diff --git a/en_US.ISO8859-1/books/handbook/basics/chapter.xml b/en_US.ISO8859-1/books/handbook/basics/chapter.xml index 0aa6635bb3..3a5544e5ed 100644 --- a/en_US.ISO8859-1/books/handbook/basics/chapter.xml +++ b/en_US.ISO8859-1/books/handbook/basics/chapter.xml @@ -1,2671 +1,2651 @@ Chris Shumway Rewritten by UNIX Basics Synopsis This chapter covers the basic commands and functionality of the &os; operating system. Much of this material is relevant for any &unix;-like operating system. New &os; users are encouraged to read through this chapter carefully. After reading this chapter, you will know: How to use the virtual consoles of &os;. How &unix; file permissions and &os; file flags work. The default &os; file system layout. The &os; disk organization. How to mount and unmount file systems. What processes, daemons, and signals are. What a shell is, and how to change your default login environment. How to use basic text editors. What devices and device nodes are. What binary format is used under &os;. How to read manual pages for more information. Virtual Consoles and Terminals virtual consoles terminals &os; can be used in various ways. One of them is typing commands to a text terminal. A lot of the flexibility and power of a &unix; operating system is readily available at your hands when using &os; this way. This section describes what terminals and consoles are, and how you can use them in &os;. The Console console Unless &os; has been configured to automatically start a graphical environment during startup, the system will boot into a command line login prompt, as seen in this example: FreeBSD/amd64 (pc3.example.org) (ttyv0) login: The first line contains some information about the system. The amd64 indicates that the system in this example is running a 64-bit version of &os;. The hostname is pc3.example.org, and ttyv0 indicates that this is the system console. The second line is the login prompt. The next section describes how to log into &os; at this prompt. Logging into &os; &os; is a multiuser, multiprocessing system. This is the formal description that is usually given to a system that can be used by many different people, who simultaneously run a lot of programs on a single machine. Every multiuser system needs some way to distinguish one user from the rest. In &os; (and all the &unix;-like operating systems), this is accomplished by requiring that every user must log into the system before being able to run programs. Every user has a unique name (the username) and a personal, secret key (the password). &os; will ask for these two before allowing a user to run any programs. startup scripts When a &os; system boots, startup scripts are automatically executed in order to prepare the system and to start any services which have been configured to start at system boot. Once the system finishes running its startup scripts, it will present a login prompt: login: Type the username that was configured during system installation and press Enter. Then enter the password associated with the username and press Enter. The password is not echoed for security reasons. Once the correct password is input, the message of the day (MOTD) will be displayed followed by a command prompt (a #, $, or % character). You are now logged into the &os; console and ready to try the available commands. Virtual Consoles &os; can be configured to provide many virtual consoles for inputting commands. Each virtual console has its own login prompt and output channel, and &os; takes care of properly redirecting keyboard input and monitor output as you switch between virtual consoles. Special key combinations have been reserved by &os; for switching consoles. Refer to &man.syscons.4;, &man.atkbd.4;, &man.vidcontrol.1; and &man.kbdcontrol.1; for a more technical description of the &os; console and its keyboard drivers.. Use AltF1, AltF2, through AltF8 to switch to a different virtual console in &os;. When switching from one console to the next, &os; takes care of saving and restoring the screen output. The result is an illusion of having multiple virtual screens and keyboards that can be used to type commands for &os; to run. The programs that are launched in one virtual console do not stop running when that console is not visible because the user has switched to a different virtual console. The <filename>/etc/ttys</filename> File By default, &os; is configured to start eight virtual consoles. The configuration can be customized to start more or fewer virtual consoles. To change the number of and the settings of the virtual consoles, edit /etc/ttys. Each uncommented line in /etc/ttys (lines that do not start with a # character) contains settings for a single terminal or virtual console. The default version configures nine virtual consoles, and enables eight of them. They are the lines that start with ttyv: # name getty type status comments # ttyv0 "/usr/libexec/getty Pc" cons25 on secure # Virtual terminals ttyv1 "/usr/libexec/getty Pc" cons25 on secure ttyv2 "/usr/libexec/getty Pc" cons25 on secure ttyv3 "/usr/libexec/getty Pc" cons25 on secure ttyv4 "/usr/libexec/getty Pc" cons25 on secure ttyv5 "/usr/libexec/getty Pc" cons25 on secure ttyv6 "/usr/libexec/getty Pc" cons25 on secure ttyv7 "/usr/libexec/getty Pc" cons25 on secure ttyv8 "/usr/X11R6/bin/xdm -nodaemon" xterm off secure For a detailed description of every column in this file and the available options for the virtual consoles, refer to &man.ttys.5;. Single User Mode Console A detailed description of single user mode can be found here. There is only one console when &os; is in single user mode as no other virtual consoles are available in this mode. The settings for single user mode are found in this section of /etc/ttys: # name getty type status comments # # If console is marked "insecure", then init will ask for the root password # when going to single-user mode. console none unknown off secure As the comments above the console line indicate, editing secure to insecure will prompt for the root password when booting into single user mode. The default setting enters single user mode without prompting for a password. Be careful when changing this setting to insecure. If you ever forget the root password, booting into single user mode is still possible, but may be difficult for someone who is not comfortable with the &os; booting process. Changing Console Video Modes The &os; console default video mode may be adjusted to 1024x768, 1280x1024, or any other size supported by the graphics chip and monitor. To use a different video mode load the VESA module: &prompt.root; kldload vesa To determine which video modes are supported by the hardware, use &man.vidcontrol.1;. To get a list of supported video modes issue the following: &prompt.root; vidcontrol -i mode The output of this command lists the video modes that are supported by the hardware. To select a new video mode, specify the mode using &man.vidcontrol.1; as the root user: &prompt.root; vidcontrol MODE_279 If the new video mode is acceptable, it can be permanently set on boot by adding it to /etc/rc.conf: allscreens_flags="MODE_279" Permissions UNIX &os;, being a direct descendant