Chapter 1. Korn Shell Basics

You’ve used your computer for simple tasks, such as invoking your favorite application programs, reading your electronic mail, and perhaps examining and printing files. You know that your machine runs the UNIX operating system, or maybe you know it under some other name, like SunOS, Ultrix, HP/UX, AIX, A/UX, UTS, or Xenix. But apart from that, you may not have given too much thought to what goes on inside the machine when you type in a command and hit RETURN.

It is true that several layers of events take place whenever you enter a command, but we’re going to consider only the top layer, known as the shell. Generically speaking, a shell is any user interface to the UNIX operating system, i.e., any program that takes input from the user, translates it into instructions that the operating system can understand, and conveys the operating system’s output back to the user.

There are various types of user interfaces. The Korn shell belongs to the most common category, known as character-based user interfaces. These interfaces accept lines of textual commands that the user types in; they usually produce text-based output. Other types of interfaces include the increasingly common graphical user interfaces (GUI), which add the ability to display arbitrary graphics (not just typewriter characters) and to accept input from mice and other pointing devices, touch-screen interfaces (such as those you see on some bank teller machines), and so on.

What Is a Shell?

The shell’s job, then, is to translate the user’s command lines into operating system instructions. For example, consider this command line:

sort -n phonelist > phonelist.sorted

This means, “Sort lines in the file phonelist in numerical order, and put the result in the file phonelist.sorted.” Here’s what the shell does with this command:

  1. Breaks up the line into the pieces sort, -n, phonelist, >, and phonelist.sorted. These pieces are called words.

  2. Determines the purpose of the words: sort is a command, -n and phonelist are arguments, and > and phonelist.sorted, taken together, are I/O instructions.

  3. Sets up the I/O according to > phonelist.sorted (output to the file phonelist.sorted) and some standard, implicit instructions.

  4. Finds the command sort in a file and runs it with the option -n (numerical order) and the argument phonelist (input filename).

Of course, each of these steps really involves several substeps, each of which includes a particular instruction to the underlying operating system.

Remember that the shell itself is not UNIX-just the user interface to it. UNIX is one of the first operating systems to make the user interface independent of the operating system.

The shell is a layer around the UNIX operating system
Figure 1-1. The shell is a layer around the UNIX operating system

Scope of This Book

In this book, you will learn about the Korn shell, which is the most recent and powerful of the major UNIX shells. There are two ways to use the Korn shell: as a user interface and as a programming environment.

This chapter and the next cover interactive use. These two chapters should give you enough background to use the shell confidently and productively for most of your everyday tasks.

After you have been using the shell for a while, you will undoubtedly find certain characteristics of your environment (the shell’s “look and feel”) that you would like to change and tasks that you would like to automate. Chapter 3 shows several ways of doing this.

Chapter 3 also prepares you for shell programming, the bulk of which is covered in Chapter 4 through Chapter 6. You need not have any programming experience to understand these chapters and learn shell programming. Chapter 7 and Chapter 8 give more complete descriptions of the shell’s I/O and process handling capabilities, while Chapter 9 discusses various techniques for debugging shell programs.

You’ll learn a lot about the Korn shell in this book; you’ll also learn about UNIX utilities and the way the UNIX operating system works in general. It’s possible to become a virtuoso shell programmer without any previous programming experience. At the same time, we’ve carefully avoided going down past a certain level of detail about UNIX internals. We maintain that you shouldn’t have to be an internals expert to use and program the shell effectively, and we won’t dwell on the few shell features that are intended specifically for low-level systems programmers.

History of UNIX Shells

The independence of the shell from the UNIX operating system per se has led to the development of dozens of shells throughout UNIX history-although only a few have achieved widespread use.

The first major shell was the Bourne shell (named after its inventor, Steven Bourne); it was included in the first popular version of UNIX, Version 7, starting in 1979. The Bourne shell is known on the system as sh. Although UNIX has gone through many, many changes, the Bourne shell is still popular and essentially unchanged. Several UNIX utilities and administration features depend on it.

The first widely-used alternative shell was the C shell, or csh. This was written by Bill Joy at the University of California at Berkeley as part of the Berkeley System Distribution (BSD) version of UNIX that came out a couple of years after Version 7. It’s included in most recent UNIX versions.

The C shell gets its name from the resemblance of its commands to statements in the C Programming Language, which makes the shell easier for programmers on UNIX systems to learn. It supports a number of operating system features (e.g., job control; see Chapter 8) that were unique to BSD UNIX but by now have migrated to most other modern versions. It also has a few important features (e.g., aliases; see Chapter 3) that make it easier to use in general.

The Korn Shell

The Korn shell, or ksh, was invented by David Korn of AT&T Bell Laboratories in the mid-1980s. It is almost entirely upwardly compatible with the Bourne shell, [1] which means that Bourne shell users can use it right away, and all system utilities that use the Bourne shell can use the Korn shell instead. In fact, some systems have the Korn shell installed as if it were the Bourne shell.

The Korn shell began its public life in 1986 as part of AT&T’s “Experimental Toolchest,” meaning that its source code was available at very low cost to anyone who was willing to use it without technical support and with the knowledge that it might still have a few bugs. Eventually, AT&T’s UNIX System Laboratories (USL) decided to give it full support as a UNIX utility. As of USL’s version of UNIX called System V Release 4 (SVR4 for short, 1989), it was distributed with all USL UNIX systems, all third-party versions of UNIX derived from SVR4, and many other versions.

