A free Basic UNIX-training with muLinux

In- and Output Redirections, Pipes

output-redirection    >

/a# ls > directory.txt

reads the contents of the working directory and writes it to directory.txt. The file directory.txt is created before the command ls is executed. An already existing file with the same name will be overwritten. If the command which is being executed does not create any output, because for example, something went wrong while executing or the current directory was empty, then the assigned file will now exist, but it will be empty!

We want to be sure that everything is fine. Firstly, with ls we want to see if the file exists. Then we look at the file directory.txt:

/a# cat directory.txt

Great!
Using this simple command, I have created most of the screenoutputs for this Unix-training manual. Lets try another command:

/a# date > date.txt

Now we already created two text-files. The command cat has a special status by in- and output-redirections. This command can concatenate more files at the same time:

/a# cat directory.txt Datum.txt > EverythingTogether.txt

Now we have added a third file, which hosts both other files.

And another variant:

/a# ls > /dev/null

What happened? Nothing! /dev/null is the Trash-bin in Unix. What gets there is gone. But why is this useful? This redirection is popular for testing programs, which means, getting error-messages on the screen, while the actual output is ignored.

We also can redirect outputs onto devices.

/a# date > /dev/tty3

To see the effect we press [Alt] [F3]. Like this we change to console 3 (tty3). There we see the output of date

Paste to a file    >>

/a# date >> date.txt

Here the actual date is pasted to the file. We can test this easily by calling this command a few times. For this of course we use the command-history (cursor-up). With this command we can easily create a log-file. When this command is linked to a certain event, like the execution of another command, this action is logged with the date.

Redirecting error-messages    2>

/a# cd Partytime 2> error.txt
/a# cat error.txt
cd: can't cd to Partytime

Redirecting keyboard-input into a file

/a# cat > Textfile01.txt
All the lines you enter now will be written to Textfile01.txt.
Like also this text.
A new line can be acheived by pressing [enter].
To end the text entry, we need to give the keys [ctrl] [d] on an empty line:

[ctrl] [d]
/a# _

Output-redirection    <

command < filename

The command sort creates a sorted output. We can redirect the file /etc/passwd to it, to get a sorted output:

sort < /etc/passwd

Pipes    |

command1 | command2

Of course the next command needs to be able to handle this data flow. Usually these are commands which work like filters. Such a filter is more, which allows an easy listing of data on the screen.

/a# ls -l | more

This command, in muLinux is terminated with q. Every time you expect a long output, you can use more. For example with dmesg. dmesg gives the kernel-messages. This provides info about the used hardware. Because the output is quite long, with more we can more easily control the output information, leading to greater viewing comfort.

/a# dmesg | more

A similar function is the command less. This one you can close with q like more.

/a# dmesg | less

A very common used command is grep. grep searches in the dataflow all the signs which contain a certain signs-chain, and then outputs these lines. We only want to be informed about the memory, we filter out the line containing the word "memory".

/a# dmesg | grep "Memory"

Another example:

/a# ls -l | grep "1999"

All files and directories which are created in 1999, are listed. And another example:

/a# cat /etc/passwd | grep "root"

All the lines which contain the word "root" are filtered out. Of course this dataflow can be redirected again. For example into a file:

/a# cat /etc/passwd | grep "root" > password.root.txt

By the way, in Unix, filenames with more dots are allowed. A filter to sort, is the already known command sort.

/a# cat /etc/passwd | sort

All lines are sorted by the first letter. But if the file /etc/passwd is too big, it's recommended to use the commands less or more:

/a# cat /etc/passwd | sort | less

The filter wc word count, counts words and signs.

/a# cat /etc/passwd | wc