I have a program that continuously runs and is piped into grep and every once in a while grep finds a match. I want my bash script to execute when grep finds the match.
Like this:
program | grep "pattern"
grep outputs -> script.sh executes
3 Answers
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 matchingpatternand do the instructions inside the following curly bracessystem("./script.sh")inside the curly braces behind the/.../executes thescript.shin current directory via awkssystemcommand
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'
-nprevents sed from printing every line inprograms out put/pattern/is the filter criteria for lines inprograms output: lines must matchpatterne ./script.shis the action that sed does when a line matches the pattern: it executes yourscript.sh
Comments
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
2 Comments
ASB
I understand the original question asks about continuous monitoring the output of program, so a single execution of grep and script.sh is not enough. Lars Fischer answer bellow addresses the continuous execution.
masterguru
@ASB, you right, I didn't catch that, thanks for clarifying.
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.
Comments
grepresults into a variable, then use bash's built in syntax forifto conditionally trigger another command