Is there a tool to dynamically redirect output to a new file on request

Question

I'm currently redirecting the output of a monitoring tool to a file, however what I'd like to do, is to redirect this output to a new file on my request (using a keybinding), without stopping the said tool.

Something like

monitor_program | handle_stdout

Where handle_stdout allows me to define a new file where to put the log at certain point.

I know I could easily write it, but I'm wondering if there's any tool that already allows this.

Answer 1

I'll suggest a named pipe.

1. Create a pipe mkfifo p (call it whatever you want, if not 'p')

2. Create a "reader" script that reads from the pipe and writes wherever you like

3. Tell the monitoring program to write its logs to the named pipe

Here's a sample reader script that reads from a named pipe 'p' and writes the data to an indexed 'mylog' file:

#!/bin/sh

INDEX=0

switchlog() {
  read INDEX < newindex
  echo now writing to "mylog.$INDEX"
}

trap switchlog USR1

while :
do
  cat p >> mylog."$INDEX"
done

Answer 2

Building up on your SIGINT idea, here using SIGQUIT (Ctrl+\) to you can still use Ctrl+C to stop the whole thing:

(trap '' QUIT; monitor_command) | (
   trap : QUIT
   ulimit -c 0 # prevent core dump so SIGQUIT behaves like SIGINT
               # for cat
   n=0; while n=$((n+1)); file=output.$n.log; do
     printf 'Outputting to "%s"\n' "$file"
     cat > "$file"
   done)

That assumes cat is not a builtin in your shell (so it does get interrupted by you press Ctrl+\).

Note that like in your approach, there's a chance that the SIGQUIT be delivered at the wrong time (in the write system call) causing some data to be lost.

Answer 3

You could probably use less and save from there by typing s then the file name you want to save to, then Enter. From How do I write all lines from less to a file?.

Answer 4

Without knowing more about your "request" its not really possible to answer. If it were based on file size or interval, then rotatelogs (which should come bundled with Apache httpd) would work.

Answer 5

Thanks to Jeff Schaller's answer, I ended up with something like this, which basically does what I need, let's call it reader.sh:

#!/bin/sh

INDEX=0
LOGNAME="$INDEX.log"

switchlog() {
  local custom_name
  read -p "Add log name: " custom_name
  INDEX=$((INDEX+1))
  LOGNAME="$(printf "%03d" $INDEX).$custom_name.log"
  echo now writing to $LOGNAME
}

trap switchlog INT
switchlog

while :
do
  read foo < p
  printf "%s\n" "$foo" >> "$LOGNAME"
done

Then it's just about creating a named pipe with mkfifo p, and using two terminals where monitor_program > p and reader.sh are running. I can then stop the reader to set a new log by using Ctrl+C, and enter a new name. Ctrl+Z, as usual then to stop and kill it.