Unix System Administration - Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

This course series targets both beginning and intermediate Linux/Unix users who want to acquire advanced system administration skills. The Linux/Unix course series is comprised of four courses that span Unix administration skills from basic to complex. Students will have access to their own Linux machines and networks throughout the series.

access mode: The access mode is the first column of an ls -l listing. The access mode is composed of a file type followed by the user permissions, group permissions, and other permissions.
access permission bits: Access permission bits are a series of 9 bits that describe various file permissions. The first 3 access permission bits describe the read, write, and execute permissions for the file's user. The second 3 bits describe the read, write, and execute permissions for the file's group. The final 3 bits describe the read, write, and execute permissions for others.
at: The at command runs a program at one specified time. It can execute several commands at once, running them in the background.
bash: The bash command starts the Bourne Again shell.
batch: The batch command executes commands entered from standard input. This command functions much like the at command. However, one key difference exists: at can execute several commands at once, running them in the background, whereas batch runs one process, waits for it to end, then runs another.
boot: To boot a system means to start it up. Many systems are configured to boot automatically after the power comes on or after a system crash.
BSD: BSD is a type of UNIX operating system derived from the work done at Computer Science laboratories at the University of California at Berkeley. The most commonly encountered BSD system is SunOS version 4.3. Berkeley's BSD version 4.4 is also fairly common.
cat: The cat command displays one or more files on the standard output. For example, the command cat file1 would display the contents of file1 on the output screen.
chgrp: The chgrp command changes the group of a file.
chmod: The chmod command changes the access mode of a file.
chown: The chown command changes the owner of a file.
cron: The cron daemon lets you automate the running of processes and applications and lets you schedule a particular time for a process to run.
crontab: The crontab command allows you to create and manage the entries that inform the cron daemon about the applications and processes you want to automate.
daemon: A daemon is a process that is not associated with any terminal and which is dedicated to handling a particular task.
device: A device is a piece of hardware used by the system, such as hard drives, CD-ROMs, and so on. Devices are treated as a special type of file.
dump: The dump utility backs up all the files within a filesystem. The dump utility has many options and arguments that allow you to back up parts of the filesystem selectively. On Solaris, the dump utility is called fsdump. On AIX, the utility is called backup.
echo: The echo command writes a string to standard output. For example, echo "THIS IS A TEST" would display THIS IS A TEST on the screen.
environment variable: An environment variable is a system variable that contains information about some aspect of the system's environment. For example, the PATH environment variable describes the search path that is used whenever a command name is entered without an explicit directory location.
exit: The exit command quits a subshell.
export: The export command allows you to give global meaning to one or more shell variables, which are by default local.
fg: The fg command resumes a suspended subshell.
file: 1) A file is any resource accessible on a UNIX system, including "regular" files, directories, and devices. 2) The file command displays information about a file's type.
file type: The file type is the first character in the access mode column of the ls -l listing. The most common file types are - for a regular file and d for a directory.
filesystem: A UNIX filesystem is a directory hierarchy. A UNIX system has one overall filesystem, with its root at /, which may be made up of a number of other filesystems.
forking: Forking is the mechanism by which the system starts a new process, waits for it to finish, then generates a new output prompt and waits for the next command.
fsck: The fsck command checks a filesystem for consistency.
fuser: The fuser command reveals all processes that have files open on a device, and therefore are preventing it from being unmounted.
grep: The grep command searches the named input files for lines containing a match to the given pattern. By default, grep prints the matching lines. The grep command reads from standard input if no files are given or when a filename of is encountered.
group: A group allows an arbitrary set of users to share files. When a user is a member of the file's group, the user will have that file's group permissions.
group permission: Group permissions are the second set of read/write/execute permissions in an access mode. Group permissions apply to users that are members of the file's group.
head: The head command displays the first part (10 lines by default) of each given file. The head command reads from standard input if no files are given or when a filename of - is encountered.
id: The id command shows the user ID and group membership of a file.
init: The init process is the first process to run on a system. All other processes on the system are derived from init by the fork and exec procedures, either directly or indirectly.
inode: An inode is the physical representation of a file on the disk. Each inode, and therefore each file, has a number that uniquely identifies it. These inode numbers can be viewed with the -i option to the ls command.
interpreter: An interpreter is a program that reads and executes line by line other programs written in the appropriate programming language.
kernel: The kernel is the core of the UNIX operating system. The kernel remains hidden from typical users.
kill: The kill command terminates processes by specifying the process identification (PID) number.
Linux: Linux is a version of UNIX derived from the work of Linus Torvalds, who wrote the basics of the operating system himself. His work was picked up by people around the world who, in a remarkable cooperative effort, wrote a complete UNIX system from scratch.
ln: The ln command creates a link. Using the -s option will create a symbolic link, or alias, to another file.
logical volume: In a logical volume system, groups of physical disks are combined together into "volume groups." These volume groups can be divided up into "logical volumes." These logical volumes may span multiple physical disks, but they act like partitions in the old sense of the word. A logical volume, for example, contains a filesystem. In the logical volume world, you mount and unmount logical volumes, not disk partitions.
ls: The ls commands lists a file or directory. With no arguments, ls lists the current directory.
major device number: Device files are categorized as "block" or "character" and have associated major and minor numbers. The major numbers typically group the devices into subsystems (IDE devices, SCSI devices, and so on).