of BSD &unix;, is based on several key &unix; concepts. The first and most pronounced is that &os; is a multi-user operating system that can handle several users working simultaneously on completely unrelated tasks. The system is responsible for properly sharing and managing requests for hardware devices, peripherals, memory, and CPU time fairly to each user. Because the system is capable of supporting multiple users, everything the system manages has a set of permissions governing who can read, write, and execute the resource. These permissions are stored as three octets broken into three pieces, one for the owner of the file, one for the group that the file belongs to, and one for everyone else. This numerical representation works like this: This section will discuss the traditional &unix; permissions. For finer grained file system access control, see the File System Access Control Lists section. permissions file permissions Value Permission Directory Listing 0 No read, no write, no execute --- 1 No read, no write, execute --x 2 No read, write, no execute -w- 3 No read, write, execute -wx 4 Read, no write, no execute r-- 5 Read, no write, execute r-x 6 Read, write, no execute rw- 7 Read, write, execute rwx ls directories Use the argument to &man.ls.1; to view a long directory listing that includes a column of information about a file's permissions for the owner, group, and everyone else. For example, a ls -l in an arbitrary directory may show: &prompt.user; ls -l total 530 -rw-r--r-- 1 root wheel 512 Sep 5 12:31 myfile -rw-r--r-- 1 root wheel 512 Sep 5 12:31 otherfile -rw-r--r-- 1 root wheel 7680 Sep 5 12:31 email.txt The first (leftmost) character in the first column indicates whether this file is a regular file, a directory, a special character device, a socket, or any other special pseudo-file device. In this example, the - indicates a regular file. The next three characters, rw- in this example, give the permissions for the owner of the file. The next three characters, r--, give the permissions for the group that the file belongs to. The final three characters, r--, give the permissions for the rest of the world. A dash means that the permission is turned off. In this example, the permissions are set so the owner can read and write to the file, the group can read the file, and the rest of the world can only read the file. According to the table above, the permissions for this file would be 644, where each digit represents the three parts of the file's permission. How does the system control permissions on devices? &os; treats most hardware devices as a file that programs can open, read, and write data to. These special device files are stored in /dev/. Directories are also treated as files. They have read, write, and execute permissions. The executable bit for a directory has a slightly different meaning than that of files. When a directory is marked executable, it means it is possible to change into that directory using cd. This also means that it is possible to access the files within that directory, subject to the permissions on the files themselves. In order to perform a directory listing, the read permission must be set on the directory. In order to delete a file that one knows the name of, it is necessary to have write and execute permissions to the directory containing the file. There are more permission bits, but they are primarily used in special circumstances such as setuid binaries and sticky directories. For more information on file permissions and how to set them, refer to &man.chmod.1;. Tom Rhodes Contributed by Symbolic Permissions permissions symbolic Symbolic permissions use characters instead of octal values to assign permissions to files or directories. Symbolic permissions use the syntax of (who) (action) (permissions), where the following values are available: Option Letter Represents (who) u User (who) g Group owner (who) o Other (who) a All (world) (action) + Adding permissions (action) - Removing permissions (action) = Explicitly set permissions (permissions) r Read (permissions) w Write (permissions) x Execute (permissions) t Sticky bit (permissions) s Set UID or GID These values are used with &man.chmod.1;, but with letters instead of numbers. For example, the following command would block other users from accessing FILE: &prompt.user; chmod go= FILE A comma separated list can be provided when more than one set of changes to a file must be made. For example, the following command removes the group and world write permission on FILE, and adds the execute permissions for everyone: &prompt.user; chmod go-w,a+x FILE Tom Rhodes Contributed by &os; File Flags In addition to file permissions, &os; supports the use of file flags. These flags add an additional level of security and control over files, but not directories. With file flags, even root can be prevented from removing or altering files. File flags are modified using &man.chflags.1;. For example, to enable the system undeletable flag on the file file1, issue the following command: &prompt.root; chflags sunlink file1 To disable the system undeletable flag, put a no in front of the : &prompt.root; chflags nosunlink file1 To view the flags of a file, use with &man.ls.1;: &prompt.root; ls -lo file1 -rw-r--r-- 1 trhodes trhodes sunlnk 0 Mar 1 05:54 file1 Several file flags may only be added or removed by the root user. In other cases, the file owner may set its file flags. Refer to &man.chflags.1; and &man.chflags.2; for more information. Tom Rhodes Contributed by The <literal>setuid</literal>, <literal>setgid</literal>, and <literal>sticky</literal> Permissions Other than the permissions already discussed, there are three other specific settings that all administrators should know about. They are the setuid, setgid, and sticky permissions. These settings are important for some &unix; operations as they provide functionality not normally granted to normal users. To understand them, the difference between the real user ID and effective user ID must be noted. The real user ID is the UID who owns or starts the process. The effective UID is the user ID the process runs as. As an example, &man.passwd.1; runs with the real user ID when a user changes their password. However, in order to update the password database, the command runs as the effective ID of the root user. This allows users to change their passwords without seeing a Permission Denied error. The setuid permission may be set by prefixing a permission set with the number four (4) as shown in the following example: &prompt.root; chmod 4755 suidexample.sh The permissions on suidexample.sh now look like the following: -rwsr-xr-x 1 trhodes trhodes 63 Aug 29 06:36 suidexample.sh Note that a s is now part of the permission set designated for the file owner, replacing the executable bit. This allows utilities which need elevated permissions, such as passwd. The nosuid &man.mount.8; option will cause such binaries to silently fail without alerting the user. That option is not completely reliable as a nosuid wrapper may be able to circumvent it. To view this in real time, open two terminals. On one, start the passwd process as a normal