Programming Forums > Script Programming > Sed and Awk

Pimp Daddy · Aug 28th, 2006, 5:51 AM

Although I've been using Linux for a while (in various forms - such as Slackware and Cygwin), I never got around to learning the likes of grep, sed and awk. That's what I've been doing over the past few days and today I noticed something with sed. I type the following:

(Toggle Plain Text)

$ sed -e '/mom/s/world/hull/g' h.txt

Naturally, it replaces 'world' with 'hull' and prints the result out on the screen. However, when I try overwriting what's in the file by doing this:

(Toggle Plain Text)

$ sed -e '/mom/s/world/hull/g' h.txt > h.txt

it just wipes the file. What do I have to do to overwrite the contents of a file?

grumpy · Aug 28th, 2006, 6:38 AM

Redirect the output to another file (preferably one that doesn't exist), delete the original file, and move (or copy) the new file back in its place.

jim mcnamara · Aug 28th, 2006, 10:19 AM

> file
truncates (sets the file pointer to the start of an empty file) a file.
It happens before grep runs. Grumpy gave you the workaround.

(Toggle Plain Text)

grep 'stuff ' file1 > tmpfile
mv tmpfile file1

free-zombie · Sep 5th, 2006, 6:58 AM

use sed -i
(edit in-place)

Aug 28th, 2006, 5:51 AM	#1
Pimp Daddy Programmer Join Date: Aug 2004 Location: Wherever the beautiful women are. Posts: 30 Rep Power: 0	Just playing Although I've been using Linux for a while (in various forms - such as Slackware and Cygwin), I never got around to learning the likes of grep, sed and awk. That's what I've been doing over the past few days and today I noticed something with sed. I type the following: (Toggle Plain Text) $ sed -e '/mom/s/world/hull/g' h.txt $ sed -e '/mom/s/world/hull/g' h.txt Naturally, it replaces 'world' with 'hull' and prints the result out on the screen. However, when I try overwriting what's in the file by doing this: (Toggle Plain Text) $ sed -e '/mom/s/world/hull/g' h.txt > h.txt $ sed -e '/mom/s/world/hull/g' h.txt > h.txt it just wipes the file. What do I have to do to overwrite the contents of a file? __________________ David Morris BSc.(Hons), MBCS Qualified Computer Engineer Administrator (SEED Software)

Aug 28th, 2006, 10:19 AM	#3
jim mcnamara Hobbyist Programmer Join Date: Jun 2005 Location: New Mexico Posts: 228 Rep Power: 4	> file truncates (sets the file pointer to the start of an empty file) a file. It happens before grep runs. Grumpy gave you the workaround. (Toggle Plain Text) grep 'stuff ' file1 > tmpfile mv tmpfile file1 grep 'stuff ' file1 > tmpfile mv tmpfile file1

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Aug 28th, 2006, 6:38 AM	#2
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,206 Rep Power: 5	Redirect the output to another file (preferably one that doesn't exist), delete the original file, and move (or copy) the new file back in its place.

Sep 5th, 2006, 6:58 AM	#4
free-zombie Programmer Join Date: May 2006 Location: Bavaria, Germany Posts: 50 Rep Power: 0	use sed -i (edit in-place)

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Programming Forums > Script Programming > Sed and Awk

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