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.