user. While it waits for a new password, check the process table and look at the user information for passwd: In terminal A: Changing local password for trhodes Old Password: In terminal B: &prompt.root; ps aux | grep passwd trhodes 5232 0.0 0.2 3420 1608 0 R+ 2:10AM 0:00.00 grep passwd root 5211 0.0 0.2 3620 1724 2 I+ 2:09AM 0:00.01 passwd As stated above, the passwd is run by a normal user, but is using the effective UID of root. The setgid permission performs the same function as the setuid permission; except that it alters the group settings. When an application or utility executes with this setting, it will be granted the permissions based on the group that owns the file, not the user who started the process. To set the setgid permission on a file, provide chmod with a leading two (2): &prompt.root; chmod 2755 sgidexample.sh In the following listing, notice that the s is now in the field designated for the group permission settings: -rwxr-sr-x 1 trhodes trhodes 44 Aug 31 01:49 sgidexample.sh In these examples, even though the shell script in question is an executable file, it will not run with a different EUID or effective user ID. This is because shell scripts may not access the &man.setuid.2; system calls. The setuid and setgid permission bits may lower system security, by allowing for elevated permissions. The third special permission, the sticky bit, can strengthen the security of a system. When the sticky bit is set on a directory, it allows file deletion only by the file owner. This is useful to prevent file deletion in public directories, such as /tmp, by users who do not own the file. To utilize this permission, prefix the permission set with a one (1): &prompt.root; chmod 1777 /tmp The sticky bit permission will display as a t at the very end of the permission set: &prompt.root; ls -al / | grep tmp drwxrwxrwt 10 root wheel 512 Aug 31 01:49 tmp Directory Structure directory hierarchy The &os; directory hierarchy is fundamental to obtaining an overall understanding of the system. The most important directory is root or, /. This directory is the first one mounted at boot time and it contains the base system necessary to prepare the operating system for multi-user operation. The root directory also contains mount points for other file systems that are mounted during the transition to multi-user operation. A mount point is a directory where additional file systems can be grafted onto a parent file system (usually the root file system). This is further described in . Standard mount points include /usr/, /var/, /tmp/, /mnt/, and /cdrom/. These directories are usually referenced to entries in /etc/fstab. This file is a table of various file systems and mount points and is read by the system. Most of the file systems in /etc/fstab are mounted automatically at boot time from the script &man.rc.8; unless their entry includes . Details can be found in . A complete description of the file system hierarchy is available in &man.hier.7;. The following table provides a brief overview of the most common directories. Directory Description / Root directory of the file system. /bin/ User utilities fundamental to both single-user and multi-user environments. /boot/ Programs and configuration files used during operating system bootstrap. /boot/defaults/ Default boot configuration files. Refer to &man.loader.conf.5; for details. /dev/ Device nodes. Refer to &man.intro.4; for details. /etc/ System configuration files and scripts. /etc/defaults/ Default system configuration files. Refer to &man.rc.8; for details. /etc/mail/ Configuration files for mail transport agents such as &man.sendmail.8;. /etc/namedb/ named configuration files. Refer to &man.named.8; for details. /etc/periodic/ Scripts that run daily, weekly, and monthly, via &man.cron.8;. Refer to &man.periodic.8; for details. /etc/ppp/ ppp configuration files as described in &man.ppp.8;. /mnt/ Empty directory commonly used by system administrators as a temporary mount point. /proc/ Process file system. Refer to &man.procfs.5;, &man.mount.procfs.8; for details. /rescue/ Statically linked programs for emergency recovery as described in &man.rescue.8;. /root/ Home directory for the root account. /sbin/ System programs and administration utilities fundamental to both single-user and multi-user environments. /tmp/ Temporary files which are usually not preserved across a system reboot. A memory-based file system is often mounted at /tmp. This can be automated using the tmpmfs-related variables of &man.rc.conf.5; or with an entry in /etc/fstab; refer to &man.mdmfs.8; for details. /usr/ The majority of user utilities and applications. /usr/bin/ Common utilities, programming tools, and applications. /usr/include/ Standard C include files. /usr/lib/ Archive libraries. /usr/libdata/ Miscellaneous utility data files. /usr/libexec/ System daemons and system utilities executed by other programs. /usr/local/ Local executables and libraries. Also used as the default destination for the &os; ports framework. Within /usr/local, the general layout sketched out by &man.hier.7; for /usr should be used. Exceptions are the man directory, which is directly under /usr/local rather than under /usr/local/share, and the ports documentation is in share/doc/port. /usr/obj/ Architecture-specific target tree produced by building the /usr/src tree. /usr/ports/ The &os; Ports Collection (optional). /usr/sbin/ System daemons and system utilities executed by users. /usr/share/ Architecture-independent files. /usr/src/ BSD and/or local source files. /var/ Multi-purpose log, temporary, transient, and spool files. A memory-based file system is sometimes mounted at /var. This can be automated using the varmfs-related variables in &man.rc.conf.5; or with an entry in /etc/fstab; refer to &man.mdmfs.8; for details. /var/log/ Miscellaneous system log files. /var/mail/ User mailbox files. /var/spool/ Miscellaneous printer and mail system spooling directories. /var/tmp/ Temporary files which are usually preserved across a system reboot, unless /var is a memory-based file system. /var/yp/ NIS maps. Disk Organization The smallest unit of organization that &os; uses to find files is the filename. Filenames are case-sensitive, which means that readme.txt and README.TXT are two separate files. &os; does not use the extension of a file to determine whether the file is a program, document, or some other form of data. Files are stored in directories. A directory may contain no files, or it may contain many hundreds of files. A directory can also contain other directories, allowing you to build up a hierarchy of directories within one another in order to organize data. Files and directories are referenced by giving the file or directory name, followed by a forward slash, /, followed by any other directory names that are necessary. For example, if the directory foo contains a directory bar which contains the file readme.txt, the full name, or path, to the file is foo/bar/readme.txt. Note that this is different from &windows; which uses \ to separate file and directory names. &os; does not use drive letters, or other drive names in the path. For example, you would not type