05 Feb 2012 [ 185 week5 hw ]

All programs have access to 3 standard streams, which when run on the command line behave as follows by default:

You can control the behavior of those streams using redirection, which can change the source of stdin, or the destination of stdout or stderr.

We'll work with examples in Microsoft Windows, but the same ideas and most of the same syntax work in Linux or OSX (usually more consistently and with more powerful features); later in the semester we'll cover the command line interfaces available on those systems (or something similar).

Output redirection

Rather than printing to the console, you can signal that output (stdout) should go to a named file by using the > operator. The following example lists the contents of C:\ into a file:

dir /b C:\ > c-contents.txt

Note that if there is no output, an empty file will be created. In either case, any existing file named c-contents.txt will be destroyed. If you want to add on to the file rather than overwrite it, use the >> operator:

dir /b C:\ >> c-contents.txt

Now the output from that dir command will be appended to the file if it already exists (otherwise it will be created just like the first case).

You can also redirect stderr; for example, the following command lists the contents of C:\XYZ into a file, and saves any error output into another file. There will probably be error output, unless you happen to have a directory named XYZ.

dir /b C:\XYZ > xyz-contents.txt 2> xyz-errors.txt

On earlier versions of Windows, redirection of stderr might not be available.

Input redirection

You can signal to a program that its input should come from a file (rather than the keyboard) by using the < operator.

For an example, let's look at the find command (Windows version). This command searches its input for a target string, and prints lines where it finds the target. For example, find "banana" will print back all the lines that you type that contain "banana" (quotes not necessary). Note that when you run the command, you'll need a way to signal that you're done. On the Windows command line, type Ctrl-Z and then Enter to signal end-of-file.

That's probably not very useful, since when you're typing you already know yourself which lines contain "banana". The command is really made to search through files, and you can send input from a file by redirection:

find "banana" < banana-info.txt

The above command will print all lines in banana-info.txt that contain the "banana".

Pipes

Finally, you might want to get output from one program and send it directly to another; so you combine output and input redirection. This is called piping output, and uses the pipe character (|) as the operator.
Here's an example:

dir /b C:\Windows\System32 | find "reg"

That particular example could be accomplished more simply by using a wildcard (dir /b C:\Windows\System32\*reg*), but in general pipes provide a powerful way to combine programs to gain new functionality.

Homework

  1. List the commands you could use to create a file (using only the command line) with the following contents:

    I created this
file using only commands;
I don't need Notepad.

    Hint: the command echo x y z will print "x y z" to the console.

  2. The findstr command works like find, except you can specify a regular expression to search for. Type find /? to see more information about its usage.

    How can you use the findstr command in combination with dir to list all files that start with a, b, c, d, e, f, or g?

  3. The assoc command can list or modify file extension associations, and the ftype command can list or modify the programs used to open a particular file type.

    How can you use the ftype and find commands to determine all file types that are opened using Notepad? Hint: depending on your setup, you might need to tell find to ignore case (to not care whether words have any capital letters or not); use find /? to see how to do that.

To submit, either do your work in your 185-hw repository in a folder called hw5 (and post to your BitBucket account when you're done), or email me your answers at jal2016@email.vccs.edu with subject CSC 185 HW5.

Other readings