USL’s distributed version of the Korn shell, dated November 16, 1988, is what this book describes. Other versions are summarized briefly in Appendix A.

Features of the Korn Shell

Although the Bourne shell is still known as the “standard” shell, the Korn shell is becoming increasingly popular and is destined to replace it. In addition to its Bourne shell compatibility, it includes the best features of the C shell as well as several advantages of its own. It also runs more efficiently than any previous shell.

The Korn shell’s command-line editing modes are the features that tend to attract people to it at first. With command-line editing, it’s much easier to go back and fix mistakes than it is with the C shell’s history mechanism-and the Bourne shell doesn’t let you do this at all.

The other major Korn shell feature that is intended mostly for interactive users is job control. As Chapter 8 explains, job control gives you the ability to stop, start, and pause any number of commands at the same time. This feature was borrowed almost verbatim from the C shell.

The rest of the Korn shell’s important advantages are mainly meant for shell customizers and programmers. It has many new options and variables for customization, and its programming features have been significantly expanded to include function definition, more control structures, built-in regular expressions and integer arithmetic, advanced I/O control, and more.

Getting the Korn Shell

You may or may not be using the Korn shell right now. Your system administrator probably set your account up with whatever shell he or she uses as the “standard” on the system. You may not even have been aware that there is more than one shell available.

Yet it’s easy for you to determine which shell you are using. Log in to your system and type echo $SHELL at the prompt. You will see a response containing sh, csh, or ksh; these denote the Bourne, C, and Korn shells respectively. (There’s also a remote chance that you’re using a third-party shell such as bash or tcsh.)

If you aren’t using the Korn shell and you want to, then first you need to find out if it exists on your system. Most major UNIX versions released since roughly 1989 come with it-especially those derived from AT&T’s System V Release 4. Just type ksh. If you get a new dollar-sign prompt ($), then all is well; type exit or press CTRL-D to go back to your normal shell.

But if you get a “not found” message, your system may not have it. Ask your system administrator or another knowledgeable user; there’s a chance that you might have some version of the Korn shell installed on the system in a place (directory) that is not normally accessible to you. But if not, read Appendix A to find out how you can obtain a version of the Korn shell.

Once you know you have the Korn shell on your system, you can invoke it from whatever other shell you use by typing ksh as above. However, it’s much better to install it as your login shell, i.e., the shell that you get automatically whenever you log in. You may be able to do the installation by yourself. Here are instructions that are designed to work on the widest variety of UNIX systems. If something doesn’t work (e.g., you type in a command and get a “not found” error message or a blank line as the response), you’ll have to abort the process and see your system administrator.

You need to find out where the Korn shell is on your system, i.e., in which directory it’s installed. You might be able to find the location by typing whereis ksh (especially if you are using the C shell); if that doesn’t work, try whence ksh, which ksh, or this complex command:

grep ksh /etc/passwd | awk -F: '{print $7}' | sort -u

You should see a response that looks like /bin/ksh or /usr/local/bin/ksh.

To install as your login shell, type chsh ksh-name, where ksh-name is the response you got to your whereis command (or whatever worked). You’ll either get an error message saying that the shell is invalid, or you’ll be prompted for your password. Type in your password, then log out and log back in again to start using the Korn shell. If you got an error message, you’ll just have to see your system administrator. (For system security reasons, only certain shells are allowed to be installed as login shells.)

Interactive Shell Use

When you use the shell interactively, you engage in a login session that begins when you log in and ends when you exit or press CTRL-D. [2] During a login session, you type command lines in to the shell; these are lines of text ending in RETURN that you type in to your terminal or workstation. By default, the shell prompts you for each command with a dollar sign, though as you will see in Chapter 3 the prompt can be changed.

Commands, Arguments, and Options

Shell command lines consist of one or more words, which are separated on a command line by blanks or TABs. The first word on the line is the command. The rest (if any) are arguments (also called parameters) to the command, which are names of things on which the command will act.

For example, the command line lp myfile consists of the command lp (print a file) and the single argument myfile. lp treats myfile as the name of a file to print. Arguments are often names of files, but not necessarily: in the command line mail billr, the mail program treats billr as the name of the user to which a message will be sent.

An option is a special type of argument that gives the command specific information on what it is supposed to do. Options usually consist of a dash followed by a letter; we say “usually” because this is a convention rather than a hard-and-fast rule. The command lp -h myfile contains the option -h, which tells lp not to print the “banner page” before it prints the file.

Sometimes options take their own arguments. For example, lp -d hp3si -h myfile has two options and one argument. The first option is -d hp3si, which means “Send the output to the printer (destination) called hp3si“. The second option and argument are as above.


Although arguments to commands aren’t always files, files are the most important types of “things” on any UNIX system. A file can contain any kind of information, and indeed there are different types of files. Three types are by far the most important:

Regular files

Also called text files; these contain readable characters. For example, this book was created from several regular files that contain the text of the book plus human-readable formatting instructions to the troff word processor.

Executable files

Also called programs; these are invoked as commands. Some can’t be read by humans; others-the shell scripts that we’ll examine in this book-are just special text files. The shell itself is a (non-human-readable) executable file called ksh.


Like folders that contain other files-possibly other directories (called subdirectories).


Let’s review the most important concepts about directories. The fact that directories can contain other directories leads to a hierarchical structure, more popularly known as a tree, for all files on a UNIX system. Figure 1.2 shows part of a typical directory tree; ovals are regular files and rectangles are directories.

