Diff for "BashFAQ/009"

What is buffering? Or, why does my command line produce no output: tail -f logfile | grep 'foo bar' | awk ...

Most standard Unix commands buffer their output when used non-interactively. This means that they don't write each character (or even each line) immediately, but instead collect a larger number of characters (often 4 kilobytes) before printing anything at all. In the case above, the grep command buffers its output, and therefore awk only gets its input in large chunks.

Buffering greatly increases the efficiency of I/O operations, and it's usually done in a way that doesn't visibly affect the user. A simple tail -f from an interactive terminal session works just fine, but when a command is part of a complicated pipeline, the command might not recognize that the final output is needed in (near) real time. Fortunately, there are several techniques available for controlling I/O buffering behavior.

The most important thing to understand about buffering is that it's the writer who's doing it, not the reader.

Eliminate unnecessary commands

In the question, we have the pipeline tail -f logfile | grep 'foo bar' | awk ... (with the actual AWK command being unspecified). There is no problem if we simply run tail -f logfile, because tail -f never buffers its output. Nor is there a problem if we run tail -f logfile | grep 'foo bar' interactively, because grep does not buffer its output if its standard output is a terminal. However, if the output of grep is being piped into something else (such as an AWK command), it starts buffering to improve efficiency.

In this particular example, the grep is actually redundant. We can remove it, and have AWK perform the filtering in addition to whatever else it's doing:

tail -f logfile | awk '/foo bar/ ...'

In other cases, this sort of consolidation may not be possible. But you should always look for the simplest solution first.

Your command may already support unbuffered output

Some programs provide special command line options specifically for this sort of problem:

Each command that writes to a pipe would have to be told to disable buffering, in order for the entire pipeline to run in (near) real time. The last command in the pipeline, if it's writing to a terminal, will not typically need any special consideration.

Disabling buffering in a C application

If the buffering application is written in C, and is either your own or one whose source you can modify, you can disable the buffering with:

setvbuf(stdout, 0, _IONBF, 0);

Often, you can simply add this at the top of the main() function, but if the application closes and reopens stdout, or explicitly calls setvbuf() later, you may need to exercise more discretion.

unbuffer

The expect package has an unbuffer program which effectively tricks other programs into always behaving as if they were being used interactively (which may often disable buffering). Here's a simple example:

tail -f logfile | unbuffer grep 'foo bar' | awk ...

expect and unbuffer are not standard POSIX tools, but they may already be installed on your system.

stdbuf

Recent versions of GNU coreutils (from 7.5 onwards) come with a nice utility called stdbuf, which can be used among other things to "unbuffer" the standard output of a command. Here's the basic usage for our example:

tail -f logfile | stdbuf -oL grep 'foo bar' | awk ...

In the above code, "-oL" makes stdout line buffered; you can even use "-o0" to entirely disable buffering. The man and info pages have all the details.

stdbuf is not a standard POSIX tool, but it may already be installed on your system (if you're using a recent GNU/Linux distribution, it will probably be present).

less

If you simply wanted to highlight the search term, rather than filter out non-matching lines, you can use the less program instead of a filtered tail -f:

$ less program.log

• Inside less, start a search with the '/' command (similar to searching in vi). Or start less with the -p pattern option.

• This should highlight any instances of the search term.

• Now put less into "follow" mode, which by default is bound to shift+f.

• You should get an unfiltered tail of the specified file, with the search term highlighted.

"follow" mode is stopped with an interrupt, which is probably control+c on your system. The '/' command accepts regular expressions, so you could do things like highlight the entire line on which a term appears. For details, consult man less.

coproc

If you're using ksh or Bash 4.0+, whatever you're really trying to do with tail -f might benefit from using coproc and fflush() to create a coprocess. Note well that coproc does not itself address buffering issues (in fact it's prone to buffering problems -- hence the reference to fflush). coproc is only mentioned here because whenever someone is trying to continuously monitor and react to a still-growing file (or pipe), they might be trying to do something which would benefit from coprocesses.

Further reading

• http://www.pixelbeat.org/programming/stdio_buffering

CategoryShell