Shell Programming and Scripting

I'm trying to change a date in a couple of large files using SED. The problem is when I use the -n parameter, it doesn't actually change the file. When I leave out the -n, it sends the whole file to the screen, but it does appear to change it.

The problem is, these files are very large and it would take forever to change it like that. I'm sure it is just an issue with my syntax, but here is what I was trying:

Code:

sed -n 's/20121201/20130101/g' filename

Should this update the existing file? If not, how can I do that?

Moderator's Comments:

Code tags please, see PM.

sed does not work "in place", not even GNU sed's "-i" works on the same file, it actually creates a copy then replaces the original.

Try:

Code:

sed 's/20121201/20130101/g' filename > filename.new

How can I edit this file then? Can I output the changes to a different filename? I just need to edit the dates, but can't figure it out.

You can edit the same file with a single command of sed:

Code:

 sed -i 's/20121201/20130101/g' filename

The -i flag allow you to do the sequence that you want in the file

And also here you can find useful information about sed:

http://www.gentoo.org/doc/en/articles/l-sed1.xml

No. The command: should not modify the file named by filename. This command requests that sed look at every line in filename and to change all occurrences of 20121201 to 20130101, but not to print anything after doing so.

To do what you want portably, try:

Code:

sed 's/20121201/20130101/g' filename > tmp && mv tmp filename

assuming that you're using a shell (like bash, sh, or ksh) that supports this use of &&. I make no claims about whether or not this will work with csh or tcsh.

On some systems you could use:

Code:

sed -i .bak 's/20121201/20130101/g' filename

instead, but not all systems' version of sed support the -i option.

Thanks a bunch.

-i didn't work on my system, but moving it to a new file did work. Thanks so much for your help.