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Full Discussion: Another Simple BASH command I don't understand. Help?
Top Forums UNIX for Dummies Questions & Answers Another Simple BASH command I don't understand. Help? Post 302503512 by robsonde on Thursday 10th of March 2011 06:39:03 PM
Old 03-10-2011
Quote:
Originally Posted by phunkypants
I have a text file called file1 which contains the text: "ls -l"
When I enter this command:
bash < file1 > file1
file1 gets erased. However if I enter this command:
bash < file1 > newfile
the output from "ls -l" is stored in newfile. My question is why doesn't file1's text ("ls -l") get replaced with the output of the ls -l command?

if we look at how the second version works:
1. bash will create a empty file called newfile, ready to accept the output from your commands.
2. bash will open the file1 to get input.
3. bash will run the ls -al that it found in file1.
4. the output of ls -al is sent to the output file called newfile.


so if we then look at the "broken" first version:
1. bash will create an empty file called file1. (over writing your "ls -al" command)
2. bash will open the file1 (now empty)
3. bash will find nothing to do.
4. bash will send the output of nothing to the new file called file1
 

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LEARN ABOUT DEBIAN

shanty

SHANTY(1)							      Shanty								 SHANTY(1)

NAME

       Shanty - Makes a PostScript file from an image and some text.

SYNOPSIS

       shanty -i image_file [-t text_file] [-o output_file] [-s paper_size] [-d density] [-m margin] [-b background_color] [-x padding] [-n title]
       [-l orientation] [-rtl] [-btt] [-f font_name] [-altgd]

DESCRIPTION

       Shanty takes a text file and an image (PNG or JPG) and creates a PostScript file where one pixel in the image becomes one character in  the
       PostScript.

OPTIONS

       -i, -image
		 Name of the image to load in. JPG and PNG images are supported. This is the only compulsory field.

       -t, -text Name of the text file to load in, if omitted STDIN is used.

       -o, -output
		 Name of the PostScript file to produce, if omitted STDOUT is used.

       -s, -size Size  of  the	paper  to  work  with.	This  field  should be one of: "a0", "a2", "a3", "a4", "a5", "a6", "letter", "broadsheet",
		 "ledger", "tabloid", "legal", "executive" and "36x36". Default is "a4".

       -d, -density
		 Density of the text. Higher numbers are more dense, default is 1.4.

       -m, -margin
		 The margins of the page in cm. Default is 1.

       -b, -background
		 The colour of a backing rectangle to place behind the text. Colours are specified as "R,G,B" with each value between 0  and  255.
		 "off" means no backing colour. Default is "off".

       -x, -p, -padding
		 Density of the text. Higher numbers are more dense, default is 1.4.

       -n, -title
		 The title of the output to write as meta-data in the PostScript file. Default is "Shanty output".

       -l, -orientation
		 The orientation of the paper, can be "portrait", "landscape" or "auto". Default is "auto".

       -rtl	 Switch to right-to-left text.

       -btt	 Switch to bottom-to-top text.

       -f, -font Specify font. The font name specified must be visible to the not just make a font magically appear. Default is "Courier-Bold".

       -altgd	 If you have problems loading the GD library, try this switch.

HOMEPAGE

       <http://www.codebunny.org/coding/shanty/>

AUTHOR

       Duncan Martin <duncan@codebunny.org>

ACKNOWLEDGEMENTS

       Many thanks to DFB <http://www.cs.nott.ac.uk/~dfb/> and contributors to comp.lang.postscript.

Duncan Martin							  6 October 2006							 SHANTY(1)
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