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| == My command line produces no output: tail -f logfile | grep 'foo bar' == Most standard Unix commands buffer their output if used non-interactively. This means, that they don't write each character (or even each line) as they are ready, but collect a larger number (e.g. 4 kilobytes) before printing it. In the case above, the {{{tail}}} command buffers its output, and therefore {{{grep}}} only gets its input in e.g. 4K blocks. |
== 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. |
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| Unfortunately there's no easy solution to this, because the behaviour of the standard programs would need to be changed. *See bottom of section before taking 'no easy solution' to heart* Some programs provide special command line options for this purpose, e.g. | 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. |
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| ||grep (e.g. GNU version 2.5.1)||{{{--line-buffered}}}|| ||sed (e.g. GNU version 4.0.6)||{{{-u,--unbuffered}}}|| ||awk (some GNU versions)||{{{-W interactive, or use the fflush() function}}}|| ||tcpdump, tethereal||{{{-l}}}|| |
The most important thing to understand about buffering is that it's the ''writer'' who's doing it, not the reader. |
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| The {{{expect}}} package (http://expect.nist.gov/) has an {{{unbuffer}}} example program, which can help here. It disables buffering for the output of a program. Example usage: {{{ unbuffer tail -f logfile | grep 'foo bar' |
==== 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: {{{#!highlight bash tail -f logfile | awk '/foo bar/ ...' |
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| In other cases, this sort of consolidation may not be possible. But you should always look for the simplest solution first. | |
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| There is another option when you have more control over the creation of the log file. If you would like to {{{grep}}} the real-time log of a text interface program which does buffered session logging by default (or you were using {{{script}}} to make a session log), then try this instead: {{{ $ program | tee -a program.log |
==== Your command may already support unbuffered output ==== Some programs provide special command line options specifically for this sort of problem: ||awk (GNU awk, nawk, busybox awk, mawk) ||use the `fflush()` function. [[https://austingroupbugs.net/view.php?id=634|It will be defined in POSIX Issue 8]]|| ||awk (mawk) ||`-W interactive` || ||find (GNU) ||use `-printf` with the `\c` escape || ||grep (e.g. GNU version 2.5.1) ||`--line-buffered` || ||jq ||`--unbuffered` || ||python ||`-u` || ||sed (e.g. GNU version 4.0.6) ||`-u,--unbuffered` || ||tcpdump, tethereal ||`-l` || |
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| In another window: $ tail -f program.log | grep whatever |
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: {{{#!highlight c setvbuf(stdout, NULL, _IONBF, 0); |
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| Apparently this works because {{{tee}}} produces unbuffered output. This has only been tested on GNU {{{tee}}}, YMMV. | 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. |
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| If you simply wanted to highlight the search term, rather than filter out non-matching lines, you can use the 'less' program instead of Bash: {{{ $ less program.log |
==== unbuffer ==== The [[http://expect.sourceforge.net/|expect]] package has an [[http://expect.sourceforge.net/example/unbuffer.man.html|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: {{{#!highlight bash tail -f logfile | unbuffer grep 'foo bar' | awk ... |
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| Inside less, start a search with the '/' command (similar to searching in vi). 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. |
`expect` and `unbuffer` are not standard POSIX tools, but they may already be installed on your system. ==== stdbuf ==== Recent versions of [[http://www.gnu.org/software/coreutils/|GNU coreutils]] (from 7.5 onwards) come with a nice utility called [[http://www.gnu.org/software/coreutils/manual/coreutils.html#stdbuf-invocation|stdbuf]], which can be used among other things to "unbuffer" the standard output of a command. Here's the basic usage for our example: {{{#!highlight bash 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`: {{{#!highlight bash 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 [[http://wiki.bash-hackers.org/syntax/keywords/coproc|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 |
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:
1 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:
awk (GNU awk, nawk, busybox awk, mawk) |
use the fflush() function. It will be defined in POSIX Issue 8 |
awk (mawk) |
-W interactive |
find (GNU) |
use -printf with the \c escape |
grep (e.g. GNU version 2.5.1) |
--line-buffered |
jq |
--unbuffered |
python |
-u |
sed (e.g. GNU version 4.0.6) |
-u,--unbuffered |
tcpdump, tethereal |
-l |
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:
1 setvbuf(stdout, NULL, _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:
1 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:
1 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:
1 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.