Modifying a file with sed using redirection

Question

this works as I expected:

$ cat in
abc 123
$ sed -E -i .bk 's/[0-9]+/(&)/' in
$ cat in 
abc (123)
$ cat in.bk
abc 123 

But not this:

$ cat in
abc 123
$ sed -E 's/[0-9]+/(&)/' in 
abc (123)
$ sed -E 's/[0-9]+/(&)/' in > in
$ cat in 

How can I modify the above set of commands such that the last command returns, instead:

abc (123)

?

Answer 1

sed -i -E 's/[0-9]+/(&)/' in

Alternatively,

sed -E 's/[0-9]+/(&)/' in | tee in

overwrites the file AND gives you the output on stdout.

Answer 2

Question: Modifying a file with sed using redirection.

Answer: obviously, the -i switch was added to sed for a reason. Just using redirection leaves you looking at a somewhat convoluted solution.

cat in | cat - | sed -E 's/[0-9]+/(&)/' > in

The following also work, but only if the input file contains a single line:

cat in | parallel 'echo {} | sed -E "s/[0-9]+/(&)/" > in'

sed -E 's/[0-9]+/(&)/' in | parallel 'echo {} >in'

Explanation: The explanation is provided by Paulo in his comment to the question (redirection is interpreted before the command is executed). My workaround is to add a level of indirection between reading the file and writing back to it.