Bash script to detect the version control system by testing command return status

bashexit-statusshell-script

I am working on a bash script that I would like to work for several types of VCS.
I am thinking of testing if a directory is a repo for a system by running a typical info command and checking the return code, success or error.
In pseudo code:

if a svn command succeded;
    Then run svn commands
elif a darcs command succeded;
    Then run darcs commands
elif a mercurial command succeded;
    then run hg commands
else 
    something else
fi

I can run a command, e.g.
darcs show repo and use $? to get its return code.

My question is: is there a neat way to run and return the return code number in one line? for example

if [ 0 -eq `darcs show repo`$? ];

Or do I have to define a function?

An added requirement is that both stderr and stdout should be printed.

Best Answer

If automatically checks the return code:

if (darcs show repo); then
  echo "repo exists"
else
  echo "repo does not exist"
fi

You could also run the command and use && (logical AND) or || (logical OR) afterwards to check if it succeeded or not:

darcs show repo && echo "repo exists"
darcs show repo || echo "repo does not exist"

Redirecting stdout and stderr can be done once with exec

exec 6>&1
exec 7>&2
exec >/dev/null 2>&1

if (darcs show repo); then
  repo="darcs"
elif (test -d .git); then
  repo="git"
fi

# The user won't see this
echo "You can't see my $repo"

exec 1>&6 6>&-
exec 2>&7 7>&-

# The user will see this
echo "You have $repo installed"

The first two exec are saving the stdin and stderr file descriptors, the third redirects both to /dev/null (or somewhere other if wished). The last two exec restore the file descriptors again. Everything in between gets redirected to nowhere.

Append other repo checks like Gilles suggested.

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