go_readrc

#! /bin/sh

# --- init ---

# leave on error
set -e


# --- external ---

basename='/bin/basename'
test ! -x ${basename}  &&  basename='/usr/bin/basename'

egrep='/bin/egrep'
test ! -x ${egrep}  &&  egrep='/usr/bin/egrep'

less='/bin/less'
test ! -x ${less}  &&  less='/usr/bin/less'
test ! -x ${less}  &&  less='/usr/local/bin/less'

sed='/bin/sed'
test ! -x ${sed}  &&  sed='/usr/bin/sed'


# --- definitions ---

prog=`${basename} $0`


# --- help ---

DisplayHelp ()
{
${xPAGER:-${less}} << EOF
$prog                     General Objects 

NAME
  $prog - read data value from a resource file
 
SYNOPSIS
  go_readrc <rcfile> <key> [<default>]
  go_readrc -h|--help
 
DESCRIPTION
  A recourcefile is a text file with key/data pairs, usefull
  to initialize programs (scripts, Fortran, etc).
  The format of the <rcfile> is chosen close to the standard X resources:

    * Comment lines start with '!'

    * A key/data pair has the format:
        <key> : <value>
      where the white space (space or tabs) is optional.
      The <key> consists of letters, numbers, '_', and '.' .
      
  Example of a valid rcfile:

    ! Specify an output directory:
    output.path : d/
  
  Given a text <key>, the <rcfile> is scanned for a line starting
  with this key; all text behind the ':' is written to the standard output. 
  Example of usage in sh script:
  
    output_root=\`go_readrc test.rc output.path\`
    
  If the <key> is not found, an error message is issued,
  unless a <default> is supplied which is then written to standard output.
  The <default> might be an empty string, e.g. '' .
  
PREPROCESSING
  The rcfile might be preprocessed by go_pprc,
  to expand environment variables.
    
EXIT STATUS
  Non zero in case of any error.
  
SEE ALSO
  X, go_pprc
  
AUTHOR
  Arjo Segers
EOF
exit 0
}

ErrorMessage ()
{
  echo "ERROR in $prog: $1" 1>&2
  echo "  Use '$prog -h' for information." 1>&2
  exit 1
}

# --- arguments ---

rcfile=''
rckey=''
with_default=''

while [ $# -gt 0 ]; do
  case "$1" in
    -h | --help )
      DisplayHelp
      ;;
    -* )
      ErrorMessage "unknown option '$1' ..."
      ;;
    * )
      if [ -z "${rcfile}" ]; then
        rcfile="$1"
      elif [ -z "${rckey}" ]; then
        rckey="$1"
      elif [ -z "${with_default}" ]; then
        default="$1"
        with_default='true'
      else
        ErrorMessage "unknown argument '$1'"
      fi
      ;;
  esac
  shift
done

if [ -z "${rcfile}" -o -z "${rckey}" ]; then
  ErrorMessage "missing arguments"
fi

# --- begin ---

# does the rcfile exist?
if [ ! -f ${rcfile} ]; then
  ErrorMessage "rcfile '${rcfile}' does not exist ..."
fi

# replace '.' in the rckey by '\.'
rckeydots=`echo ${rckey} | ${sed} -e 's/\./\\\\./g'`

# 10 Apr 06: Andy Jacobson
#   [[:space:]] indicates a space or tab character
#wspace='[[:space:]]*'
#
# 26 Apr 06: Arjo Segers
# The egrep on SGI system does not support the '[:space:]' ;
# use a real tab character instead ...
tab='	'
wspace="[ ${tab}]*"

# A key-data line has the following synopsis:
#
#    <begin-of-line><key>[<wspace>]:[<wspace>]<data>
#
# where <wspace> denote tabs or spaces.
# Set regular expression for such a line except the <data> part;
# this expression is used to search for a key and to extract
# the data part:
#
re="^${rckeydots}${wspace}:${wspace}"

# set grep command to select matching lines:
selectlinecmd="${egrep} '${re}' ${rcfile}"

# count number of hits; should be exactely 1 ...
nfound=`eval "${selectlinecmd}" | /usr/bin/wc -l`
if [ ${nfound} -eq 0 ]; then
  if [ -z "${with_default}" ]; then
    ErrorMessage "key '${rckey}' not found in ${rcfile} and no default specified ..."
  else
    echo "${default}"
    exit 0
  fi
elif [ ${nfound} -gt 1 ]; then
  ErrorMessage "key '${rckey}' found ${nfound} times in $rcfile ..."
fi

# extract the data part for this key;
# substitute an empty string for the 'key : ' part;
# remove trailing blanks;
# output is written to standard output:
eval "${selectlinecmd}" | ${sed} -e "s/${re}//" -e "s/${wspace}$//"