Execute command when grep finds a match

If I understand your question correct (the "every once in a while grep finds a match" part), your program produces the positive pattern several times during its longish runtime and you want script.sh run at those points in time .

When you pipe program to grep and watch on greps exit code with &&, then you execute script.sh only once and only at the end of programs runtime.

If my assumptions are correct (that you want script.sh run several times), then you need "if then logic" behind the | symbol. One way to do it is using awk instead of grep:

program | awk '/pattern/ {system("./script.sh")}'

• /pattern/ {system("./script.sh")} in single quotes is the awk script
  • /pattern/ instructs awk to look for lines matching pattern and do the instructions inside the following curly braces
  • system("./script.sh") inside the curly braces behind the /.../ executes the script.sh in current directory via awks system command

The overall effect is: script.sh is executed as often as there are lines matching pattern in the output of program.

Another way to this is using sed:

program | sed -n '/pattern/ e ./script.sh'

• -n prevents sed from printing every line in programs out put
• /pattern/ is the filter criteria for lines in programs output: lines must match pattern
• e ./script.sh is the action that sed does when a line matches the pattern: it executes your script.sh

Use -q grep parameter and && syntax, like this:

program | grep -q "pattern" && sh script.sh

where:

-q: Runs in quiet mode but exit immediately with zero status if any match is found

&&: is used to chain commands together, such that the next command is run if and only if the preceding command exited without errors (or, more accurately, exits with a return code of 0). Explained here.

sh script.sh: Will run only if "pattern" is found

Equivalent to @masterguru's answer, but perhaps more understandable:

if program | grep -q "pattern"
then
    # execute command here
fi

grep returns a zero exit status when it finds a match, so it can be used as a conditional in if.