Bourne Shell Scripting/Files and streams

The Unix world: one file after another
When you think of a computer and everything that goes with it, you usually come up with a mental list of all sorts of different things:


 * The computer itself
 * The monitor
 * The keyboard
 * The mouse
 * Your hard drive with your files and directories on it
 * The network connection leading to the Internet
 * The printer
 * The DVD player
 * et cetera

Here's a surprise for you: Unix doesn't have any of these things. Well, almost. Unix certainly has files. Unix has endless reams of files. And since Unix has files, it also has a concept of "between files" (think of it this way: if your universe consists only of boxes, you automatically know about spaces where there are no boxes as well). But Unix knows nothing else than that. Everything in the whole (Unix) universe is a file.

Everything is a file. Even things that are really weird things to think of as files, are files. Your (data) files are files. Your directories are files. Your hard drive is a file. Your keyboard, monitor and printer are files. Yes, really: your keyboard is a read-only file of infinite size. Your monitor and printer are infinitely sized write-only files. Your network connection is a read/write file.

At this point you're probably asking: Why? Why would the designers of the Unix system have come up with this madness? Why is everything a file? The answer is: because if everything is a file, you can treat everything like a file. Or, put a different way, you can treat everything in the Unix world the same way. And, as we will see shortly, that means you can also combine virtually everything using file operations.

Before we move on, here's an extra level of weirdness for you: everything in Unix is a file. Including the processes that run programs. Effectively this means that running programs are also files. Including the interactive shell session that you've been running to practice scripting in. Yes, really, that text screen with the blinking cursor is also a file. And we can prove it too. You might recall that in the chapter on Running Commands we mentioned you can exit the shell using the Ctrl+d key combination. Because that combination produces the Unix character for... that's right, end-of-file!

Streams: what goes between files
As we mentioned in the previous section, everything in Unix is a file -- except that which sits between files. Between files Unix defines a mechanism that allows data to move, bit by bit, from one file to another: the stream. A stream is literally what it sounds like: a little river of bits pouring from one file into another. Although actually a bridge would probably have been a better name because unlike a stream (which is a constant flow of water) the flow of bits between files need not be constant, or even used at all.

The standard streams
Within the Unix world it is a general convention that each file is connected to at least three streams (that's because that turned out to be the most useful number for those files that are processes, or running programs). There can be more and in fact each file can cause itself to be connected to any number of streams (a program can print and open a network connection, for instance). But there are three basic streams available to all files, even though they may not always be useful or used. These streams are called the "standard" streams:


 * Standard in (stdin):the standard stream for input into a file.
 * Standard out (stdout):the standard stream for output out of a file.
 * Standard error (stderr):the standard stream for error output from a file.

As you can probably tell, these streams are very geared towards those files that are actually processes of the system. In fact many programming languages (like C, C++, Java and Pascal) use exactly these conventions for their standard I/O operations. And since the Unix operating system family includes them in the core of the system definition, these streams are also central to the Bourne Shell.

Getting hold of the standard streams in your scripts
So now we know that there's a general mechanism for basic input and output in Unix; but how do you get hold of these streams in a script? What do you have to do to hook your script up to the standard out, or read from the standard in? Well, the happy answer is: nothing. Your scripts are automatically connected to the standard in, out and error stream of the process that is running them. When you read input, it automatically comes from the standard in. Your output goes straight to the standard out. And program errors go right to the standard error. In fact you've already used these streams: every example so far that has printed anything has done so to the standard output stream of your script.

And what about the shell in interactive mode? Does that use those standard streams as well? Yes, it does. In interactive mode, the standard in stream is connected to the keyboard file. And the standard output and standard error are connected to the monitor file.

Okay... But what good is it?
This discussion on files and streams has been very interesting so far and a nice insight into the depths of Unix. But what good does it do you to know all this? Ah, glad you asked!

The Bourne Shell has some built-in features that allow you to do neat tricks involving files and their streams. You see, files don't just have streams -- you can also cross-connect the streams of two files. At the end of the previous section we said that the standard input of the interactive session is connected to the keyboard file. In fact it is connected to the standard output stream of the keyboard file. And the standard output and error of the interactive session are connected to the standard input of the monitor file. So you can connect the streams of the interactive session to the streams of devices.

