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I'm curious.

You can use less to read through a 100 GB file with no problem. But if you cat a 100 GB file into less, I'm thinking that less must buffer the whole file because there is a pipe (was it 64K?) between the file and less.

So, anyone know about the buffering policy in less, will it start buffering from the beginning until memory runs out?

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1 Answer 1

up vote 3 down vote accepted

The manpage of less covers the topic:

   -bn or --buffers=n
          Specifies  the  amount of buffer space less will use for each file,
          in units of kilobytes (1024 bytes).  By default 64K of buffer space
          is  used  for  each  file  (unless  the  file is a pipe; see the -B
          option).  The -b option specifies instead that n kilobytes of  buf‐
          fer  space  should be used for each file.  If n is -1, buffer space
          is unlimited; that is, the entire file can be read into memory.

   -B or --auto-buffers
          By default, when data is read from a pipe,  buffers  are  allocated
          automatically  as  needed.   If a large amount of data is read from
          the pipe, this can cause a large amount of memory to be  allocated.
          The  -B  option  disables  this automatic allocation of buffers for
          pipes, so that only 64K (or the amount of space specified by the -b
          option)  is  used  for  the pipe.  Warning: use of -B can result in
          erroneous display, since only the most recently viewed part of  the
          piped data is kept in memory; any earlier data is lost.

The manpage implies that the buffer will eventually grow as big as the whole input, if you do not restrict it by -B and -b options.

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