why redirecting to same file results in a empty file in unix

Question

Hi,

if i redirect to same file it places null in the resultant file.

sed 's/abd/def/g' a.txt > a.txt

i know it put null in to the file.

But can anyone explain why?

Answer 1

You need to use the -i option to edit the file in place:

sed -i .bck 's/abd/def/g' a.txt

EDIT: as noted by neil, the redirection first opens the file for writing thus clears it.

EDIT2: it might be interesting for some reader

On OSX, if you want to use -i with an empty extension to prevent the creation of a backup file, you need to use the -eswitch as well otherwise it fails to parse the arguments correctly:

sed -i -e 's/abc/def/g' a.txt

Answer 2

The first thing the redirection does is to open the file for writing, thus clearing any existing contents. sed then tries to read this empty file you have just created, and does nothing. The file is then closed, containing nothing.

Answer 3

The redirection operations <, >, etc. are handled by the shell. When you give a command to the shell that includes redirection, the shell will first open the file. In the case of > the file will be opened for writing, which means it gets truncated to zero size. After the redirection files have been opened, the shell starts a new process, binding its standard input, output, and error to any possible redirected files, and only then executes the command you gave. So when the sed command in your example begins execution, a.txt has already been truncated by the shell.

Incidentally, and somewhat tangentially, this is also the reason why you cannot use redirection directly with sudo because it is the shell that needs the permissions to open the redirection file, not the command being executed.