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3947 posts

Uber Geek

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# 260189 15-Nov-2019 09:23
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Hi GZ,


I'm banging my head against a wall trying to achieve this, so I'm crowd sourcing some help :-)


I need way to invert the exit code of a command, so that the $? exit code variable contains 0 for a failure and 1 or higher for success.


Using grep as an example (but the solution should work with anything, not just grep):


if /tmp/test.txt contains 10 lines, one of which is hello, grepping the file for hello will return 0, and return 1 if it didn't contain it.



ubuntu:/tmp$ grep hello test.txt;echo $?
ubuntu:/tmp$ grep hesllo test.txt;echo $?



I need to invert that, and the solution needs to work with a single command line rather than a shell script.


I've tried things like grep hello test.txt && false || true, using (exit 1) instead of true/false, using | [ $? -eq 0 ] etc without much luck.


I need this for AWS cfn-init command tests. From the documentation,


A test command that determines whether cfn-init runs commands that are specified in the command key. If the test passes, cfn-init runs the commands. The cfn-init script runs the test in a command interpreter, such as Bash or cmd.exe. Whether a test passes depends on the exit code that the interpreter returns.


For Linux, the test command must return an exit code of 0 for the test to pass



E.g. if I want an init command that adds an entry to a file, I need the test to return 0 if the file doesn't have that entry, and 1+ if it does. If I don't have a test that works like this, it won't be idempotent, which is pretty vital.









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28 posts


  # 2353744 15-Nov-2019 09:38
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How about


if grep hello test.txt ; then false ; else true ; fi



120 posts

Master Geek

  # 2353760 15-Nov-2019 09:45
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Just use negate operator before the command.


~$ grep blah examples.desktop; echo $?


~$ ! grep blah examples.desktop; echo $?




3947 posts

Uber Geek

Lifetime subscriber

  # 2353765 15-Nov-2019 09:56
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Damn that was fast.. thanks folks :-)

Information wants to be free. The Net interprets censorship as damage and routes around it.

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