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How do I take optional parameters in my shell script.

Command Line argument would be: ./abc.sh parm1 parm2 parm3 parm4

Here parm3 and parm4 are optional depending upon parm1(config)

How to write a shell script which will take mandatorily parm1 and parm2 as argument and treat the other two as optional parameters

Prashant Singh
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  • I am new to shell and handling this scenario for the first time. – Prashant Singh Sep 01 '15 at 05:17
  • Don't forget to reject `./abc.sh p1 p2 p3 p4 p5` if, as your question indicates, only 2, 3 or 4 arguments are acceptable. You should know about `$#`, the special parameter (variable) that reports the number 'command line arguments' ('positional parameters' in the formal jargon). – Jonathan Leffler Sep 01 '15 at 06:12
  • No this is not a duplicate as this question is at its core about mandatory parameters (despite the title) whereas the other is about optional ones. – Paul Childs May 01 '19 at 02:27

1 Answers1

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Use ${var?} syntax for required parameters:

#!/bin/sh

: ${1:?first argument is required}
: ${2:?second argument is required}

You can check the number of argument with $#, so you can do things like:

#!/bin/sh
die() { echo "$@" >&2; exit 1; }
case ${1?} in
foo) test $# = 4 || die When first argument is foo, you must give 4 args;;
bar) test $# = 2 || die When first argument is bar, only give 2 args;;
*) ;;
esac
William Pursell
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    `test -n "$3"` will pass if the empty string is passed as an argument, so checking `$#` is the only way to handle optional values that could be empty. – chepner Sep 01 '15 at 12:00
  • Is there a reason you chose not to quote the arguments? That is, as you indicated, bad general practice and doesn't help clarify anything here. – Etan Reisner Sep 01 '15 at 12:06
  • I'd use `: ${1?first arg is required}` to display an error only if the parameter is missing altogether. Use `: ${1:?...}` if a null string is not permitted as the value of the argument. – chepner Sep 01 '15 at 13:09
  • @chepner: `test -n "$3"` fails when $3 is the empty string, and I'm making the tacit assumption that the empty string is unacceptable throughout. – William Pursell Sep 01 '15 at 16:05
  • @Etan I think the code is cleaner without quotes, and I'm being meticulous in using them where necessary. The only impact is in the possible difference with echo, and those statements are debug in nature and don't really impact anything. I believe that overquoting is as much of a problem as underquoting, and leads to a failure to learn when quotes are required and how they are used, so I err on the side of underquoting in answers on this forum. And now the echos with unquoted arguments have been removed, so that point is moot! – William Pursell Sep 01 '15 at 16:09
  • I agree about not quoting where quoting actually isn't necessary (like in assignments, the rhs of `=~`, etc.) but anywhere a variable is used normally I think quoting is appropriate (especially in a venue where failing to quote is a routine problem). But yes, the issue isn't present here anymore. – Etan Reisner Sep 01 '15 at 16:21