How to avoid calling external utility (grep) twice while maintaining return code & output?

Question

I have following Bash function which return property value from Java-style property file. It property wasn't found, it should return non-zero. If found, property's value will be printed & return code must be zero.

function property_get() {
    local pfile="$1"
    local pname="$2"
    if egrep "^${pname}=" "$pfile" 2>&1 >/dev/null; then
        local line="$(egrep "^${pname}=" "$pfile")"
        printf "${line#*=}"
        return 0 # success
    else
        return 1 # property not found
    fi
}

The question is: how to avoid from calling egrep twice? First exec is for status code, 2nd is for property value. If I use $(grep parameters) notation, then grep will be launched in subshell and I can't get it's return code and won't be able to determine success or failure of property searching.

@matt that's looks like indirect check for me. grep already report it's success or failure by providing us return code and I would like to get advantage of that fact — Andrey Brindeyev, Jun 29 '16 at 09:21
Something like `$?`, as per this [how to check the exit status](http://linuxcommando.blogspot.co.uk/2008/03/how-to-check-exit-status-code.html)? — matt, Jun 29 '16 at 09:26
@matt it won't work with `$()`. I need to find a way to capture both exit code from grep & it's output from single grep invocation — Andrey Brindeyev, Jun 29 '16 at 09:28
@AndreyBrindeyev Yes it will work. See this: http://stackoverflow.com/questions/20157938/bash-exit-code-of-variable-assignment-to-command-substitution — Julien Lopez, Jun 29 '16 at 09:35
@julien-lopez well, it certainly failing for me: https://gist.github.com/abrindeyev/10a57177b9181dc82f30518a580f1885 — Andrey Brindeyev, Jun 29 '16 at 09:52
Did you see the difference. It is the local command as stated in Julien's answer. — matt, Jun 29 '16 at 12:38

score 2 · Answer 1 · answered Jun 29 '16 at 11:06

2

This should work:

...
local line
if line=$(egrep "^${pname}=" "$pfile" 2>/dev/null); then
...

answered Jun 29 '16 at 11:06

mouviciel

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score 0 · Answer 2 · edited May 23 '17 at 11:52

As @matt points out, you can just get the line then check the exit status of your command:

line=$(...)
if test $? -eq 0; then
  # success
  ...
else
  # property not found
  ...
fi

Edit:

To summarize:

You can use var=$(cmd), to set var to the standard output of cmd and $? to the exit status of cmd
With local var=$(cmd), $? will be set to the exit status of local which is 0 as explained here: Why does "local" sweep the return code of a command?

How to avoid calling external utility (grep) twice while maintaining return code & output?

2 Answers2