stdbuf

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stdbuf runs a command with altered buffering on its standard streams.

stdbuf [options] command...

$ slow-producer | stdbuf -oL grep error

What buffering is, and why change it #

A program usually does not write each piece of output the instant it is produced — it collects output in a buffer and writes it in batches, which is faster. The cost is delay: when a program's output feeds a pipe, that batching can hold lines back for a long time, which is unhelpful when you are watching a pipeline live.

stdbuf lets you override that batching for a command, so its output appears sooner.

Setting the buffering #

MODE is one of:

-oL — line-buffered output — is the common case: it makes a command in a pipeline emit each line as it is finished.

How it works #

stdbuf works by preloading a small helper library, libstdbuf.so, into the command, so the C library's buffering defaults are changed before the command's main runs. The library is installed with the utilities at /usr/libexec/peiosutils/libstdbuf.so; stdbuf looks for it beside its own executable first and in that directory second, and refuses to run the command if it is missing rather than run it unbuffered. stdbuf then waits for the command and exits with its status.

Nothing is written anywhere at run time. Earlier builds embedded the library and extracted it to $TMPDIR on every invocation, which is where two upstream security advisories lived; the shipped build no longer does that.

A limitation #

stdbuf adjusts the default buffering. A command that manages its own stream buffering will override what stdbuf sets, and some commands do not use buffered streams at all — for those, stdbuf has no effect.

Exit status #

When stdbuf runs a command, it exits with that command's status. The exception is a failure in stdbuf itself:

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