::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact

Self Redirection

January 28th, 2012
tech  [html]
Trying to append a file to another, I was surprised when the terminal just sat there, taking seconds for something that should have been instant. When I canceled the operation I found that the file I was trying to append to hadn't changed and the original file had ballooned to several megabytes. Looking back, I had redirected a file to itself:
  $ cat file1 >> file1 # should have been file2
You can try this yourself, but be prepared to hit Ctrl+C if it isn't instant:
   $ echo "abc" > tmp.txt
   $ cat tmp.txt >> tmp.txt
With BSD cat, as I have on my mac, this just sits there, filling up the disk at around 2MB/s. With GNU cat, on the server where I read email, I get:
   cat: tmp.txt: input file is output file
It's even smart enough to detect hardlinks:
   $ echo "abc" > tmp.txt
   $ ln tmp.txt tmp2.txt
   $ cat tmp.txt &gt> tmp2.txt
   cat: tmp.txt: input file is output file
The code for this in GNU cat, unchanged except for whitespace since at least 1992-11-08 when it was first put under source control, is:
   /* Input file can be output file for non-regular files.
      fstat on pipes returns S_IFSOCK on some systems, S_IFIFO
      on others, so the checking should not be done for those types,
      and to allow things like cat < /dev/tty > /dev/tty, checking
      is not done for device files either. */

   if (S_ISREG (stat_buf.st_mode))
     {
       out_dev = stat_buf.st_dev;
       out_ino = stat_buf.st_ino;
     }
   else
     check_redirection = 0;

   ...

   /* Compare the device and i-node numbers of this input file with
      the corresponding values of the (output file associated with)
      stdout, and skip this input file if they coincide.  Input
      files cannot be redirected to themselves.  */

   if (check_redirection
       && stat_buf.st_dev == out_dev && stat_buf.st_ino == out_ino)
     {
       error (0, 0, "%s: input file is output file", infile);
       exit_stat = 1;
       goto contin;
     }
The reason hardlinks don't fool cat is that it checks the device and inode values, which all hardlinks to a file will have the same:
   $ echo "abc" > tmp.txt
   $ ln tmp.txt tmp2.txt
   $ stat tmp.txt | grep Inode
   Device: 803h/2051dInode: 18645001    Links: 2
   $ $ stat tmp2.txt | grep Inode
   Device: 803h/2051dInode: 18645001    Links: 2

Comment via: google plus, facebook

Recent posts on blogs I like:

The Private Sector’s Role in Transit Innovation

The United States has long had private success and public failure – not just the sense of private affluence and public squalor, in which household income is high but the state of public services lags, but also in that the private sector is more productive…

via Pedestrian Observations June 17, 2019

Unintended pregnancy in folk songs

I’ve been listening to a lot of the Watersons and Waterson:Carthy this week. It’s reminded me how absolutely full British folk music is of songs about unintended pregnancy. Most commonly the result is unhappy motherhood: “But if I had kent that I now ken …

via The whole sky June 1, 2019

Programmer migration patterns

I made a little flow chart of mainstream programming languages and how programmers seem to move from one to another. There's a more common kind of chart, which shows how the languages themselves evolved. I didn't want to show the point of view of …

via apenwarr March 18, 2019

more     (via openring)

More Posts:


  ::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact