How to make a command write to the file it's reading from?

kofucii · 01-07-2010, 03:07 PM

Also it can be done with "perl", if you insist on one command solution:

Code:

perl -pi -e "s/foo/bar/g" myfile

Tinkster · 01-07-2010, 03:08 PM

Quote:

Just for fun I will try to make a little C program that will store up all it's input from stdin in an array, and when it gets an EOF it will dump everything it stored up and exit.

Just make sure you ave plenty of RAM on the stack ;}

MTK358 · 01-07-2010, 03:36 PM

Here it is, but it didn't seem to solve the problem. Here is a little commented command-line session with it:

Code:

$ cat test.txt
buffer is a very silly little program.
$ sed 's:silly:clever:g' test.txt | ./buffer > test.txt
$ cat test.txt
$ # Why didn't it work? Let's test it using a program called "test" that
$ # prints out 5 lines containing the character 'a', over 5 seconds.
$ ./test
a
a
a
a
a
$ # There was a 1 second delay between the appearance of each "a".
$ ./test | ./buffer
a
a
a
a
a
$ # This time it just sat for 5 seconds and then all the "a"'s suddenly
$ # appeared all at once , as expected. I wonder why the "sed" test
$ # didn't work?

buffer basically consists of a loop that gets a character, and if it is not EOF, it appends it to an allocated list. If the list is to small, it is reallocated 50% larger. When the EOF is reached, it prints out the buffer and frees it.

Here is the C source code for buffer:

whizje · 01-07-2010, 04:57 PM

echo "$(sed 's/silly/clever/g' < silly)" > silly

This seems to work file is closed before opening for writing i think.

MTK358 · 01-07-2010, 05:07 PM

Quote:

Originally Posted by whizje

echo "$(sed 's/silly/clever/g' < silly)" > silly

This seems to work file is closed before opening for writing i think.

That not only clears the file but it also complains about it not existing.

whizje · 01-07-2010, 05:24 PM

Are you using bash?
bash-3.1$ cat silly
silly
bash-3.1$ echo "$(sed 's/silly/clever/g' < silly)" > silly
bash-3.1$ cat silly
clever

MTK358 · 01-07-2010, 07:27 PM

Quote:

Are you using bash?

Yes, sure.

EDIT: I tried your example and it worked this time.

konsolebox · 01-07-2010, 08:28 PM

That's just like trying to push a cart while riding it or plugging the wire of an AVR to itself. You'll need another object like a temporary file, a named pipe, an allocated part of the memory, etc. to make that work.

konsolebox · 01-07-2010, 08:36 PM

Quote:

Originally Posted by whizje

Are you using bash?
bash-3.1$ cat silly
silly
bash-3.1$ echo "$(sed 's/silly/clever/g' < silly)" > silly
bash-3.1$ cat silly
clever

It appears that bash interprets the inner command in $() first, allocate the output as a string then send it to the file before truncating silly. It's a pretty odd or delicate implementation.