But wait. Do you remember the remark above that the point of Unix considering everything to be a file was that everything gets treated like a file? This is why that was important: you can connect a stream from any file to a stream of any other file. You can connect your interactive shell session to the printer or the network rather than to the monitor (or in addition to the monitor) using streams. You can run a program and have its output go directly to the printer by reconnecting the standard output stream of the program. You can connect the standard output stream of one program directly to the standard input stream of another program and make chains of programs. And the Bourne Shell makes it really simple to do all that.

Do you suddenly feel like you've stuck your fingers in the electrical socket? That's the feeling of the raw power of the shell flowing through your body....

Redirecting: using streams in the shell
As explained in the previous section, the shell process is connected by standard streams to (by default) the keyboard and the monitor. But very often you will want to change this linking. Connecting a file to a stream is a very common operation, so would expect it to be called something like "connecting" or "linking". But since the Bourne Shell has default connections and everything you do is always a change in the default connections, connecting a file to a (different) stream using the shell is actually called redirecting.

There are several operators built in to the Bourne Shell that relate to redirecting. The most basic and general one is the pipe operator, which we will examine in some detail further on. The others are related to redirecting to file.

Redirecting to file
As we explained (or rather: hinted at) in the previous section, one of the enormously powerful features of the Bourne Shell on top of a Unix operating system is the ability to chain programs together. Execute a program, have it produce output, then automatically send that output to another program as input. The possible combinations are endless, as is the power of what you can achieve.

One of the most common places where you might want to send a program's output is to a file in the file system. And this time by file we mean a regular, classic data file and not a Unix "everything is a file including your hardware" file. In order to achieve this you can imagine that we can use the chaining mechanism described above: let a program generate output through the standard output stream, then connect that stream (i.e. redirect the output) to the standard input stream of a program that creates a data file in the file system. And this would absolutely work. However, redirecting to a data file is such a common operation that you don't need a separate end-of-chain program for it. Redirecting to file is built straight into the Bourne Shell, through the following operators:


 * process > data file:redirect the output of process to the data file; create the file if necessary, overwrite its existing contents otherwise.
 * process >> data file:redirect the output of process to the data file; create the file if necessary, append to its existing contents otherwise.
 * process < data file:read the contents of the data file and redirect that contents to process as input.

Redirecting output
Let's take a closer look at these operators through some examples. Take the simple Bourne shell script below called 'hello.sh':

A simple shell script that generates some output

This code may be run in any of the ways described in the chapter Running Commands. When we run the script, it simply outputs the string "Hello" to the screen and then returns us to our prompt. But let's say we want to redirect the output to a file instead. We can use the redirect operators to do that easily:

Redirecting the output to a data file

This time, we don't see the string 'Hello' on the screen. Where's it gone? Well, exactly where we wanted it to: into the (new) data file called 'myfile.txt'. Let's examine this file using the 'cat' command:

Examining the results of redirecting some output

Let's run the program again, this time using the '>>' operator instead, and then examine 'myfile.txt' again using the 'cat' command:

Redirecting using the append redirect

You can see that 'myfile.txt' now consists of two lines &mdash; the output has been added to the end of the file (or concatenated); this is due to the use of the '>>' operator. If we run the script again, this time with the single greater-than operator, we get:

Redirecting using the overwrite redirect

Just one 'Hello' again, because the '>' will always overwrite the contents of an existing file if there is one.

Redirecting input
Okay, so it's clear we can redirect output to a data file. But what about reading from a data file? That's also pretty common. The Bourne Shell helps us here as well: the entire process of reading a file and pumping its data into a stream is captured by the '<' operator.

By default 'stdin' is fed from your keyboard; run the 'cat' command without any arguments and it will just sit there, waiting for you to type something: cat ???

In fact 'cat' will sit there all day until you type a 'Ctrl+D' (the 'End of File Character' or 'EOF' for short). To redirect our standard input from somewhere else use the '<' (less-than operator):

Redirecting into the standard input

So 'cat' will now read from the text file 'myfile.txt'; the 'EOF' character is also generated at the end of file, so 'cat' will exit as before.

