Is it possible to specify the order getopts condit

问题:

In a bash script, I'd like to load settings from a config file and override individual settings with command-line options. In cases where a setting is specified both in the config file and also on the command line, the command-line setting should take precedence.

How do you ensure the config file is loaded before the other getopts blocks? Here's what I've got:

#!/bin/bash
# ...

while getopts “c:l:o:b:dehruwx” OPTION
do
   case $OPTION in
      c)  
         echo "load"
         CONFIG_FILE=$OPTARG
         # load_config is a function that sources the config file
         load_config $CONFIG_FILE
         ;;  
      l)  
         echo "set local"
         LOCAL_WAR_FILE=$OPTARG
         ;;  

# ...

   esac
done
shift $(($OPTIND - 1))

No matter what order I put the handler for the -c option, it always loads the config file AFTER the other options are set. This makes it more of a pain to merge the config file settings with the command-line options.

回答1:

Each call to getopts always processes the "next" option (as determined by examining $OPTIND), so your while-loop will necessarily process the options in the order they appear.

Since you want -c to be partly superseded by other options, even if it appears after them on the command-line, there are a few approaches you can take.

One is to loop over the options twice:

#!/bin/bash
# ...

optstring='c:l:o:b:dehruwx'

while getopts "$optstring" OPTION
do
   case $OPTION in
      c)
         echo "load"
         CONFIG_FILE=$OPTARG
         # load_config is a function that sources the config file
         load_config $CONFIG_FILE
   esac
done

OPTIND=1

while getopts "$optstring" OPTION
do
   case $OPTION in
      l)  
         echo "set local"
         LOCAL_WAR_FILE=$OPTARG
         ;;
      # ...
   esac
done
shift $(($OPTIND - 1))

Another is to save options in variables that -c won't override, and then copy them over:

#!/bin/bash
# ...

while getopts c:l:o:b:dehruwx OPTION
do
   case $OPTION in
      c)
         echo "load"
         CONFIG_FILE=$OPTARG
         # load_config is a function that sources the config file
         load_config $CONFIG_FILE
         ;;  
      l)  
         echo "set local"
         LOCAL_WAR_FILE_OVERRIDE=$OPTARG
         ;;
      # ...
   esac
done
shift $(($OPTIND - 1))

LOCAL_WAR_FILE="${LOCAL_WAR_FILE_OVERRIDE-${LOCAL_WAR_FILE}}"

(Or, conversely, the config file can set options like LOCAL_WAR_FILE_DEFAULT, and then you'd write LOCAL_WAR_FILE="${LOCAL_WAR_FILE-${LOCAL_WAR_FILE_DEFAULT}}".)

Another option is to require that -c, if present, come first. You can do that by handling it first yourself:

if [[ "$1" = -c ]] ; then
   echo "load"
   CONFIG_FILE="$2"
   # load_config is a function that sources the config file
   load_config "$CONFIG_FILE"
   shift 2
fi

and then in your main while-loop, just handle -c by printing an error message.

Another is simply to document your existing behavior and call it a "feature". A lot of Unix utilities have later options supersede earlier ones, so this behavior isn't really a problem.

回答2:

Assuming your config file contains the default options for your program, you should always use those options by default unless they are overridden by their equivalent command-line options. This is reasonable. In your case, simply source/load the config file first and then parse command-line options - and assigning new values to them in the parseopts loop as needed.