, but the remote computer must be configured to support remote logins—otherwise, you won't be able to log in.