UNIX for Dummies Questions & Answers

Hi,

Sorry for a dummy question I believe. I am just wanting to know how do I assign a default $1 argument if one is not provided.

At the moment, I am doing something like below:

Code:

arg1="${1:-foo}"

And then I check $arg1 in case/esac. I am just wondering if there is a way for me to simply do ${1:-foo} and check $1 in case/esac.

At the moment, if I simply do ${1:-foo} and run the script, it seem to be trying to run the foo command instead. Script is working fine just curious for whether I can set $1 to a default value without having to assign it to a variable.

FYI, the script is in ksh.

Thanks in advance.

You don't need to assign to a variable. Anywhere in your script you can use ${1:-foo} in lieu of $1.

While this works well on short script, with just a single invocation of it, but as soon there are more than one occourence of this syntax, you'll need to change all of them rather than just a single variable assignment.

---------- Post updated at 18:38 ---------- Previous update was at 18:33 ----------

But anyway, have a look at:

Code:

echo "1:$1 2:$2 3:$3"
set 15 30 45
echo "1:$1 2:$2 3:$3"
shift
echo "1:$1 2:$2 3:$3"

Have fun

In fact you are doing it more right then wrong: declare (sensible) default values for variables not explicitly stated at the commandline. You might want to explore the "getopts" command in ksh for passing certain values to a script, which works mostly the same way as the "getopt" OS command.

In short: not at all! You see, "$1" is the first parameter passsed on the commandline and if this is empty, it is empty. Analogous for "$2", "$3", etc..

Consider the OS command line:

Code:

command abc 123 def

The command "command" is passed "abc" as the first parameter, "123" as the second and "def" as the third. If "command" wouldl be a ksh script "$1" would be set to "abc" and so on. If you pass fewer parameters then less of these variables are set. Otherwise, if you pass more then "$4" and so on also will have values.

But usually you want parameters to have a certain meaning and ideally this meaning is not dependent on the place where the variable is passed:

Code:

command file1 file2

If the first parameter passed is a configuration file and the second one an output destination this:

Code:

command file2 file1

would not only fail but probably first try to read file2 as configuration, then destroy your real configuration in file1 by overwriting it. Better would perhaps be:

Code:

command -c configfile -o outputfile

because what is config and what is output will be stated by the option preceeding it. These two calls:

Code:

command -c configfile -o outputfile
command -o outputfile -c configfile

will both work the same. This mechanism is indeed standardized and "getopts" is a command to standardizedly parsing your commandline. Here is code for the above example

Code:

#! /bin/ksh

typeset chOpt=""

while getopts "o:c:" chOpt ; do                  # process commandline
     
    case "$chOpt" in
          c)                                       # set config file
               fConfig="$OPTARG"
               ;;

          o)                                       # set output file
               fOutput="$OPTARG"
               ;;

     esac
done

This leaves just one problem: what if the user does not set one of the necessary options? You can either do it like you already did: assign a default value in case the values is not already set. This could be done via adding a last line in the code above with your construct. The other way is to set the default before:

Code:

#! /bin/ksh

typeset fOutput="/my/default/output"
typeset fConfig="/default/config.file"
typeset chOpt=""

while getopts "o:c:" chOpt ; do                  # process commandline
     
    case "$chOpt" in
          c)                                       # set config file
               fConfig="$OPTARG"
               ;;

          o)                                       # set output file
               fOutput="$OPTARG"
               ;;

     esac
done

print - "final config file is: $fConfig"
print - "final output file is: $fOutput"

I suggest you try this script with various invocations, with and without one or all parameters.

Hint: robust script programming would mean to actually test files for existence/non-existence and issue warnings about an output file that already exists or a config file which doesn't (or is not readable). I have skipped that here to make the example as simple as possible.

I hope this helps.

bakunin