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?

Daenyth · Accepted Answer · 2010-09-28 16:40:23Z

9

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.

edited Sep 28, 2010 at 16:40

answered Sep 28, 2010 at 16:33

Daenyth

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1 Comment

Andrew Over a year ago

Useful to know: ${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null. tldp.org/LDP/abs/html/parameter-substitution.html

iniju · Accepted Answer · 2010-09-28 16:44:09Z

1

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

edited Sep 28, 2010 at 16:44

answered Sep 28, 2010 at 16:36

iniju

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3 Comments

Daenyth Over a year ago

Since he mentions bash specifically, you should be using [[ over [

Elpezmuerto Over a year ago

Why is [[ more appropriate than [ ?

Daenyth Over a year ago

@Elpezmuerto: You can find that on the bash wiki at FAQ #31

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