A tree of directories and files
Figure 1-2. A tree of directories and files

The top of the tree is a directory called “root” that has no name on the system. [3] All files can be named by expressing their location on the system relative to root; such names are built by listing all of the directory names (in order from root), separated by slashes (/), followed by the file’s name. This way of naming files is called a full (or absolute) pathname.

For example, say there is a file called memo that is in the directory fred, which is in the directory users, which is in the root directory. This file’s full pathname is /users/fred/memo.

The working directory

Of course, it’s annoying to have to use full pathnames whenever you need to specify a file. So there is also the concept of the working directory (sometimes called the current directory), which is the directory you are “in” at any given time. If you give a pathname with no leading slash, then the location of the file is worked out relative to the working directory. Such pathnames are called relative pathnames; you’ll use them much more often than full pathnames.

When you log in to the system, your working directory is initially set to a special directory called your home (or login) directory. System administrators often set up the system so that everyone’s home directory name is the same as their login name, and all home directories are contained in a common directory under root.

For example, /users/billr is a typical home directory. If this is your working directory and you give the command lp memo, then the system looks for the file memo in /users/billr. If you have a directory called bob in your home directory, and it contains the file statreport, then you can print it with the command lp bob/statreport.

Tilde notation

As you can well imagine, home directories occur often in pathnames. Although many systems are organized so that all home directories have a common parent (such as /home or /users), you should not have to rely on that being the case, nor should you even have to know what the absolute pathname of someone’s home directory is.

Therefore, the Korn shell has a way of abbreviating home directories: just precede the name of the user with a tilde (~). For example, you could refer to the file memo in user fred’s home directory as ~fred/memo. This is an absolute pathname, so it doesn’t matter what your working directory is when you use it. If fred’s home directory has a subdirectory called bob and the file is in there instead, you can use ~fred/bob/memo as its name.

Even more convenient, a tilde by itself refers to your own home directory. You can refer to a file called notes in your home directory as ~/notes (note the difference between that and ~notes, which the shell would try to interpret as user notes home directory). If notes is in your bob subdirectory, then you can call it ~/bob/notes. This notation is handiest when your working directory is not in your home directory tree, e.g., when it’s some “system” directory like /tmp.

Changing working directories

If you want to change your working directory, use the command cd. If you don’t remember your working directory, the command pwd tells the shell to print it.

cd takes as argument the name of the directory you want to become your working directory. It can be relative to your current directory, it can contain a tilde, or it can be absolute (starting with a slash). If you omit the argument, cd changes to your home directory (i.e., it’s the same as cd ~ ).

Table 1.1 gives some sample cd commands. Each command assumes that your working directory is /users/billr just before the command is executed, and that your directory structure looks like Figure 1.2.

Table 1-1. Sample cd Commands
CommandNew Working Directory
cd bob /users/billr/bob
cd bob/dave /users/billr/bob/dave
cd ~/bob/dave /users/billr/bob/dave
cd /usr/lib /usr/lib
cd .. /users
cd ../pete /users/pete
cd ~pete /users/pete
cd billr pete /users/pete
cd illr arry /users/barry

The first four are straightforward. The next two use a special directory called .. (two dots), which means “parent of this directory.” Every directory has one of these; it’s a universal way to get to the directory above the current one in the hierarchy-which is called the parent directory. [4]

The last two examples in the table use a new form of the cd command, which is not included in most Bourne shells. The form is cd old new. It takes the full pathname of the current working directory and tries to find the string old in it. If it finds the string, it substitutes new and changes to the resulting directory.

In the first of the two examples, the shell substitutes pete for billr in the current directory name and makes the result the new current directory. The last example shows that the substitution need not be a complete filename: substituting arry for illr in /users/billr yields /users/barry. (If the old string can’t be found in the current directory name, the shell prints an error message.)

Another new feature of the Korn shell’s cd command is the form cd -, which changes to whatever directory you were in before the current one. For example, if you start out in /usr/lib, type cd without an argument to go to your home directory, and then type cd -, you will be back in /usr/lib.

Filenames and Wildcards

Sometimes you need to run a command on more than one file at a time. The most common example of such a command is ls, which lists information about files. In its simplest form, without options or arguments, it lists the names of all files in the working directory except special hidden files, whose names begin with a dot (.).

If you give ls filename arguments, it will list those files-which is sort of silly: if your current directory has the files bob and fred in it and you type ls bob fred, the system will simply parrot the filenames back at you.

Actually, ls is more often used with options that tell it to list information about the files, like the -l (long) option, which tells ls to list the file’s owner, size, time of last modification, and other information, or -a (all), which also lists the hidden files described above. But sometimes you want to verify the existence of a certain group of files without having to know all of their names; for example, if you use the WordPerfect word processor, you might want to see which files in your current directory have names that end in .wp.

Filenames are so important in UNIX that the shell provides a built-in way to specify the pattern of a set of filenames without having to know all of the names themselves. You can use special characters, called wildcards, in filenames to turn them into patterns. We’ll show the three basic types of wildcards that all major UNIX shells support, and we’ll save the Korn shell’s set of advanced wildcard operators for Chapter 4. Table 1.2 lists the basic wildcards.