Note that we previously used 'cat' in this format:

Which is functionally identical to

However, these are two fundamentally different mechanisms: one uses an argument to the command, the other is more general and redirects 'stdin' – which is what we're concerned with here. It's more convenient to use 'cat' with a filename as argument, which is why the inventors of 'cat' put this in. However, not all programs and scripts are going to take arguments so this is just an easy example.

Combining file redirects
It's possible to redirect 'stdin' and 'stdout' in one line:

Redirecting input to and output from cat at the same time

The command above will copy the contents of 'myfile.txt' to 'mynewfile.txt' (and will overwrite any previous contents of 'mynewfile.txt'). Once again this is just a convenient example as we normally would have achieved this effect using 'cp myfile.txt mynewfile.txt'.

Redirecting standard error (and other streams)
So far we have looked at redirecting the "normal" standard streams associated with files, i.e. the files that you use if everything goes correctly and as planned. But what about that other stream? The one meant for errors? How do we go about redirecting that? For example, if we wanted to redirect error data into a log file.

As an example, consider the ls command. If you run the command 'ls myfile.txt', it simply lists the filename 'myfile.txt' – if that file exists. If the file 'myfile.txt' does NOT exist, 'ls' will return an error to the 'stderr' stream, which by default in Bourne Shell is also connected to your monitor.

So, lets run 'ls' a couple of times, first on a file which does exist and then on one that doesn't:

Listing an existing file

and then:

Listing a non-existent file

And again, this time with 'stdout' redirected only:

Trying to redirect...

We still see the error message; 'logfile.txt' will be created but will be empty. This is because we have now redirected the stdout stream, while the error message was written to the error stream. So how do we tell the shell that we want to redirect the error stream?

In order to understand the answer, we have to cover a little more theory about Unix files and streams. You see, deep down the reason that we can redirect stdin and stdout with simple operators is that redirecting those streams is so common that the shell lets us use a shorthand notation for those streams. But actually, to be completely correct, we should have told the shell in every case which stream we wanted to redirect. In general you see, the shell cannot know: there could be tons of streams connected to any file. And in order to distinguish one from the other each stream connected to a file has a number associated with it: by convention 0 is the standard in, 1 is the standard out, 2 is standard error and any other streams have numbers counting on from there. To redirect any particular stream you prepend the redirect operator with the stream number (called the file descriptor. So to redirect the error message in our example, we prepend the redirect operator with a 2, for the stderr stream:

Redirecting the stderr stream

No output to the screen, but if we examine 'logfile.txt':

Checking the logfile

As we mentioned before, the operator without a number is a shorthand notation. In other words, this:

is actually short for

We can also redirect both 'stdout' and 'stderr' independently like this:

'stdio.txt' will be blank, 'logfile.txt' will contain the error as before.

If we want to redirect stdout and stderr to the same file, we can use the file descriptor as well:

Here '2>&1' means something like 'redirect stderr to the same file stdout has been redirected to'. Be careful with the ordering! If you do it this way:

you will redirect stderr to the file that stdout points to, then send stdout somewhere else &mdash; and both streams will end up being redirected to different locations.

Special files
We said earlier that the redirect operators discussed so far all redirect to data files. While this is technically true, Unix magic still means that there's more to it than just that. You see, the Unix file system tends to contain a number of special files called "devices", by convention collected in the /dev directory. These device files include the files that represent your hard drive, DVD player, USB stick and so on. They also include some special files, like /dev/null (also known as the bit bucket; anything you write to this file is discarded). You can redirect to device files as well as to regular data files. Be careful here; you really don't want to redirect raw text data to the boot sector of your hard drive (and you can!). But if you know what you're doing, you can use the device files by redirecting to them (this is how DVDs are burned in Linux, for instance). As an example of how you might actually use a device file, in the 'Solaris' flavour of Unix the loudspeaker and its microphone can be accessed by the file '/dev/audio'. So:

Will play a sound, whereas:

Will record a sound.(you will need to CTRL-C this to finish...)

This is fun:

Now wave the microphone around whilst shouting - Jimi Hendrix style feedback. Great stuff. You will probably need to be logged in as 'root' to try this by the way.

Some redirect warnings
The astute reader will have noticed one or two things in the discussion above. First of all, a file can have more than just the standard streams associated with it. Is it legal to redirect those? Is it even possible? The answer is, technically, yes. You can redirect stream 4 or 5 of a file (if they exist). Don't try it though. If there's more than a few streams in any direction, you won't know which stream you're redirecting. Plus, if a program needs more than the standard streams it's a good bet that program also needs its extra streams going to a specific location.

