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bashwatchtee

Use multiple tee commands redirections in a watch call

The following command processes the output of the pipe twice by using tee:

echo -e "ALPHA\nBRAVO" | tee >(head -n 1) >(tail) >/dev/null

As expected it outputs:

ALPHA
ALPHA
BRAVO

When trying to call it with watch like this:

watch 'echo -e "ALPHA\nBRAVO" | tee >(head -n 1) >(tail) >/dev/null'

It returns:

sh: -c: line 0: syntax error near unexpected token `('
sh: -c: line 0: `echo -e "ALPHA\nBRAVO" | tee >(head -n 1) >(tail) >/dev/null'

How should I escape my command to use it with watch?

Solution

  • Process substitutions are an extension, not all sh implementations support them. You can use redirections to circumvent this restriction though. Like

    watch '{ { printf '\''ALPHA\nBRAVO\n'\'' |
           tee /proc/self/fd/3 |
           head -n 1 >&4
         } 3>&1 | tail >&4
       } 4>&1'

    Just note that this is no more portable than doing watch 'bash -c ...'.