Table 1-2. Basic Wildcards
? Any single character
* Any string of characters
[set]Any character in set
[!set]Any character not in set

The ? wildcard matches any single character, so that if your directory contains the files program.c, program.log, and program.o, then the expression program . ? matches program.c and program.o but not program.log.

The asterisk (*) is more powerful and far more widely-used; it matches any string of characters. The expression program. * will match all three files in the previous paragraph; WordPerfect users can use the expression *.wp to match their input files. [5]

Table 1.3 should give you a better idea of how the asterisk works. Assume that you have the files bob, darlene, dave, ed, frank, and fred in your working directory.

Notice that * can stand for nothing: both * ed and * e * match ed. Also notice that the last example shows what the shell does if it can’t match anything: it just leaves the string with the wildcard untouched.

Table 1-3. Using the * Wildcard
fr* frank fred
*ed ed fred
b* bob
*e* darlene dave ed fred
*r* darlene frank fred
* bob darlene dave ed frank fred
d*e darlene dave
g* g*

The remaining wildcard is the set construct. A set is a list of characters (e.g., abc), an inclusive range (e.g., a-z), or some combination of the two. If you want the dash character to be part of a list, just list it first or last. Table 1.4 should explain things more clearly.

Table 1-4. Using the Set Construct Wildcards
[abc] a, b, or c
[.,;] Period, comma, or semicolon
[-_] Dash and underscore
[a-c] a, b, or c
[a-z] All lowercase letters
[!0-9] All non-digits
[0-9!] All digits and exclamation point
[a-zA-Z] All lower- and uppercase letters
[a-zA-Z0-9_-] All letters, all digits, underscore, and dash

In the original wildcard example, program.[co] and program.[a-z] both match program.c and program.o, but not program.log.

An exclamation point after the left bracket lets you “negate” a set. For example, [! .; ] matches any character except period and semicolon; [!a-zA-Z] matches any character that isn’t a letter.

The range notation is handy, but you shouldn’t make too many assumptions about what characters are included in a range. It’s safe to use a range for uppercase letters, lowercase letters, digits, or any subranges thereof (e.g., [f-q], [2-6]). Don’t use ranges on punctuation characters or mixed-case letters: e.g., [a-Z] and [A-z] should not be trusted to include all of the letters and nothing more. The problem is that such ranges are not entirely portable between different types of computers. [6]

The process of matching expressions containing wildcards to filenames is called wildcard expansion. This is just one of several steps the shell takes when reading and processing a command line; another that we have already seen is tilde expansion, where tildes are replaced with home directories where applicable. We’ll see others in later chapters, and the full details of the process are enumerated in Chapter 7.

However, it’s important to be aware that the commands that you run only see the results of wildcard expansion. That is, they just see a list of arguments, and they have no knowledge of how those arguments came into being. For example, if you type ls fr* and your files are as on the previous page, then the shell expands the command line to ls fred frank and invokes the command ls with arguments fred and frank. If you type ls g*, then (because there is no match) ls will be given the literal string g* and will complain with the error message, g* not found. [7]

Here is another example that should help you understand why this is important. Suppose you are a C programmer. This just means that you deal with files whose names end in .c (programs, a.k.a. source files), .h (header files for programs), and .o (object code files that aren’t human-readable) as well as other files.

Let’s say you want to list all source, object, and header files in your working directory. The command ls * .[cho] does the trick. The shell expands * .[cho] to all files whose names end in a period followed by a c, h, or o and passes the resulting list to ls as arguments.

In other words, ls will see the filenames just as if they were all typed in individually-but notice that we assumed no knowledge of the actual filenames whatsoever! We let the wildcards do the work.

As you gain experience with the shell, reflect on what life would be like without wildcards. Pretty miserable, we would say.

Input and Output

The software field-really, any scientific field-tends to advance most quickly and impressively on those few occasions when someone (i.e., not a committee) comes up with an idea that is small in concept yet enormous in its implications. The standard input and output scheme of UNIX has to be on the short list of such ideas, along with such classic innovations as the LISP language, the relational data model, and object-oriented programming.

The UNIX I/O scheme is based on two dazzlingly simple ideas. First, UNIX file I/O takes the form of arbitrarily long sequences of characters (bytes). In contrast, file systems of older vintage have more complicated I/O schemes (e.g., “block,” “record,” “card image,” etc.). Second, everything on the system that produces or accepts data is treated as a file; this includes hardware devices like disk drives and terminals. Older systems treated every device differently. Both of these ideas have made systems programmers’ lives much more pleasant.

Standard I/O

By convention, each UNIX program has a single way of accepting input called standard input, a single way of producing output called standard output, and a single way of producing error messages called standard error output, usually shortened to standard error. Of course, a program can have other input and output sources as well, as we will see in Chapter 7.

Standard I/O was the first scheme of its kind that was designed specifically for interactive users at terminals, rather than the older batch style of use that usually involves decks of punch-cards. Since the UNIX shell provides the user interface, it should come as no surprise that standard I/O was designed to fit in very neatly with the shell.

All shells handle standard I/O in basically the same way. Each program that you invoke has all three standard I/O channels set to your terminal or workstation, so that standard input is your keyboard, and standard output and error are your screen or window. For example, the mail utility prints messages to you on the standard output, and when you use it to send messages to other users, it accepts your input on the standard input. This means that you view messages on your screen and type new ones in on your keyboard.

When necessary, you can redirect input and output to come from or go to a file instead. If you want to send the contents of a pre-existing file to someone as mail, you redirect mail’s standard input so that it reads from that file instead of your keyboard.