man: The man command displays pages of the UNIX online manual. For example, man mount would display the manual page for the mount command.
minor device number: Device files are categorized as ""block"" or ""character"" and have associated major and minor numbers. The minor numbers identify the individual devices within the subsystem.
mkfs: The mkfs command creates a new filesystem on a formatted partition.
mount: The mount command attaches a new filesystem to the existing filesystem.
mount-point: A mount-point is the directory to which a filesystem will be mounted.
octal: An octal number is a number based on 8s, just as a decimal number is based on 10s. For example, the decimal number 10 means the number consists of one 10 and zero ones. In octal, the decimal number 10 would be represented as 12--one 8 and two 1s.
other permission: Other permissions are the third set of read/write/execute permissions in an access mode. Other permissions apply to everyone besides the file owner and the group members.
owner: The file owner is typically the user who created the file. You can change permissions only on files you own.
PATH: The PATH environment variable contains the default search path for the current shell. Colons separate the various directories in the path. This search path is used to locate a command when the command is entered without an explicit directory.
periodic process: A periodic process is any process that runs at a specified time.
permission type: A permission type grants users a particular type of access. The three basic permission types are r (read), w (write), and x (execute).
pipe: A pipe is a way of using the output of one command as the input for another command and is designated by the character |. For example, who | wc -l would take the output of the who command and use that as the input for the wc -l command. This would give you the number ofusers currently logged in.
process: A UNIX process is a running program.
process ID (PID): A process ID (PID) is a unique number associated with each process on the system. PIDs start with number 1, which is the init process.
ps: The ps command lists the running processes on a system, together with useful information about their status.
root account: A special user account that has absolute power over all aspects of system operation.
root privileges: When one has root privileges, one has the powers associated with the root account.
save text permission: Setting the save text permission bit (indicated by t) on an executable file is supposed to tell the kernel to leave a program in memory after it terminates. This use is now largely obsolete. The save text permission on a directory means something slightly different. When this permission bit is set on a directory, a user may delete a file only if he or she has write permission (w) for that file, even if he or she has write permission on the directory.
SGID permission: The SGID permission sets a process's group ID on execution.
shell: The shell is a program that interprets your commands and passes them to the operating system for further processing. You enter commands at the shell prompt (also called the system prompt or command prompt).
shell script: A shell script is a set of commands stored in a file. You can run the file as a program, much like a batch script is run on DOS.
sleep: The sleep command allows you to specify a certain period of time (usually in seconds) to wait before the system executes a command. You can use this program within a shell script to execute certain commands. The syntax is sleep followed by a number that represents the number of seconds you want to wait.
source: The source command allows you to execute programs that have not yet been compiled. This is because source opens a C shell script, then executes the program. You can use the source command to test shell text files before you make them executable.
su: The su command allows a user to gain access to the account of another user, as long as the user's password is known. Its main purpose, however, is in asserting root privileges, which is accomplished by using the su command without any arguments.
subshell: A subshell is any shell that is started from another shell.
SUID permission: The SUID permission sets a process's user ID on execution.
superblock: The superblock contains high-level information about the disk partition, such as its label. It also contains tables of information laying out available space on the partition. Every filesystem has one active superblock, but the information there is so critical that the commonly used UNIX filesystems replicate the superblock data in other locations on the disk.
superuser: A special user account that has root privileges. You can become a superuser by typing the su command without arguments and giving the system's root password.
suspend: The suspend command temporarily suspends a subshell with root permissions and returns to the shell with normal user privileges.
symbolic link: A symbolic link is an alias for a file.
System V: System V is a type of UNIX operating system that arose from an alliance of ATT and Sun. Solaris, AIX, and HP-UX are the most commonly encountered System V operating systems.
tail: The tail command displays the last part (10 lines by default) of each given file. The tail command reads from standard input if no files are given or when a filename of - is encountered.
tar: The tar utility is an archiving program that allows you to back up directories or files quickly. It is used in many smaller backups and less sophisticated archiving procedures.
touch: The touch command creates a new, empty text file. For example, touch file1 will create a new, empty text file named file1.
umask: The umask command sets the default permissions of a new file.
umount: The umount command unmounts a filesystem.
uname: The uname command displays (depending on the options used) the operating system name and version and the machine name and architecture.
unmount: To unmount is to detach a filesystem from another filesystem.
user permission: User permissions are the first set of read/write/execute permissions in an access mode. User permissions apply to the file owner.
wc: The wc command displays a character, word, or line count for a specified file. Use the -c option for a character count, the -l option for a line count, and the -w for a word count. The wc command reads from standard input if no filenames are given. For example, wc -c file1would print the number of characters in file1.
whereis: The whereis command displays the location of the source, executable, and manual page for a command.
which: The which command displays which command gets runs under a given name.
xargs: The xargs command arranges for another command to read arguments from standard input instead of the command line. This command is used to pipe output from find into other commands.
/etc/checkpoint: The configuration file on HP-UX that describes partitions and mount-points. This file controls which filesystems get mounted at boot time.
/etc/filesystems: The configuration file on AIX that describes partitions and mount-points. This file controls which filesystems get mounted at boot time.
/etc/fstab: The configuration file on Linux that describes partitions and mount-points. This file controls which filesystems get mounted at boot time.
/etc/vfstab: The configuration file on Solaris that describes partitions and mount-points. This file controls which filesystems get mounted at boot time.