Live program output monitoring tool

command linemonitoring

I would like to know if there is a tool that would enable you to watch how the program output changes live. Something like tail -f but instead of monitoring the file changes, it would repeatedly call some executable and display it live.

For example, if the tool is caled foobar and I would call foobar 'ps -Al', it would behave kind of like top – displaying the output in real time.

Best Answer

Try watch. From the manpage:

Name

watch - execute a program periodically, showing output fullscreen

Synopsis
watch [-dhvt] [-n <seconds>] [--differences[=cumulative]] [--help] [--interval=<seconds>] [--no-title] [--version] <command>
Description

watch runs command repeatedly, displaying its output (the first screenfull). This allows you to watch the program output change over time. By default, the program is run every 2 seconds; use -n or --interval to specify a different interval.

The -d or --differences flag will highlight the differences between successive updates. The --cumulative option makes highlighting "sticky", presenting a running display of all positions that have ever changed. [...]

watch will run until interrupted.

Note that "realtime" would have to be approximated by "once a second" (for example) here...

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Command Line – Output Common Lines of Two Text Files

How about using diff, even though you don't want a diff? Try this:

diff --unchanged-group-format='@@ %dn,%df 
  %<' --old-group-format='' --new-group-format='' \
  --changed-group-format='' a.txt b.txt

Here is what I get with your sample data:

$ cat a.txt 
Foo Bar
X
Hello
World
42
$ cat b.txt 
Foo Baz
Hello
World
23
$ diff --unchanged-group-format='@@ %dn,%df
%<' --old-group-format='' --new-group-format='' \
  --changed-group-format='' a.txt b.txt
@@ 2,3
Hello
World

Get latest output of a running command

I would use strace for that:

strace -qfp PID -e trace=write -e write=1,2

That will trace all write(2) system calls of PID and its child processes, and hexdump the data written to file descriptors 1 and 2.

Of course, that won't let you see what the process has already written to the tty, but will start monitoring all writes from a point on.

Also, strace is not amenable to change its output format -- you should explore using gdb(1) or write a small program using ptrace(2) if you need more flexibility.

Best Answer

Related Solutions

Command Line – Output Common Lines of Two Text Files

Get latest output of a running command

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