You can also hook up programs into a pipeline, in which the standard output of one program feeds directly into the standard input of another; for example, you could feed mail output directly to the lp program so that messages are printed instead of shown on the screen.

This makes it possible to use UNIX utilities as building blocks for bigger programs. Many UNIX utility programs are meant to be used in this way: they each perform a specific type of filtering operation on input text. Although this isn’t a textbook on UNIX utilities, they are essential to productive shell use. The more popular filtering utilities are listed in Table 1.5.

Table 1-5. Popular UNIX Data Filtering Utilities
cat Copy input to output
grep Search for strings in the input
sort Sort lines in the input
cut Extract columns from input
sed Perform editing operations on input
tr Translate characters in the input to other characters

You may have used some of these before and noticed that they take names of input files as arguments and produce output on standard output. You may not know, however, that all of them (and most other UNIX utilities) accept input from standard input if you omit the argument. [8]

For example, the most basic utility is cat, which simply copies its input to its output. If you type cat with a filename argument, it will print out the contents of that file on your screen. But if you invoke it with no arguments, it will expect standard input and copy it to standard output. Try it: cat will wait for you to type a line of text; when you type RETURN, cat will parrot the text back at you. To stop the process, hit CTRL-D at the beginning of a line (see below for what this character means). You will see ^ D when you type CTRL-D. Here’s what this should look like:

$ cat
                  Here is a line of text.
Here is a line of text.
This is another line of text.
This is another line of text.

I/O Redirection

cat is actually short for “catenate,” i.e., link together. It accepts multiple filename arguments and copies them to the standard output. But let’s pretend, for the moment, that cat and other utilities don’t accept filename arguments and accept only standard input. As we said above, the shell lets you redirect standard input so that it comes from a file. The notation command < filename does this; it sets things up so that command takes standard input from a file instead of from a terminal.

For example, if you have a file called fred that contains some text, then cat < fred will print fred’s contents out onto your terminal. sort < fred will sort the lines in the fred file and print the result on your terminal (remember: we’re pretending that utilities don’t take filename arguments).

Similarly, command > filename causes the command’s standard output to be redirected to the named file. The classic “canonical” example of this is date > now: the date command prints the current date and time on the standard output; the above command saves it in a file called now.

Input and output redirectors can be combined. For example: the cp command is normally used to copy files; if for some reason it didn’t exist or was broken, you could use cat in this way:

$ cat  < 

This would be similar to cp file1 file2.


It is also possible to redirect the output of a command into the standard input of another command instead of a file. The construct that does this is called the pipe, notated as |. A command line that includes two or more commands connected with pipes is called a pipeline.

Pipes are very often used with the more command, which works just like cat except that it prints its output screen by screen, pausing for the user to type SPACE (next screen), RETURN (next line), or other commands. If you’re in a directory with a large number of files and you want to see details about them, ls -l | more will give you a detailed listing a screen at a time.

Pipelines can get very complex (see, for example, the lsd function in Chapter 4 or the pipeline version of the C compiler driver in Chapter 7); they can also be combined with other I/O directors. To see a sorted listing of the file fred a screen at a time, type sort < fred | more. To print it instead of viewing it on your terminal, type sort < fred | lp.

Here’s a more complicated example. The file /etc/passwd stores information about users’ accounts on a UNIX system. Each line in the file contains a user’s login name, user ID number, encrypted password, home directory, login shell, and other info. The first field of each line is the login name; fields are separated by colons (:). A sample line might look like this:

billr:5Ae40BGR/tePk:284:93:Bill Rosenblatt:/home/billr:/bin/ksh

To get a sorted listing of all users on the system, type:

$ cut -d: -f1 < /etc/passwd | sort

(Actually, you can omit the <, since cut accepts input filename arguments.) The cut command extracts the first field (-f1), where fields are separated by colons (-d:), from the input. The entire pipeline will print a list that looks like this:


If you want to send the list directly to the printer (instead of your screen), you can extend the pipeline like this:

$ cut -d: -f1 < /etc/passwd | sort | lp

Now you should see how I/O directors and pipelines support the UNIX building block philosophy. The notation is extremely terse and powerful. Just as important, the pipe concept eliminates the need for messy temporary files to store output of commands before it is fed into other commands.

For example, to do the same sort of thing as the above command line on other operating systems (assuming that equivalent utilities were available...), you would need three commands. On DEC’s VAX/VMS system, they might look like this:

$ cut [etc]passwd /d=":" /f=1 /out=temp1
$ sort temp1 /out=temp2
$ print temp2

After sufficient practice, you will find yourself routinely typing in powerful command pipelines that do in one line what it would take several commands (and temporary files) in other operating systems to accomplish.

Background Jobs

Pipes are actually a special case of a more general feature: doing more than one thing at a time. This is a capability that many other commercial operating systems don’t have, because of the rigid limits that they tend to impose upon users. UNIX, on the other hand, was developed in a research lab and meant for internal use, so it does relatively little to impose limits on the resources available to users on a computer-as usual, leaning towards uncluttered simplicity rather than overcomplexity.

“Doing more than one thing at a time” means running more than one program at the same time. You do this when you invoke a pipeline; you can also do it by logging on to a UNIX system as many times simultaneously as you wish. (If you try that on an IBM VM/CMS system, for example, you will get an obnoxious “already logged in” message.)

