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I have destroyed a bunch of non-essential files and I don't know why. I have been executing commands like:

tr -sc 'A-Za-z' '\n' > somefile.txt | less

there is no output (blank page with flashing END) and upon checking all the content from the file is erased.

Another command that erased a full text file 

grep someword > someotherfile.txt | less
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3 Answers 3

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19

The > operator means "take the output of the command, truncate the named file, and then write the output of the command to that.

Reading that command line I guess you want <, which is "read standard input from this file, and feed it to the command" instead.

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    this has to be the stupidest question on the site. thanks. Commented Mar 10, 2012 at 2:13
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    There is nothing wrong with not knowing things. Everyone starts out from zero. Commented Mar 10, 2012 at 2:17
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    “There is no shame in not knowing; the shame lies in not finding out.” -- Russian Proverb Commented Mar 10, 2012 at 15:04
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    Ooooh. I've never had the redirect operators explained in that way, comparatively. I've been a casual linux user for years (not full time, obviously), and never learned properly how to use the < operator. Now I know, and this answer is what did it. @kuchnahi, even the veterans and long-time users are learning new things, every day. Don't sweat it. :) Commented Mar 12, 2012 at 17:55
  • Still, surely it would make some sense to read the documentation for the language features you use, no? Commented Nov 25, 2014 at 9:35
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While you make yourself familiar with I/O redirection, you might find it "safer" to enable noclobber shell setting. This prevents unintentional clobbering of your files. See your shell man page and http://en.wikipedia.org/wiki/Clobbering

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  • +1 great tip, and this is something I do, even with many years of experience. Typos happen, and this helps. Commented Mar 10, 2012 at 14:34
  • I still have to think twice with < and >. Thanks for the tip. Commented Mar 15, 2012 at 20:33
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These commands have clobbered the text file because you told it to (> file will truncate any existing file before writing to it). You are probably looking for <, which means "redirect standard input from here".

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