Accessing Memory Using DMA

Direct Memory Access (DMA) allows internal peripherals to access memory without the processor needing to execute instructions for each transfer. There are two types of DMA; one uses the DMA controller on the motherboard and the other uses a busmaster controller on the peripheral card.




How to convert images between formats

Use convert to convert the file format of an image. Give the name of the file to convert as the first argument, and the destination file as the second argument. When you convert a file, the original is not altered.




Sending e-mail from bash

To send an email message with mail, give the email addresses to which you are sending as arguments, and then type the message proper in the lines that follow; type C−d on a line by itself to signify the end of the message body, and to send the message.




How to verify any e-mail account using shell

Use vrfy to determine whether or not a given email address works. This is useful when you are unsure whether or not you have the right email address for someone. If the address works, vrfy outputs a message indicating that the recipient exists; if the address is not valid, vrfy outputs a message saying that the user is unknown.




How to save files from URLs using shell

To download a single file from the Web, give the URL of the file as an argument to wget.

For example, to download ftp://ftp.neuron.net/pub/spiral/septembr.mp3 to a file, type: $ wget ftp://ftp.neuron.net/pub/spiral/septembr.mp3 RET




How to archive an Entire Web Site

To archive a single Web site, use the `−m' ("mirror") option, which saves files with the exact timestamp of the original, if possible, and sets the "recursive retrieval" option to download everything. To specify the number of retries to use when an error occurs in retrieval, use the `−t' option with a numeric argument−−−`−t3' is usually good for safely retrieving across the net; use `−t0' to specify an infinite number of retries, good for when a network connection is really bad but you really want to archive something, regardless of how long it takes. Finally, use the `−o' with a file name as an argument to write a progress log to the file−−examining it can be useful in the event that something goes wrong during the archiving; once the archival process is complete and you've determined that it was successful, you can delete the log file.




How to use dig

When you know the name of a particular host, and you want to find the IP address that corresponds to it, ping the host in question; this will output the IP address of the host in parenthesis.




Copying Files and Directories

Use cp ("copy") to copy files. It takes two arguments: the source file, which is the existing file to copy, and the target file, which is the file name for the new copy. cp then makes an identical copy of the source file, giving it the specified target name. If a file with the target name already exists, cp overwrites it. It does not alter the source file.




Changing File Names to Lowercase

To change the uppercase letters in a file name to lowercase (or vice versa), use chcase. It takes as arguments the files whose names it should change.




Renaming Multiple Files with the Same Extension

To give a different file name extension to a group of files that share the same file name extension, use chcase with the `−x' option for specifying a Perl expression; give the patterns to match the source and target files as a quoted argument.




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