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

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.

As others have already mentioned, if command tests whether command succeeds. In fact [ … ] is an ordinary command, which can be used outside of an if or while conditional although it's uncommon.

However, for this application, I would test the existence of the characteristic directories. This will be correct in more edge cases. Bash/ksh/zsh/dash version (untested):

vc=
if [ -d .svn ]; then
  vc=svn
elif [ -d CVS ]; then
  vc=cvs
else
  d=$(pwd -P)
  while [ -n "$d" ]; do
    if [ -d "$d/.bzr" ]; then
      vc=bzr
    elif [ -d "$d/_darcs" ]; then
      vc=darcs
    elif [ -d "$d/.git" ]; then
      vc=git
    elif [ -d "$d/.hg" ]; then
      vc=hg
    fi
    if [ -n "$vc" ]; then break; fi
    d=${d%/*}
  done
fi
if [ -z "$vc" ]; then
  echo 1>&2 "This directory does not seem to be under version control."
  exit 2
fi

Well, it's not very pretty, but it's one way to do it inline:

if darcs show repo > /dev/null 2>&1; then <do something>; fi

By definition, if tests the exit code of a command, so you don't need to do an explicit comparison, unless you want more than success or failure. There's probably a more elegant way to do this.