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Asked: 2017-08-25 05:31:29 +0800 CST

How to evaluate grep result in bash?

  • 772

I want to grep a specific status of a service (tomcat8.service).

Only if the string was found, I want to execute some logic.

Problem: even if I execute the script on service name that does not exist ("asd" in this example), the if $status still matches and prints out. But why?

status = $(systemctl status asd.service | grep 'active')
echo $status

if $status
then 
    echo "$SERVICE was active"
    exit 0
fi 
exit 0

Result output is: asd.service was active, which is certainly not true.

The echo $status prints: status: Unknown job: =

bash

2 Answers

  1. Best Answer

    You can make use of grep's return status.

    systemctl status asd.service | grep 'active' \
    && status=active \
    || status=not_active

if [ "$status" == "active" ]; then
    [...]
fi

    or even easier:

    test $(systemctl status asd.service | grep 'active') \
    && echo "$SERVICE was active"

    or if you prefer if:

    if $(systemctl status asd.service | grep 'active'); then
    echo "$SERVICE was active"
fi

    Anyways, take care about the keywords inactive, not active, active (exited) or alike. This will also match your grep statement. See the comments. Thanks @ Terrance for the hint.

    Update:

    No need for grep. systemctl has the command is-active included.

    systemctl -q is-active asd.service \
    && echo "$SERVICE was active"

    or:

    if systemctl -q is-active asd.service; then
    echo "is active";
fi
    • 3

  2. Some code review comments:

    • sh/bash/ksh/zsh variable assignments look like var=value -- no spaces around the = are allowed. (documentation)
    • status=$(some command) -- the status variable hold the output of the command, not the exit status. The exit status is in the $? variable

    • the if statement acts on the exit status of the subsequent command (documentation)

      if some_comment; then action1; else action2; fi

      Most often, the command is [ or [[ to test some condition.

      However, grep has a clear exit status: 0 if the pattern was found, 1 otherwise. So you want this:

      if systemctl status asd.service | grep -q 'active'; then
    echo "$SERVICE was active"
fi
    • 1