c:/foo/bar/readme.txt on &os;. Directories and files are stored in a file system. Each file system contains exactly one directory at the very top level, called the root directory for that file system. This root directory can contain other directories. One file system is designated the root file system or /. Every other file system is mounted under the root file system. No matter how many disks you have on your &os; system, every directory appears to be part of the same disk. Suppose you have three file systems, called A, B, and C. Each file system has one root directory, which contains two other directories, called A1, A2 (and likewise B1, B2 and C1, C2). Call A the root file system. If you used ls to view the contents of this directory you would see two subdirectories, A1 and A2. The directory tree looks like this: / | +--- A1 | `--- A2 A file system must be mounted on to a directory in another file system. When mounting file system B on to the directory A1, the root directory of B replaces A1, and the directories in B appear accordingly: / | +--- A1 | | | +--- B1 | | | `--- B2 | `--- A2 Any files that are in the B1 or B2 directories can be reached with the path /A1/B1 or /A1/B2 as necessary. Any files that were in /A1 have been temporarily hidden. They will reappear if B is unmounted from A. If B had been mounted on A2 then the diagram would look like this: / | +--- A1 | `--- A2 | +--- B1 | `--- B2 and the paths would be /A2/B1 and /A2/B2 respectively. File systems can be mounted on top of one another. Continuing the last example, the C file system could be mounted on top of the B1 directory in the B file system, leading to this arrangement: / | +--- A1 | `--- A2 | +--- B1 | | | +--- C1 | | | `--- C2 | `--- B2 Or C could be mounted directly on to the A file system, under the A1 directory: / | +--- A1 | | | +--- C1 | | | `--- C2 | `--- A2 | +--- B1 | `--- B2 Typically you create file systems when installing &os; and decide where to mount them, and then never change them unless you add a new disk. It is entirely possible to have one large root file system, and not need to create any others. There are some drawbacks to this approach, and one advantage. Benefits of Multiple File Systems Different file systems can have different mount options. For example, the root file system can be mounted read-only, making it impossible for users to inadvertently delete or edit a critical file. Separating user-writable file systems, such as /home, from other file systems allows them to be mounted nosuid. This option prevents the suid/guid bits on executables stored on the file system from taking effect, possibly improving security. &os; automatically optimizes the layout of files on a file system, depending on how the file system is being used. So a file system that contains many small files that are written frequently will have a different optimization to one that contains fewer, larger files. By having one big file system this optimization breaks down. &os;'s file systems are very robust should you lose power. However, a power loss at a critical point could still damage the structure of the file system. By splitting data over multiple file systems it is more likely that the system will still come up, making it easier to restore from backup as necessary. Benefit of a Single File System File systems are a fixed size. If you create a file system when you install &os; and give it a specific size, you may later discover that you need to make the partition bigger. This is not easily accomplished without backing up, recreating the file system with the new size, and then restoring the backed up data. &os; features the &man.growfs.8; command, which makes it possible to increase the size of file system on the fly, removing this limitation. File systems are contained in partitions. This does not have the same meaning as the common usage of the term partition (for example, &ms-dos; partition), because of &os;'s &unix; heritage. Each partition is identified by a letter from a through to h. Each partition can contain only one file system, which means that file systems are often described by either their typical mount point in the file system hierarchy, or the letter of the partition they are contained in. &os; also uses disk space for swap space to provide virtual memory. This allows your computer to behave as though it has much more memory than it actually does. When &os; runs out of memory, it moves some of the data that is not currently being used to the swap space, and moves it back in (moving something else out) when it needs it. Some partitions have certain conventions associated with them. Partition Convention a Normally contains the root file system. b Normally contains swap space. c Normally the same size as the enclosing slice. This allows utilities that need to work on the entire slice, such as a bad block scanner, to work on the c partition. You would not normally create a file system on this partition. d Partition d used to have a special meaning associated with it, although that is now gone and d may work as any normal partition. Disks in &os; are divided into slices, referred to in &windows; as partitions, which are numbered from 1 to 4. These are then then divided into partitions, which contain file systems, and are labeled using letters. slices partitions dangerously dedicated Slice numbers follow the device name, prefixed with an s, starting at 1. So da0s1 is the first slice on the first SCSI drive. There can only be four physical slices on a disk, but you can have logical slices inside physical slices of the appropriate type. These extended slices are numbered starting at 5, so ad0s5 is the first extended slice on the first IDE disk. These devices are used by file systems that expect to occupy a slice. Slices, dangerously dedicated physical drives, and other drives contain partitions, which are represented as letters from a to h. This letter is appended to the device name, so da0a is the a partition on the first da drive, which is dangerously dedicated. ad1s3e is the fifth partition in the third slice of the second IDE disk drive. Finally, each disk on the system is identified. A disk name starts with a code that indicates the type of disk, and then a number, indicating which disk it is. Unlike slices, disk numbering starts at 0. Common codes that you will see are listed in . When referring to a partition, include the disk name, s, the slice number, and then the partition letter. Examples are shown in . shows a conceptual model of a disk layout. When installing &os;, configure the disk slices, create partitions within the slice to be used for &os;, create a file system or swap space in each partition, and decide where each file system will be mounted. Disk Device Codes Code Meaning ad ATAPI (IDE) disk da SCSI direct access disk acd ATAPI (IDE) CDROM cd SCSI CDROM fd Floppy disk
