Set default user variable in bash scripting

Question

I am writing a bash shell script, run where the user is expected to pass a path into it as the $1 variable. How can I determine if $1 is missing and define a default value instead for it?

Answer 1

You can check the length of the variable.

if [[ -z $1 ]]; then
    echo '$1 is zero-length. Please provide a value!'
fi

If you just want to use a default value, you can use a brace expansion.

first_param=${1:-defaultvalue}

The ${varname:-foo} construct will use the value of varname if it is set, or use what follows the :- if it is not set.

Answer 2

Do you mean detect if a value is missing, or if the directories in the path are missing? For the former:

MYPATH=$1
if [[ -z $MYPATH ]]
then
    MYPATH=$MYDEFAULTPATH
fi

for the latter:

MYPATH=$1
if [[ ! -d $MYPATH ]]
then
    MYPATH=$MYDEFAULTPATH
fi