The shell also lets you run more than one command at a time during a single login session. Normally, when you type a command and hit RETURN, the shell will let the command have control of your terminal until it is done; you can’t type in further commands until the first one is done. But if you want to run a command that does not require user input and you want to do other things while the command is running, put an ampersand (&) after the command.

This is called running the command in the background, and a command that runs in this way is called a background job; for contrast, a job run the normal way is called a foreground job. When you start a background job, you get your shell prompt back immediately, enabling you to enter other commands.

The most obvious use for background jobs is programs that take a long time to run, such as sort or uncompress on large files. For example, assume you just got an enormous compressed file loaded into your directory from magnetic tape. Compressed files are created by the compress utility, which packs files into smaller amounts of space; they have names of the form filename.Z, where filename is the name of the original uncompressed file. Let’s say the file is gcc.tar.Z, which is a compressed archive file that contains well over 10 MB of source code files.

Type uncompress gcc.tar & (you can omit the .Z), and the system will start a job in the background that uncompresses the data “in place” and ends up with the file gcc.tar. Right after you type the command, you will see a line like this:

[1]     4692

followed by your shell prompt, meaning that you can enter other commands. Those numbers give you ways of referring to your background job; Chapter 8 explains them in detail.

You can check on background jobs with the command jobs. For each background job, jobs prints a line similar to the above but with an indication of the job’s status:

[1]  +  Running                 uncompress gcc.tar

When the job finishes, you will see a message like this right before your shell prompt:

[1] +  Done                     uncompress gcc.tar

The message changes if your background job terminated with an error; again, see Chapter 8 for details.

Background I/O

Jobs you put in the background should not do I/O to your terminal. Just think about it for a moment and you’ll understand why.

By definition, a background job doesn’t have control over your terminal. Among other things, this means that only the foreground process (or, if none, the shell itself) is “listening” for input from your keyboard. If a background job needs keyboard input, it will often just sit there doing nothing until you do something about it (as described in Chapter 8).

If a background job produces screen output, the output will just appear on your screen. If you are running a job in the foreground that produces output too, then the output from the two jobs will be randomly (and often annoyingly) interspersed.

If you want to run a job in the background that expects standard input or produces standard output, the obvious solution is to redirect it so that it comes from or goes to a file. The only exception is that some programs produce small, one-line messages (warnings, “done” messages, etc.); you may not mind if these are interspersed with whatever other output you are seeing at a given time.

For example, the diff utility examines two files, whose names are given as arguments, and prints a summary of their differences on the standard output. If the files are exactly the same, diff is silent. Usually, you invoke diff expecting to see a few lines that are different.

diff, like sort and compress, can take a long time to run if the input files are very large. Suppose you have two large files that are called warandpeace.wp and warandpeace.wp.old. The command diff warandpeace.wp warandpeace.wp.old [9] reveals that the author decided to change the name “Ivan” to “Aleksandr” throughout the entire file-i.e., hundreds of differences, resulting in large amounts of output.

If you type diff warandpeace.wp warandpeace.wp.old &, then the system will spew lots and lots of output at you, which it will be very difficult to stop-even with the techniques explained in Chapter 7. However, if you type:

$ diff warandpeace.wp warandpeace.wp.old > wpdiff &

then the differences will be saved in the file wpdiff for you to examine later.

Background Jobs and Priorities

Background jobs can save you a lot of thumb-twiddling time (or can help you diet by eliminating excuses to run to the candy machine). Just remember that such jobs eat up lots of system resources like memory and the processor (CPU). Just because you’re running several jobs at once doesn’t mean that they will run faster than they would if run sequentially-in fact, it’s usually worse.

Every job on the system is assigned a priority, a number that tells the operating system how much priority to give the job when it doles out resources (the higher the number, the lower the priority). Foreground commands that you enter from the shell usually have the same, standard priority. But background jobs, by default, have lower priority. [10]

You’ll find out in Chapter 3 how you can override this priority assignment so that background jobs run at the same priority as foreground jobs. However, if you’re on a multiuser system, then running lots of background jobs may eat up more than your fair share of resources, and you should consider whether having your job run as fast as possible is really more important than being a good citizen.


Speaking of good citizenship, there is also a shell command that lets you lower the priority of any job: the aptly-named nice. If you type nice command, where command can be a complex shell command line with pipes, redirectors, etc., then the command will run at a lower priority. You can control just how much lower by giving nice a numerical argument; consult the man page for details. [11]

Special Characters and Quoting

The characters <, >, |, and & are four examples of special characters that have particular meanings to the shell. The wildcards we saw earlier in this chapter (*, ?, and [...]) are also special characters.

Table 1.6 gives indications of the meanings of all special characters within shell command lines only. Other characters have special meanings in specific situations, such as the regular expressions and string-handling operators we’ll see in Chapter 3 and Chapter 4.

Table 1-6. Special Characters
CharacterMeaningSee Chapter
~ Home directory1
lsquo; Command substitution (archaic)4
# Comment4
$ Variable expression3
& Background job1
* String wildcard1
( Start subshell8
) End subshell8
\ Quote next character1
| Pipe1
[ Start character-set wildcard1
] End character-set wildcard1
{ Start code block7
} End code block7
; Shell command separator3
' Strong quote1
" Weak quote1
< Input redirect1
> Output redirect1
/ Pathname directory separator1
? Single-character wildcard1


Sometimes you will want to use special characters literally, i.e., without their special meanings. This is called quoting. If you surround a string of characters with single quotes, you strip all characters within the quotes of any special meaning they might have.

