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Old 03-25-2006, 04:32 AM   #1
Ander
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A quick bash question


I'm trying to understand pipes.

Why does 'kate mytextfile' work, but 'mytextfile | kate' doesn't?
 
Old 03-25-2006, 04:39 AM   #2
cs-cam
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http://sig9.com/articles/unix-pipes-intro

The reason that doesn't work is obvious but I can't think of a decent way of explaining them so here's a link, it'll do a beter job than I would
 
Old 03-25-2006, 05:29 AM   #3
pixellany
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Quote:
Originally Posted by Ander
I'm trying to understand pipes.

Why does 'kate mytextfile' work, but 'mytextfile | kate' doesn't?
A pipe connects the output of one command to the input of the next.
"mytextfile" does not produce any output---it actually does nothing. to prove this, just type it in at a prompt.
The first example works because the kate command is set up to handle what are called "command-line arguments"

If you like to experiment, try entering:
"cat mytextfile | <somecommand>" This reads the content of mytextfile and gives it to <somecommand>

A common usage would be "cat mytextfile | grep hello"
This prints any line from the file that contains the word "hello"
 
Old 03-25-2006, 06:48 AM   #4
jschiwal
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Pipes are often used with a type of program called a filter. Grep, sed, tr & awk are common filters. They are placed after a pipe, taking the standard input from the pipe and printing the result to the standard output.

Another type of program like kate, or kghostview expect a filename argument. Some of these will accept the "-" dash character argument as a signal to take the input from standard input.

For example, try:
man -t bash | kghostview -

Then read page 5 to learn about pipes!

Or suppose that you want a text file output:
man bash | cat >bash.txt

A program such as "enscript" will be setup to write a file instead of sending the output to stdout.
With enscript and others you can use the output filename of "/dev/stdout" to generate a stdout stream.
examples:
man bash | enscript -o /dev/stdout >bash.ps
man bash | enscript -o /dev/stdout | kghostview -

A handy command to learn is "xargs". This command will take the standard output of a pipe and use them as arguments to another command.

Just minutes ago, I used pipes to take the contents of the unziped .k3b file to get the filelist of files I backed up to cdrom.
unzip name.k3b.
The resultant maindata.xml contains the filenames that were backed up. So I can delete them like this:
sed -e '/^<url>/!d'-e 's/<url>\(.*\)<\/url>/\1/' -e 's/\&/\\&/' maindata.xml | tr '\n' '\000' | xargs -0 rm -v

The sed commands extract just the filenames from the xml file and escape an "evil" character in the filenames. The tr command replaces newlines with nulls. The -0 option to xargs tells it to use nulls instead of returns. This helps with filenames that contains spaces. These filenames are used as arguments to the "rm" command and are deleted.

Last edited by jschiwal; 03-25-2006 at 06:51 AM.
 
Old 03-25-2006, 12:44 PM   #5
nopicexpert
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Hi does anyone know how to merge three two photos into one by chance. If anyone can help mea dn give me suggestions i would greatyl appreciate it thanks a ton.

Thansk
 
Old 03-25-2006, 03:37 PM   #6
jschiwal
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ImageMagick is a package of several programs. One of them will combine photos.
I think the program you want is called mogrify.
Quote:
NAME
mogridy (sic) - resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more. Mogrify overwrites
the original image file, whereas, convert(1) writes to a different image file.
...
For more information about this command, point your browser to file:///usr/share/doc/packages/ImageMagick/index.html.

Run `mogrify -help' to get a summary of the mogrify command options.

Last edited by jschiwal; 03-25-2006 at 03:39 PM.
 
Old 05-05-2006, 02:13 AM   #7
Ander
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Quote:
cs-cam > The reason that doesn't work is obvious but I can't think of a decent way of explaining them so here's a link...
Yup, thanks... It is indeed obvious to me too, now that I've been using Linux a month longer (and having taken this long to check this thread).

That's good, though... If such originally-confusing stuff seems so simple now, it means there's hope for all of us as we confront the truly hard stuff. (Er, if anybody understands what I just said, explain it to me, okay?)
 
  


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