stdbuf
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 #
| Option | Stream |
|---|---|
-i, --input=MODE | Standard input. |
-o, --output=MODE | Standard output. |
-e, --error=MODE | Standard error. |
MODE is one of:
| MODE | Buffering |
|---|---|
0 | Unbuffered — every write goes out immediately. |
L | Line-buffered — output is flushed at the end of each line. Not valid for input. |
| a size | Fully buffered with a buffer of that many bytes. Accepts suffixes — K, M, and so on. |
-oL — line-buffered output — is the common case: it makes a command in a pipeline emit each line as it is finished.
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:
| Code | Meaning |
|---|---|
| (command's own) | The command ran; this is its exit status. |
(a stdbuf-level error code) | The command could not be started. |