CODEWITHSUNDEEP
linuxbash
chris01
chris01

Reputation: 12387

Output-redirection should recreate the destination-file

I can redirect the output of a process to a file

./prog > a.txt

But if I delete a.txt and do not restart prog, then no more output will get into a.txt. The same is the case if I use the append-redirect >>.

Is there a way to make my redirection recreate the file when it is deleted during the runtime of prog?

Redirection is part of the OS I think and not of prog. So maybe there are some tools or settings.

Thanks!

Upvotes: 1

Views: 401

Answers (2)

sungtm
sungtm

Reputation: 577

You can use gdb to redirect the output of program to file when original file is deleted.

Refer to this post.

For later references, I give the only excerpt from the post:

  1. Find the files that are opened by the process using /proc/<pid>/fd.
  2. Attach the PID of program to gdb.
  3. Close the file descriptor of the deleted file through gdb session.
  4. Redirect the program output to another file using gdb calls.

Examples

Suppose that PID of program is 19080 and file descriptor of deleted file is 2.

gdb attach 19080

ls -l /proc/19080/fd

gdb> p close(2) 
$1 = 0
gdb> p fopen("/tmp/file", "w")
$2 = 20746416
(gdb) p fileno($2)
$3 = 7
gdb> quit

N.B.: If data of the deleted file is required, recover the deleted text file before closing the file handle: cp -pv /proc/19080/fd/2 recovered_file.txt

Upvotes: 1

axiac
axiac

Reputation: 72266

At the OS level, a file is made up of many components:

  • the content, stored somewhere on the storage device;
  • an i-node that keeps all file information except the name;
  • the name, listed in a directory (also stored on the storage device);
  • when the file is open, each application that opens it handle memory buffers that keep some of the file content.

All these are linked and the OS keeps their booking.

If you delete the file while it is open by another application (the redirect operator > keeps it open until ./prog completes), only the name is removed from the directory. The other pieces of the puzzle are still there and they keep working until the last application that keeps the file open closes it. This is when the file content is discarded on the storage medium.

If you delete the file, while ./prog keeps running and producing output the file grows and uses space on the storage medium but it cannot be open again because there is no way to access it. Only the programs that have it already open when it was deleted can still access the file until they close it.

Even if you re-create the file, it is a different file that happens to have the same name as the deleted one. ./prog is not affected, its output goes to the old, deleted file.

When its output is redirected, apart from restarting ./prog, there is no way to persuade it to store its output in a different file when a.txt is deleted.

There are several ways to make this happen if ./prog writes itself into a.txt (they all require changing the code of ./prog).

Upvotes: 3

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