23.5 stdbuf: Run a command with modified I/O stream buffering

stdbuf allows one to modify the buffering operations of the three standard I/O streams associated with a program. Synopsis:

stdbuf optioncommand

command must start with the name of a program that

  1. uses the ISO C FILE streams for input/output, and
  2. does not adjust the buffering of its standard streams.

Not every command operates in this way. For example, dd does not use FILE streams, and tee adjusts its streams’ buffering.

Any additional args are passed as additional arguments to the command.

The program accepts the following options. Also see Common options.

-i mode

Adjust the standard input stream buffering.

-o mode

Adjust the standard output stream buffering.

-e mode

Adjust the standard error stream buffering.

The mode can be specified as follows:


Set the stream to line buffered mode. In this mode data is coalesced until a newline is output or input is read from any stream attached to a terminal device. This option is invalid with standard input.


Disable buffering of the selected stream. In this mode, data is output immediately and only the amount of data requested is read from input. Disabling buffering for input does not necessarily influence the responsiveness or blocking behavior of the stream input functions. For example, fread will still block until EOF or error or the amount requested is read, even if the underlying read returns less data than requested.


Specify the size of the buffer to use in fully buffered mode. size may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:

KB’ =>           1000 (KiloBytes)
‘K’  =>           1024 (KibiBytes)
‘MB’ =>      1000*1000 (MegaBytes)
‘M’  =>      1024*1024 (MebiBytes)
‘GB’ => 1000*1000*1000 (GigaBytes)
‘G’  => 1024*1024*1024 (GibiBytes)

and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on.

stdbuf is installed only on platforms that use the Executable and Linkable Format (ELF) and support the constructor attribute, so portable scripts should not rely on its existence.

Exit status:

125 if stdbuf itself fails
126 if command is found but cannot be invoked
127 if command cannot be found
the exit status of command otherwise