The most obvious situation where you might need to quote a string is with the print command, which just takes its arguments and prints them to the standard output. What is the point of this? As you will see in later chapters, the shell does quite a bit of processing on command lines—most of which involves some of the special characters listed in Table 1.6. print is a way of making the result of that processing available on the standard output.

But what if we wanted to print the string, 2 * 3 > 5 is a valid inequality? Suppose you typed this:

$ print 2 * 3 > 5 is a valid inequality.

You would get your shell prompt back, as if nothing happened! But then there would be a new file, with the name 5, containing “2”, the names of all files in your current directory, and then the string 3 is a valid inequality. Make sure you understand why. [12]

However, if you type:

$ print '2 * 3 > 5 is a 
valid inequality.'

the result is the string, taken literally. You needn’t quote the entire line, just the portion containing special characters (or characters you think might be special, if you just want to be sure):

$ print '2 * 3 > 5' is a valid inequality.

This has exactly the same result.

Notice that Table 1.6 lists double quotes (") as weak quotes. A string in double quotes is subjected to some of the steps the shell takes to process command lines, but not all. (In other words, it treats only some special characters as special.) You’ll see in later chapters why double quotes are sometimes preferable; Chapter 7 contains the most comprehensive explanation of the shell’s rules for quoting and other aspects of command-line processing. For now, though, you should stick to single quotes.


Another way to change the meaning of a character is to precede it with a backslash (\). This is called backslash-escaping the character. In most cases, when you backslash-escape a character, you quote it. For example:

$ print 2 \* 3 \> 5 is a valid inequality.

will produce the same results as if you surrounded the string with single quotes. To use a literal backslash, just surround it with quotes ('\') or, even better, backslash-escape it (\\).

Here is a more practical example of quoting special characters. A few UNIX commands take arguments that often include wildcard characters, which need to be escaped so the shell doesn’t process them first. The most common such command is find, which searches for files throughout entire directory trees.

To use find, you supply the root of the tree you want to search and arguments that describe the characteristics of the file(s) you want to find. For example, the command find . -name string searches the directory tree whose root is your current directory for files whose names match the string. (Other arguments allow you to search by the file’s size, owner, permissions, date of last access, etc.)

You can use wildcards in the string, but you must quote them, so that the find command itself can match them against names of files in each directory it searches. The command find . -name '*.c' will match all files whose names end in .c anywhere in your current directory, subdirectories, sub-subdirectories, etc.

Quoting Quotation Marks

You can also use a backslash to include double quotes within a quoted string. For example:

$ print \"2 \* 3 \> 5\" is a valid inequality.

produces the following output:

"2 * 3 > 5" is a valid inequality.

However, this won’t work with single quotes inside quoted expressions. For example, print ‘Bob\’s hair is brown’ will not give you Bob’s hair is brown. You can get around this limitation in various ways. First, try eliminating the quotes:

$ print Bob\'s hair is brown

If no other characters are special (as is the case here), this works. Otherwise, you can use the following command:

$ print 'Bob'\''s hair is brown'

That is, '\'' (i.e., single quote, backslash, single quote, single quote) acts like a single quote within a quoted string. Why? The first ' in '\'' ends the quoted string we started with ( ' Bob), the \' inserts a literal single quote, and the next ' starts another quoted string that ends with the word “brown”. If you understand, then you will have no trouble resolving the other bewildering issues that arise from the shell’s often cryptic syntax.

Continuing Lines

A related issue is how to continue the text of a command beyond a single line on your terminal or workstation window. The answer is conceptually simple: just quote the RETURN key. After all, RETURN is really just another character.

You can do this in two ways: by ending a line with a backslash, or by not closing a quote mark (i.e., by including RETURN in a quoted string). If you use the backslash, there must be nothing between it and the end of the line-not even spaces or TABs.

Whether you use a backslash or a single quote, you are telling the shell to ignore the special meaning of the RETURN character. After you press RETURN, the shell understands that you haven’t finished your command line (i.e., since you haven’t typed a “real” RETURN), so it responds with a secondary prompt, which is > by default, and waits for you to finish the line. You can continue a line as many times as you wish.

For example, if you want the shell to print the first sentence of Thomas Hardy’s The Return of the Native, you can type this:

$ print A Saturday afternoon in November was approaching the \
> time of twilight, and the vast tract of unenclosed wild known \
> as Egdon Heath embrowned itself moment by moment.

Or you can do it this way:

$ print ' A Saturday afternoon in November was approaching the  
> time of twilight, and the vast tract of unenclosed wild known 
> as Egdon Heath embrowned itself moment by moment.'

Control Keys

Control keys-those that you type by holding down the CONTROL (or CTRL) key and hitting another key-are another type of special character. These normally don’t print anything on your screen, but the operating system interprets a few of them as special commands. You already know one of them: RETURN is actually the same as CTRL-M (try it and see). You have probably also used the BACKSPACE or DEL key to erase typos on your command line.

Actually, many control keys have functions that don’t really concern you-yet you should know about them for future reference and in case you type them by accident.

Perhaps the most difficult thing about control keys is that they can differ from system to system. The usual arrangement is shown in Table 1.7 which lists the control keys that all major modern versions of UNIX support. Note that CTRL-\ and CTRL-| (control-backslash and control-pipe) are the same character notated two different ways; the same is true of DEL and CTRL-?.

