Wednesday, 4 February 2009


If you've understood pipes reasonably well, redirection should be easy to understand as well. It's a similar concept in that it involves doing something with the output of a command.

Redirection allows you to send the output of a command into a text file, or send the contents of a file as input to a command. It's used in a similar fashion to pipes, and is represented by the > symbol.

Here's an example:

ls -A > list.txt

Try running this and it will generate a list of all the files and folders in your home directory and save it as list.txt. If you then open the created text file using less or cat, you can see the effect for yourself.

Please note, however, that if you use this with an existing file it will be overwritten. However, by using the double redirection sign(>>) you can append the results to the file, so they are added to whatever is there. Try running a variant of the same command:

ls -A >> list.txt

If you've done it right, there will be two entries for each item in list.txt.

You can also use redirection in the opposite direction to send the contents of a file to a command in order to be written to standard output, as in this example:

matthew@trinity:~/Development/Python/PPAB2ED$ grep game < list.txt

In this case I'd already used ls -A > list.txt to provide a list of the files in a directory I'd used for Python programming. This command allows grep to sort through the file and pick out those lines that have the word "game" in them. OK, there are other ways to do this using pipes, such as cat list.txt | grep game, but this demonstrates how redirection works

You can also combine redirection each way within one command. In this case, I use grep to sort through list.txt for lines featuring the word "game", but I then send the output of that to a new file, game_list.txt:

matthew@trinity:~$ grep game < list.txt > game_list.txt
matthew@trinity:~$ cat game_list.txt

I've had to shorten the path to fit this in, but it's still in the same directory. This demonstrates how you can combine redirection in both ways.

It's quite common to see redirection used in shell scripts (programs written in bash) for various purposes, such as capturing errors, editing text files (I've seen it used to edit your /etc/apt/sources.list in order to add new repositories) and other purposes. It's not of use to the average user as often as pipes are, but it's still worth knowing about.

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