Another Simple BASH command I don't understand. Help? Post: 302503512

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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

03-10-2011

Registered User

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

robsonde

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lzcmp

XZDIFF(1)							     XZ Utils								 XZDIFF(1)

NAME

       xzcmp, xzdiff, lzcmp, lzdiff - compare compressed files

SYNOPSIS

       xzcmp [cmp_options] file1 [file2]
       xzdiff [diff_options] file1 [file2]
       lzcmp [cmp_options] file1 [file2]
       lzdiff [diff_options] file1 [file2]

DESCRIPTION

       xzcmp  and  xdiff invoke cmp(1) or diff(1) on files compressed with xz(1), lzma(1), gzip(1), or bzip2(1).  All options specified are passed
       directly to cmp or diff.  If only one file is specified, then the files compared are file1 (which must have a suffix of	a  supported  com-
       pression  format)  and  file1 from which the compression format suffix has been stripped.  If two files are specified, then they are uncom-
       pressed if necessary and fed to cmp(1) or diff(1).  The exit status from cmp or diff is preserved.

       The names lzcmp and lzdiff are provided for backward compatibility with LZMA Utils.

SEE ALSO

       cmp(1), diff(1), xz(1), gzip(1), bzip2(1), zdiff(1)

BUGS

       Messages from the cmp(1) or diff(1) programs refer to temporary filenames instead of those specified.

Tukaani 							    2009-07-05								 XZDIFF(1)