Sample Disk, Slice, and Partition Names Name Meaning ad0s1a The first partition (a) on the first slice (s1) on the first IDE disk (ad0). da1s2e The fifth partition (e) on the second slice (s2) on the second SCSI disk (da1). Conceptual Model of a Disk This diagram shows &os;'s view of the first IDE disk attached to the system. Assume that the disk is 4 GB in size, and contains two 2 GB slices (&ms-dos; partitions). The first slice contains a &ms-dos; disk, C:, and the second slice contains a &os; installation. This example &os; installation has three data partitions, and a swap partition. The three partitions will each hold a file system. Partition a will be used for the root file system, e for the /var/ directory hierarchy, and f for the /usr/ directory hierarchy. .-----------------. --. | | | | DOS / Windows | | : : > First slice, ad0s1 : : | | | | :=================: ==: --. | | | Partition a, mounted as / | | | > referred to as ad0s2a | | | | | :-----------------: ==: | | | | Partition b, used as swap | | | > referred to as ad0s2b | | | | | :-----------------: ==: | Partition c, no | | | Partition e, used as /var > file system, all | | > referred to as ad0s2e | of FreeBSD slice, | | | | ad0s2c :-----------------: ==: | | | | | : : | Partition f, used as /usr | : : > referred to as ad0s2f | : : | | | | | | | | --' | `-----------------' --' Mounting and Unmounting File Systems The file system is best visualized as a tree, rooted, as it were, at /. /dev, /usr, and the other directories in the root directory are branches, which may have their own branches, such as /usr/local, and so on. root file system There are various reasons to house some of these directories on separate file systems. /var contains the directories log/, spool/, and various types of temporary files, and as such, may get filled up. Filling up the root file system is not a good idea, so splitting /var from / is often favorable. Another common reason to contain certain directory trees on other file systems is if they are to be housed on separate physical disks, or are separate virtual disks, such as Network File System mounts, or CDROM drives. The <filename>fstab</filename> File file systems mounted with fstab During the boot process, file systems listed in /etc/fstab are automatically mounted except for the entries containing . This file contains entries in the following format: device /mount-point fstype options dumpfreq passno device An existing device name as explained in . mount-point An existing directory on which to mount the file system. fstype The file system type to pass to &man.mount.8;. The default &os; file system is ufs. options Either for read-write file systems, or for read-only file systems, followed by any other options that may be needed. A common option is for file systems not normally mounted during the boot sequence. Other options are listed in &man.mount.8;. dumpfreq Used by &man.dump.8; to determine which file systems require dumping. If the field is missing, a value of zero is assumed. passno Determines the order in which file systems should be checked. File systems that should be skipped should have their passno set to zero. The root file system needs to be checked before everything else and should have its passno set to one. The other file systems should be set to values greater than one. If more than one file system has the same passno, &man.fsck.8; will attempt to check file systems in parallel if possible. Refer to &man.fstab.5; for more information on the format of /etc/fstab and its options. The <command>mount</command> Command file systems mounting File systems are mounted using &man.mount.8;. The most basic syntax is as follows: &prompt.root; mount device mountpoint This command provides many options which are described in &man.mount.8;, The most commonly used options include: Mount Options Mount all the file systems listed in /etc/fstab, except those marked as noauto, excluded by the flag, or those that are already mounted. Do everything except for the actual mount system call. This option is useful in conjunction with the flag to determine what &man.mount.8; is actually trying to do. Force the mount of an unclean file system (dangerous), or the revocation of write access when downgrading a file system's mount status from read-write to read-only. Mount the file system read-only. This is identical to using . fstype Mount the specified file system type or mount only file systems of the given type, if is included. ufs is the default file system type. Update mount options on the file system. Be verbose. Mount the file system read-write. The following options can be passed to as a comma-separated list: nosuid Do not interpret setuid or setgid flags on the file system. This is also a useful security option. The <command>umount</command> Command file systems unmounting To unmount a filesystem use &man.umount.8;. This command takes one parameter which can be a mountpoint, device name, or . All forms take to force unmounting, and for verbosity. Be warned that is not generally a good idea as it might crash the computer or damage data on the file system. To unmount all mounted file systems, or just the file system types listed after , use or . Note that does not attempt to unmount the root file system. Processes &os; is a multi-tasking operating system. Each program running at any one time is called a process. Every running command starts at least one new process and there are a number of system processes that are run by &os;. Each process is uniquely identified by a number called a process ID (PID). Similar to files, each process has one owner and group, and the owner and group permissions are used to determine which files and devices the process can open. Most processes also have a parent process that started them. For example, the shell is a process, and any command started in the shell is a process which has the shell as its parent process. The exception is a special process called &man.init.8; which is always the first process to start at boot time and which always has a PID of 1. To see the processes on the system, use &man.ps.1; and &man.top.1;. To display a static list of the currently running processes, their PIDs, how much memory they are using, and the command they were started with, use ps. To display all the running processes and update the display every few seconds so that you can interactively see what the computer is doing, use top. By default, ps only shows the commands that are running and owned by the user. For example: &prompt.user; ps PID TT STAT TIME COMMAND 298 p0 Ss 0:01.10 tcsh 7078 p0 S 2:40.88 xemacs mdoc.xsl (xemacs-21.1.14) 37393 p0 I 0:03.11 xemacs freebsd.dsl (xemacs-21.1.14) 72210 p0 R+ 0:00.00 ps 390 p1 Is 0:01.14 tcsh 7059 p2 Is+ 1:36.18 /usr/local/bin/mutt -y 6688 p3 IWs 0:00.00 tcsh 10735 p4 IWs 0:00.00 tcsh 20256 p5 IWs 0:00.00 tcsh 262 v0 IWs 0:00.00 -tcsh (tcsh) 270 v0 IW+ 0:00.00 /bin/sh /usr/X11R6/bin/startx -- -bpp 16 280 v0 IW+ 0:00.00 xinit /home/nik/.xinitrc -- -bpp 16 284 v0 IW 0:00.00 /bin/sh /home/nik/.xinitrc 285 v0 S 0:38.45 /usr/X11R6/bin/sawfish The output from &man.ps.1; is organized into a number of columns. The PID column displays the process ID. PIDs are assigned starting at 1, go up to 99999, then wrap around back to the beginning. However, a PID is not reassigned if it is already in use. The TT column shows the tty the program is running on and STAT shows the program's state. TIME is the amount of time the