Difference between revisions of "BASH/C3/Using-File-Descriptors/English"

• Assign an output file descriptor
• Assign an imput file descriptor
• Close the file descriptor (fd)
• With the help of some examples

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System requirements

• Ubuntu Linux 12.04 Operating System and
• GNU BASH version 4.2

Please note, GNU Bash version 4 or above is recommended to practice this tutorial.

Slide 5

We have already studied about the file descriptors in the previous tutorial.

• 0, 1 and 2 are the standard file descriptors for stdin, stdout and stderr
• File descriptors are used for i/o redirection

Syntax:

exec [File descriptor] > filename

exec [File descriptor] > filename

Let us see an example.

I have a code file with the name fdassign.sh

It will be executed in the place of the current shell without creating a new process.

We know that 0, 1, and 2 are standard file descriptors.

For any newly opened file, we have additional file descriptors from 3 to 9.

Here, 3 is the file descriptor.

This will write the output to output.txt file.

This is done via a file descriptor 3.

This is similar to redirecting a string to a file.

For example:

We will append the current system date to the output.txt file.

The syntax is:

date SPACE a greater-than sign followed by ampersand followed by a file descriptor 3

After this line the descriptor cannot write anything to the output.txt file.

Open the temrinal using CTRL+ALT+T keys.

chmod +x fdassign.sh

./fdassign

chmod +x fdassign.sh

./fdassign

Type:

cat output.txt

We can see that the string,

Welcome to BASH learning and the current system date is displayed.

echo “Hi” > &3

Now I will type echo at the end, after the descriptor is closed.

Type:

echo “Hi” > &3

Press the uparrow key, to recall the previous command

./fdassign

press Enter.

./fdassign.sh: line 6: 3: Bad file descriptor

echo "hiii" >&3

Paste above

Come back to the editor.

Now

I will cut the last line of code and paste it below the date command

Let us execute the code once again.

On the terminal, recall the previous command:

./fdassign.sh

press Enter.

Type:

cat output.txt

Fri Aug 8 15:02:51 IST 2014

Hi

The string Hi is displayed at the end.

<<PAUSE>>

Let us see an example.

I have a file named fdread.sh

Let us go through it.

exec 3< output.txt

cat <&3

exec 3<&-

Here we will read the file output.txt

The line

exec 3< output.txt

will open the file for reading.

cat command will display the content of the file.

And finally we close the file descriptor.

./fdread.sh

On the terminal type:

chmod +x fdread.sh

./fdread.sh

Fri Aug 8 15:12:06 IST 2014

Hi

The content of output.txt file is displayed.

Let us summarize

In this tutorial we learnt to,

• Assign the output file descriptor

• Assign the input file descriptor
• Closing the file descriptor.

Try to

• append a few lines to a file test.txt
  using file descriptors covered in this tutorial.
• Display the content of the file using file descriptors

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Acknowledgement

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This is Ashwini from IIT Bombay.

Thank you for joining.