Second, you might have noticed that file descriptor 0 is, by convention, the standard input stream. Does that mean you can redirect a program's standard input away from the program? Could you do the following?

The answer is, yes you can. And yes, things will break if you do.

Pipes, Tees and Named Pipes
So, after all this talk about redirecting to file, we finally get to it: general redirecting by cross-connecting streams. The most general form of redirecting and the most powerful one to boot. It's called a pipe and is performed using the pipe operator '|'. Pipes allow you to join two processes together through a "pipeline", which directly connects the stdout of one file to the stdin of another.

As an example let's consider the 'grep' command which returns a matching string, given a keyword and some text to search. And let's also use the ps command, which lists running processes on the machine. If you give the command

it will generally list pagefuls of running processes on your machine, which you would have to sift through manually to find what you want. Let's say you are looking for a process which you know contains the word 'oracle'; use the output of 'ps' to pipe into grep, which will only return the matching lines:

Now you will only get back the lines you need. What happens if there's still loads of these ? No problem, pipe the output to the command 'more' (or 'pg'), which will pause your screen if it fills up:

What about if you want to kill all those processes? You need the 'kill' program, plus the process number for each process (the second column returned by the ps command). Easy:

In this command, 'ps' lists the processes and 'grep' narrows the results down to oracle. The 'awk' tool pulls out the second column of each line. And 'xargs' feeds each line, one at a time, to 'kill' as a command line argument.

Pipes can be used to link as many programs as you wish within reasonable limits (and we don't know what these limits are!)

Don't forget you can still use the redirectors in combination:

There is another useful mechanism that can be used with pipes: the 'tee'. To understand tee, imagine a pipe shaped like a 'T' - one input, two outputs:

The 'tee' will copy whatever is given to its stdin and redirect this to the argument given (a file); it will also then send a further copy to its stdout - which means you can effectively intercept the pipe, take a copy at this stage, and carry on piping up other commands; useful maybe for outputting to a logfile, and copying to the screen.

A note on piped commands: piped processes run in parallel on the Unix environment. Sometimes one process will be blocked, waiting for input from another process. But each process in a pipeline is, in principle, running simultaneously with all the others.

Named pipes
There is a variation on the in-line pipe which we have been discussing called the 'named pipe'. A named pipe is actually a file with its own 'stdin' and 'stdout' - which you attach processes to. This is useful for allowing programs to talk to each other, especially when you don't know exactly when one program will try and talk to the other (waiting for a backup to finish etc.) and when you don't want to write a complicated network-based listener or do a clumsy polling loop.

To create a 'named pipe', you use the 'mkfifo' command (fifo=first in, first out; so data is read out in the same order as it is written into).

This creates a named pipe called 'mypipe'; next we can start using it.

This test is best run with two terminals logged in:

1. From 'terminal a'

The 'cat' will sit there waiting for an input.

2. From 'terminal b'

This should finish immediately. Flick back to 'terminal a'; this will now have read from the pipe and received an 'EOF', and you will see the data on the screen; the command will have finished, and you are back at the command prompt.

Now try the other way round:

1. From terminal 'b'

This will now sit there, as there isn't another process on the other end to 'drain' the pipe - it's blocked.

2. From terminal 'a'

As before, both processes will now finish, the output showing on terminal 'a'.

Here documents
So far we have looked at redirecting from and to data files and cross-connecting data streams. All of these shell mechanisms are based on having a "physical" source for data &mdash; a process or a data file. Sometimes though, you want to feed some data into a target without having a source for it. In these cases you can use an "on the fly" document called a here document. A here document means that you open a virtual text document (in memory), type into it as usual, close it and then treat it like any normal file.

Creating a here document is done using a variation on the input redirect operator: the '<<' operator. Like the input redirect operator, the here document operator takes an argument. For the input redirect operator this operand is the name of the file to be streamed in. For the here document operator it is the string that will terminate the here document. So using the here document operator looks like this:

For example:

Using a here document

When using here documents in combination with variable or command substitution, it is important to realize that substitutions are carried out before the here document is passed on. So for example:

Using a here document with substitutions