program has been running on the CPU. This is usually not the elapsed time since the program was started, as most programs spend a lot of time waiting for things to happen before they need to spend time on the CPU. Finally, COMMAND is the command that was used to start the program. &man.ps.1; supports a number of different options to change the information that is displayed. One of the most useful sets is auxww. displays information about all the running processes of all users. displays the username of the process' owner, as well as memory usage. displays information about daemon processes, and causes &man.ps.1; to display the full command line for each process, rather than truncating it once it gets too long to fit on the screen. The output from &man.top.1; is similar. A sample session looks like this: &prompt.user; top last pid: 72257; load averages: 0.13, 0.09, 0.03 up 0+13:38:33 22:39:10 47 processes: 1 running, 46 sleeping CPU states: 12.6% user, 0.0% nice, 7.8% system, 0.0% interrupt, 79.7% idle Mem: 36M Active, 5256K Inact, 13M Wired, 6312K Cache, 15M Buf, 408K Free Swap: 256M Total, 38M Used, 217M Free, 15% Inuse PID USERNAME PRI NICE SIZE RES STATE TIME WCPU CPU COMMAND 72257 nik 28 0 1960K 1044K RUN 0:00 14.86% 1.42% top 7078 nik 2 0 15280K 10960K select 2:54 0.88% 0.88% xemacs-21.1.14 281 nik 2 0 18636K 7112K select 5:36 0.73% 0.73% XF86_SVGA 296 nik 2 0 3240K 1644K select 0:12 0.05% 0.05% xterm 175 root 2 0 924K 252K select 1:41 0.00% 0.00% syslogd 7059 nik 2 0 7260K 4644K poll 1:38 0.00% 0.00% mutt ... The output is split into two sections. The header (the first five lines) shows the PID of the last process to run, the system load averages (which are a measure of how busy the system is), the system uptime (time since the last reboot) and the current time. The other figures in the header relate to how many processes are running (47 in this case), how much memory and swap space has been used, and how much time the system is spending in different CPU states. Below the header is a series of columns containing similar information to the output from &man.ps.1;, such as the PID, username, amount of CPU time, and the command that started the process. By default, &man.top.1; also displays the amount of memory space taken by the process. This is split into two columns: one for total size and one for resident size. Total size is how much memory the application has needed and the resident size is how much it is actually using at the moment. In this example, mutt has required almost 8 MB of RAM, but is currently only using 5 MB. &man.top.1; automatically updates the display every two seconds. A different interval can be specified with . Daemons, Signals, and Killing Processes When using an editor, it is easy to control the editor and load files because the editor provides facilities to do so, and because the editor is attached to a terminal. Some programs are not designed to be run with continuous user input and disconnect from the terminal at the first opportunity. For example, a web server responds to web requests, rather than user input. Mail servers are another example of this type of application. These programs are known as daemons. The term daemon comes from Greek mythology and represents an entity that is neither good or evil, and which invisibly performs useful tasks. This is why the BSD mascot is the cheerful-looking daemon with sneakers and a pitchfork. There is a convention to name programs that normally run as daemons with a trailing d. BIND is the Berkeley Internet Name Domain, but the actual program that executes is named. The Apache web server program is httpd and the line printer spooling daemon is lpd. This is only a naming convention. For example, the main mail daemon for the Sendmail application is sendmail, and not maild. One way to communicate with a daemon, or any running process, is to send a signal using &man.kill.1;. There are a number of different signals; some have a specific meaning while others are described in the application's documentation. A user can only send a signal to a process they own and sending a signal to someone else's process will result in a permission denied error. The exception is the root user, who can send signals to anyone's processes. &os; can also send a signal to a process. If an application is badly written and tries to access memory that it is not supposed to, &os; will send the process the Segmentation Violation signal (SIGSEGV). If an application has used the &man.alarm.3; system call to be alerted after a period of time has elapsed, it will be sent the Alarm signal (SIGALRM). Two signals can be used to stop a process: SIGTERM and SIGKILL. SIGTERM is the polite way to kill a process as the process can read the signal, close any log files it may have open, and attempt to finish what it is doing before shutting down. In some cases, a process may ignore SIGTERM if it is in the middle of some task that can not be interrupted. SIGKILL can not be ignored by a process. This is the I do not care what you are doing, stop right now signal. Sending a SIGKILL to a process will usually stop that process there and then. There are a few tasks that can not be interrupted. For example, if the process is trying to read from a file that is on another computer on the network, and the other computer is unavailable, the process is said to be uninterruptible. Eventually the process will time out, typically after two minutes. As soon as this time out occurs the process will be killed. . Other commonly used signals are SIGHUP, SIGUSR1, and SIGUSR2. These are general purpose signals and different applications will respond differently. For example, after changing a web server's configuration file, the web server needs to be told to re-read its configuration. Restarting httpd would result in a brief outage period on the web server. Instead, send the daemon the SIGHUP signal. Be aware that different daemons will have different behavior, so refer to the documentation for the daemon to determine if SIGHUP will achieve the desired results. Sending a Signal to a Process This example shows how to send a signal to &man.inetd.8;. The inetd configuration file is /etc/inetd.conf, and inetd will re-read this configuration file when it is sent a SIGHUP. Find the PID of the process you want to send the signal to using &man.pgrep.1;. In this example, the PID for &man.inetd.8; is 198: &prompt.user; pgrep -l inetd 198 inetd -wW Use &man.kill.1; to send the signal. Because &man.inetd.8; is owned by root, use &man.su.1; to become root first. &prompt.user; su Password: &prompt.root; /bin/kill -s HUP 198 Like most &unix; commands, &man.kill.1; will not print any output if it is successful. If you send a signal to a process that you do not own, you will instead see kill: PID: Operation not permitted. Mistyping the PID will either send the signal to the wrong process, which could have negative results, or will send the signal to a PID that is not currently in use, resulting in the error kill: PID: No such process. Why Use <command>/bin/kill</command>? Many shells provide kill as a built in command, meaning that the shell will send the signal directly, rather