You can use the stty command to find out what your settings are and change them if you wish; see Chapter 8 for details. If the version of UNIX on your system is one of those that derive from BSD (such as SunOS and Ultrix), type stty all to see your control-key settings; you will see something like this:

erase  kill   werase rprnt  flush  lnext  susp   intr   quit   stop   eof
^?     ^U     ^W     ^R     ^O     ^V     ^Z/^Y  ^C     ^|     ^S/^Q  ^D

Table 1-7. Control Keys
Control Keystty NameFunction Description
CTRL-C intr

Stop current command

CTRL-D eof

End of input

CTRL-\ or CTRL-| quit

Stop current command, if CTRL-C doesn’t work

CTRL-S stop

Halt output to screen


Restart output to screen

DEL or CTRL-? erase

Erase last character

CTRL-U kill

Erase entire command line

CTRL-Z susp

Suspend current command (see Chapter 8)

The ^X notation stands for CTRL-X. If your UNIX version derives from System III or System V (this includes AIX, HP/UX, SCO, and Xenix), type stty -a; the resulting output will include this information:

intr = ^c; quit = ^|; erase = DEL; kill = ^u; eof = ^d; eol = ^`; swtch = ^`
susp = ^z; dsusp <undef>;

The control key you will probably use most often is CTRL-C, sometimes called the interrupt key. This stops-or tries to stop-the command that is currently running. You will want to use this when you enter a command and find that it’s taking too long, you gave it the wrong arguments by mistake, you change your mind about wanting to run it, or whatever.

Sometimes CTRL-C doesn’t work; in that case, if you really want to stop a job, try CTRL-\. But don’t just type CTRL-\; always try CTRL-C first! Chapter 8 explains why in detail. For now, suffice it to say that CTRL-C gives the running job more of a chance to clean up before exiting, so that files and other resources are not left in funny states.

We’ve already seen an example of CTRL-D. When you are running a command that accepts standard input from your keyboard, CTRL-D tells the process that your input is finished-as if the process were reading a file and it reached the end of the file. mail is a utility in which this happens often. When you are typing in a message, you end by typing CTRL-D. This tells mail that your message is complete and ready to be sent. Most utilities that accept standard input understand CTRL-D as the end-of-input character, though many such programs accept commands like q, quit, exit, etc. The shell itself understands CTRL-D as the end-of-input character: as we saw earlier in this chapter, you can normally end a login session by typing CTRL-D at the shell prompt. You are just telling the shell that its command input is finished.

CTRL-S and CTRL-Q are called flow-control characters. They represent an antiquated way of stopping and restarting the flow of output from one device to another (e.g., from the computer to your terminal) that was useful when the speed of such output was low. They are rather obsolete in these days of high-speed local networks and dialup lines. In fact, under the latter conditions, CTRL-S and CTRL-Q are basically a nuisance. The only thing you really need to know about them is that if your screen output becomes “stuck,” then you may have hit CTRL-S by accident. Type CTRL-Q to restart the output; any keys you may have hit in between will then take effect.

The final group of control characters gives you rudimentary ways to edit your command line. DEL acts as a backspace key (in fact, some systems use the actual BACKSPACE or CTRL-H key as “erase” instead of DEL); CTRL-U erases the entire line and lets you start over. Again, these are outmoded. [13] Instead of using these, go to the next chapter and read about Korn shell’s editing modes, which are among its most exciting features.

[1] With a few extremely minor exceptions. See Appendix A for the only important one.

[2] You can set up your shell so that it doesn’t accept CTRL-D, i.e., it requires you to type exit to end your session. We recommend this, because CTRL-D is too easy to type by accident; see the section on options in Chapter 3.

[3] Most introductory UNIX tutorials say that root has the name /. We stand by this alternative explanation because it is more logically consistent.

[4] Each directory also has the special directory . (single dot), which just means “this directory.” Thus, cd . effectively does nothing. Both . and .. are actually special hidden files in each directory that point to the directory itself and to its parent directory, respectively. Root is its own parent.

[5] MS-DOS and VAX/VMS users should note that there is nothing special about the dot (.) in UNIX filenames (aside from the leading dot, which “hides” the file); it’s just another character. For example, ls * lists all files in the current directory; you don’t need *.* as you do on other systems.

[6] Specifically, ranges depend on the character encoding scheme your computer uses. The vast majority use ASCII, but IBM mainframes use EBCDIC.

[7] This is different from the C shell’s wildcard mechanism, which prints an error message and doesn’t execute the command at all.

[8] If a particular UNIX utility doesn’t accept standard input when you leave out the filename argument, try using - as the argument.

[9] You could use diff warandpeace * as a shorthand to save typing-as long as there are no other files with names of that form. Remember that diff doesn’t see the arguments until after the shell has expanded the wildcards. Many people overlook this use of wildcards.

[10] This feature was borrowed from the C shell; it is not present in most Bourne shells.

[11] If you are a system administrator logged in as root, then you can also use nice to raise a job’s priority.

[12] This should also teach you something about the flexibility of placing I/O redirectors anywhere on the command line-even in places where they don’t seem to make sense.

[13] Why are so many outmoded control keys still in use? They have nothing to do with the shell per se; instead, they are recognized by the tty driver, an old and hoary part of the operating system’s lower depths that controls input and output to/from your terminal.

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