I try to display a named pipe in a Terminal with this command:
tail -f textFile | cut -d " " -f 3- | sed -e "s/a/g&g/"
For some reason this produces no output.
It does work as expected if the -f is removed:
tail textFile | cut -d " " -f 3- | sed -e "s/a/g&g/"
or the cut statement is removed:
tail -f textFile | sed -e "s/a/g&g/"
or the sed statement is removed:
tail -f textFile | cut -d " " -f 3-
Only when all of these three things are there together, it suddenly doesn't produce any output anymore. The order of sed and cut don't make a difference. All of this makes it hard for me to blame the input or output buffering behaviour of any one or pair of these programms.
A possible solution to get the required functionality would be a while read line structure, but I would like to avoid initializing a command for every line, if at all possible.
I was experiencing something similar with a ping command that I wanted to filter.
The following webpage seems to explain what the issue is (stdio buffering) http://www.pixelbeat.org/programming/stdio_buffering/
The website points to a solution which involves disabling the buffering with the "stdbuf" command
tail -f filename | stdbuf -o0 cut -d " " -f 3- | sed -e "s/a/g&g/"
The above works well for me, and removing "stdbuf -o0" causes no output to be displayed.
>stdbuf --help
Usage: stdbuf OPTION... COMMAND
Run COMMAND, with modified buffering operations for its standard streams.
Mandatory arguments to long options are mandatory for short options too.
-i, --input=MODE adjust standard input stream buffering
-o, --output=MODE adjust standard output stream buffering
-e, --error=MODE adjust standard error stream buffering
--help display this help and exit
--version output version information and exit