than running /bin/kill. Be aware that different shells have a different syntax for specifying the name of the signal to send. Rather than try to learn all of them, it can be simpler to use /bin/kill ... directly. When sending other signals, substitute TERM or KILL in the command line as necessary. Killing a random process on the system can be a bad idea. In particular, &man.init.8;, PID 1, is special. Running /bin/kill -s KILL 1 is a quick, and unrecommended, way to shutdown the system. Always double check the arguments to &man.kill.1; before pressing Return. Shells shells command line &os; provides a command line interface called a shell. A shell receives commands from the input channel and executes them. Many shells provide built in functions to help with everyday tasks such as file management, file globbing, command line editing, command macros, and environment variables. &os; comes with several shells, including sh, the Bourne Shell, and tcsh, the improved C-shell. Other shells are available from the &os; Ports Collection, such as zsh and bash. The shell that is used is really a matter of taste. A C programmer might feel more comfortable with a C-like shell such as tcsh. A Linux user might prefer bash. Each shell has unique properties that may or may not work with a user's preferred working environment, which is why there is a choice of which shell to use. One common shell feature is filename completion. After a user types the first few letters of a command or filename and presses Tab, the shell will automatically complete the rest of the command or filename. Consider two files called foobar and foo.bar. To delete foo.bar, type rm fo[Tab].[Tab]. The shell should print out rm foo[BEEP].bar. The [BEEP] is the console bell, which the shell used to indicate it was unable to complete the filename because there is more than one match. Both foobar and foo.bar start with fo. By typing ., then pressing Tab again, the shell would be able to fill in the rest of the filename. environment variables Another feature of the shell is the use of environment variables. Environment variables are a variable/key pair stored in the shell's environment. This environment can be read by any program invoked by the shell, and thus contains a lot of program configuration. Here is a list of common environment variables and their meanings: Variable Description USER Current logged in user's name. PATH Colon-separated list of directories to search for binaries. DISPLAY Network name of the Xorg display to connect to, if available. SHELL The current shell. TERM The name of the user's type of terminal. Used to determine the capabilities of the terminal. TERMCAP Database entry of the terminal escape codes to perform various terminal functions. OSTYPE Type of operating system. MACHTYPE The system's CPU architecture. EDITOR The user's preferred text editor. PAGER The user's preferred text pager. MANPATH Colon-separated list of directories to search for manual pages. Bourne shells How to set an environment variable differs between shells. In tcsh and csh, use setenv to set environment variables. In sh and bash, use export to set the current environment variables. This example sets the default EDITOR to /usr/local/bin/emacs for the tcsh shell: &prompt.user; setenv EDITOR /usr/local/bin/emacs The equivalent command for bash would be: &prompt.user; export EDITOR="/usr/local/bin/emacs" To expand an environment variable in order to see its current setting, type a $ character in front of its name on the command line. For example, echo $TERM displays the current $TERM setting. Shells treat special characters, known as meta-characters, as special representations of data. The most common meta-character is *, which represents any number of characters in a filename. Meta-characters can be used to perform filename globbing. For example, echo * is equivalent to ls because the shell takes all the files that match * and echo lists them on the command line. To prevent the shell from interpreting a special character, escape it from the shell by starting it with a backslash (\). For example, echo $TERM prints the terminal setting whereas echo \$TERM literally prints the string $TERM. Changing Your Shell The easiest way to permanently change the default shell is to use chsh. Running this command will open the editor that is configured in the EDITOR environment variable, which by default is set to vi. Change the Shell: line to the full path of the new shell. Alternately, use chsh -s which will set the specified shell without opening an editor. For example, to change the shell to bash: &prompt.user; chsh -s /usr/local/bin/bash The new shell must be present in /etc/shells. If the shell was installed from the &os; Ports Collection, it should be automatically added to this file. If it is missing, add it using this command, replacing the path with the path of the shell: &prompt.root; echo /usr/local/bin/bash >> /etc/shells Then rerun chsh. Text Editors text editors editors Most &os; configuration is done by editing text files. Because of this, it is a good idea to become familiar with a text editor. &os; comes with a few as part of the base system, and many more are available in the Ports Collection. ee editors ee A simple editor to learn is ee, which stands for easy editor. To start this editor, type ee filename where filename is the name of the file to be edited. Once inside the editor, all of the commands for manipulating the editor's functions are listed at the top of the display. The caret ^ represents Ctrl, so ^e expands to Ctrle. To leave ee, press Esc, then choose the leave editor option from the main menu. The editor will prompt you to save any changes if the file has been modified. vi editors vi emacs editors emacs &os; also comes with more powerful text editors such as vi as part of the base system. Other editors, like editors/emacs and editors/vim, are part of the &os; Ports Collection. These editors offer more functionality at the expense of being a more complicated to learn. Learning a more powerful editor such as vim or Emacs can save more time in the long run. Many applications which modify files or require typed input will automatically open a text editor. To alter the default editor used, set the EDITOR environment variable as described in the shells section. Devices and Device Nodes A device is a term used mostly for hardware-related activities in a system, including disks, printers, graphics cards, and keyboards. When &os; boots, the majority of the boot messages refer to devices being detected. A copy of the boot messages are saved to /var/run/dmesg.boot. Each device has a device name and number. For example, acd0 is the first IDE CD-ROM drive, while kbd0 represents the keyboard. Most devices in a &os; must be accessed through special files called device nodes, which are located in /dev. - - - Creating Device Nodes - - When adding a new device to your system, or compiling - in support for additional devices, new device nodes must - be created. - - - <literal>DEVFS</literal> (DEVice File System) - - The device file system, DEVFS, - provides access to the kernel's device namespace in the - global file