Bash Scripting - Parsing Options

bman

Hi lads,

What's the best and / or easiest way to parse options passed through to a bash script? I've had a look at getopt(s) but not sure how to separate the options from the regular arguments easily.

For example, how would I separate the following args:

./myscript -n -ab someparam -c someoption

into

a=true
b=true
n=true
c="someoption"
myparam="someparam"

I would imagine this to be a pretty common task.

Any help appreciated.

Naikon

bman wrote: »

Hi lads,

What's the best and / or easiest way to parse options passed through to a bash script? I've had a look at getopt(s) but not sure how to separate the options from the regular arguments easily.

For example, how would I separate the following args:

./myscript -n -ab someparam -c someoption

into

a=true
b=true
n=true
c="someoption"
myparam="someparam"

I would imagine this to be a pretty common task.

Any help appreciated.

getopt(s) and such have their uses, but for general shell tasks, it's easier
to just allow the shell to work it's stuff. It's more automated than you think.

Like in C programming when you run the script a runtime array is set up
for the script to deal with command line args.

For reference, say an array called *argv[] (pointer to string array) is used.
argv[0] is the name of the script being run, argv[1] is the first command line
argument. So argv[2] would corrospond to the second string.

The great thing about the shell is the notation to access the parameters is
quite simple. Instead of argv[1] like in C, you simply refer to the element
with $1 which is the first argument. $0 is always the name of the script.

Quick breakdown:

$# number of arguments passed as an integer
$@ returns each argument string as seperate strings
$* returns each argument string as one single concatenated string
$0 name of script being executed
$1 first argument
$2 second argument

So, c=$5 for example. things like "-n" are treated as params with
increasing int values up to 9. There are workarounds for more.

The special variables are handy if you want to do something like:
echo "the number of parameters passed is $#"

ravydavygravy

I never looked into it for bash - I normally just expect a certain number of args, and display a usage message if the number isn't correct - not exactly what you want.

if [ $# -ne 2 ]
then
    echo 
    echo "Usage: myscript.sh <arg1> <arg2>"
    echo "   where"
    echo "       <arg1> is blah..."
    echo "       <arg2> is blah..."
    echo
 
    exit 1
fi

Tomorrow in work, I can check a few scripts that do this better than this...

EDIT: this any help? http://www.linux.com/archive/feature/118031

Naikon

Just to note, if you pass in params in this fashion:

./script -al -cy -zx

you will need to use getopt to seperate the options following
the dash. That or you come up with your own logic for parsing.

./script -a -l -c -y -z -x may be better for portability, or if you
just want to avoid getopts(s) alltogether until you need more
sophistication.

Capt'n Midnight

just like to mention in passing that parsing batch file scripting in windows is truly painful , Iv'e used sed instead because it well nigh impossible to handle stuff like file names unless you resourted to 8.3 and even then it was hairy.

Naikon

Capt'n Midnight wrote: »

just like to mention in passing that parsing batch file scripting in windows is truly painful , Iv'e used sed instead because it well nigh impossible to handle stuff like file names unless you resourted to 8.3 and even then it was hairy.

Cygwin is your friend;) Not really a 100% reliable method though.
I suppose PowerShell should alleviate these problems. At least in theory.

bman

Thanks for all the replies.

I understand the basics of bash scripting and how $#, $*, $0, $1, etc. work; just don't want to have to start checking all the parameters to see if they start with a "-" and all the other messing required to break up the options.

I'll mess about with getopt(s) and see what I come up with. I'm sure it'll only take a few lines of code anyway, but I hate reinventing the wheel. Just that this seems like something people would want to do a lot of the time.

It's only for a small script for my own use anyway so worse comes to worse I'll hardcode it and it'll be a bit flaky but work for me .

Skrynesaver

Running the following:

#! /bin/bash

while getopts "abc:n" opt; do
   case $opt in
      a )   a="true";;
      b )   b="true";;
      c )   c=$OPTARG;;
      n )   n="true";;
   esac
done

shift $(($OPTIND - 1)) # remove all options from the argument array

echo "a is $a
b is $b
c is $c
n is $n
remaining args are $*";

gives:

~$ ./test.sh -abc setting -n  filename otherfile directory
a is true
b is true
c is setting
n is true
remaining args are filename otherfile directory

Limitation is that options MUST preceed operands on the commandline ie

~$ test.sh filename -a will fail to set $a

So having someparam among the options would require writing your own parser:(

bman

Skrynesaver wrote: »

Running the following:

#! /bin/bash

while getopts "abc:n" opt; do
   case $opt in
      a )   a="true";;
      b )   b="true";;
      c )   c=$OPTARG;;
      n )   n="true";;
   esac
done

shift $(($OPTIND - 1)) # remove all options from the argument array

echo "a is $a
b is $b
c is $c
n is $n
remaining args are $*";

gives:

~$ ./test.sh -abc setting -n  filename otherfile directory
a is true
b is true
c is setting
n is true
remaining args are filename otherfile directory

Limitation is that options MUST preceed operands on the commandline ie

~$ test.sh filename -a will fail to set $a

So having someparam among the options would require writing your own parser:(

Exactly what I was looking for. The shift $(($OPTIND -1)) command is what I needed. Will rememeber that for again.

Having to have the regular params after the options is something I can live with .