system namespace. Instead of having to - manually create and modify device nodes, - DEVFS automatically maintains this - particular file system. Refer to &man.devfs.5; for - more information. - - Binary Formats To understand why &os; uses the &man.elf.5; format,the three currently dominant executable formats for &unix; must be described: &man.a.out.5; The oldest and classic &unix; object format. It uses a short and compact header with a &man.magic.5; number at the beginning that is often used to characterize the format. It contains three loaded segments: .text, .data, and .bss, plus a symbol table and a string table. COFF The SVR3 object format. The header comprises a section table which can contain more than just .text, .data, and .bss sections. &man.elf.5; The successor to COFF, featuring multiple sections and 32-bit or 64-bit possible values. One major drawback is that ELF was designed with the assumption that there would be only one ABI per system architecture. That assumption is actually incorrect, and not even in the commercial SYSV world (which has at least three ABIs: SVR4, Solaris, SCO) does it hold true. &os; tries to work around this problem somewhat by providing a utility for branding a known ELF executable with information about its compliant ABI. Refer to &man.brandelf.1; for more information. &os; comes from the classic camp and used the &man.a.out.5; format, a technology tried and proven through many generations of BSD releases, until the beginning of the 3.X branch. Though it was possible to build and run native ELF binaries and kernels on a &os; system for some time before that, &os; initially resisted the push to switch to ELF as the default format. Why? When Linux made its painful transition to ELF, it was due to their inflexible jump-table based shared library mechanism, which made the construction of shared libraries difficult for vendors and developers. Since ELF tools offered a solution to the shared library problem and were generally seen as the way forward, the migration cost was accepted as necessary and the transition made. &os;'s shared library mechanism is based more closely on the &sunos; style shared library mechanism and is easy to use. So, why are there so many different formats? Back in the PDP-11 days when simple hardware supported a simple, small system, a.out was adequate for the job of representing binaries. As &unix; was ported, the a.out format was retained because it was sufficient for the early ports of &unix; to architectures like the Motorola 68k or VAXen. Then some hardware engineer decided that if he could force software to do some sleazy tricks, a few gates could be shaved off the design and the CPU core could run faster. a.out was ill-suited for this new kind of hardware, known as RISC. Many formats were developed to get better performance from this hardware than the limited, simple a.out format could offer. COFF, ECOFF, and a few others were invented and their limitations explored before settling on ELF. In addition, program sizes were getting huge while disks and physical memory were still relatively small, so the concept of a shared library was born. The virtual memory system became more sophisticated. While each advancement was done using the a.out format, its usefulness was stretched with each new feature. In addition, people wanted to dynamically load things at run time, or to junk parts of their program after the init code had run to save in core memory and swap space. Languages became more sophisticated and people wanted code called before the main() function automatically. Lots of hacks were done to the a.out format to allow all of these things to happen, and they basically worked for a time. In time, a.out was not up to handling all these problems without an ever increasing overhead in code and complexity. While ELF solved many of these problems, it would be painful to switch from the system that basically worked. So ELF had to wait until it was more painful to remain with a.out than it was to migrate to ELF. As time passed, the build tools that &os; derived their build tools from, especially the assembler and loader, evolved in two parallel trees. The &os; tree added shared libraries and fixed some bugs. The GNU folks that originally wrote these programs rewrote them and added simpler support for building cross compilers and plugging in different formats. Those who wanted to build cross compilers targeting &os; were out of luck since the older sources that &os; had for as and ld were not up to the task. The new GNU tools chain (binutils) supports cross compiling, ELF, shared libraries, and C++ extensions. In addition, many vendors release ELF binaries, and &os; should be able to run them. ELF is more expressive than a.out and allows more extensibility in the base system. The ELF tools are better maintained and offer cross compilation support. ELF may be a little slower than a.out, but trying to measure it can be difficult. There are also numerous details that are different between the two such as how they map pages and handle init code. For More Information Manual Pages manual pages The most comprehensive documentation on &os; is in the form of manual pages. Nearly every program on the system comes with a short reference manual explaining the basic operation and available arguments. These manuals can be viewed using man: &prompt.user; man command where command is the name of the command you wish to learn about. For example, to learn more about ls, type: &prompt.user; man ls The online manual is divided into numbered sections: User commands. System calls and error numbers. Functions in the C libraries. Device drivers. File formats. Games and other diversions. Miscellaneous information. System maintenance and operation commands. Kernel developers. In some cases, the same topic may appear in more than one section of the online manual. For example, there is a chmod user command and a chmod() system call. To tell man which section to display, specify the section number: &prompt.user; man 1 chmod This will display the manual page for the user command chmod. References to a particular section of the online manual are traditionally placed in parenthesis in written documentation, so &man.chmod.1; refers to the chmod user command and &man.chmod.2; refers to the system call. If you do not know the command name, use man -k to search for keywords in the command descriptions: &prompt.user; man -k mail This command displays a list of commands that have the keyword mail in their descriptions. This is equivalent to using &man.apropos.1;. To determine what the commands in /usr/bin do, type: &prompt.user; cd /usr/bin &prompt.user; man -f * or &prompt.user; cd /usr/bin &prompt.user; whatis * GNU Info Files Free Software Foundation &os; includes many applications and utilities produced by the Free Software Foundation (FSF). In addition to manual pages, these programs may include hypertext documents called info files. These can be viewed using info or, if editors/emacs is installed, the info mode of emacs. To use &man.info.1;, type: &prompt.user; info For a brief introduction, type h. For a quick command reference, type ?.