Adding default to bash script variable

Question

I have a bash script that uses $1 to process a command line argument.

I want to modify this script to work even when a command line argument isn't given; in that case I want the script to use a default value.

I don't know how to do this; basically I figure I need to replace $1 with my own variable and have a line at the start of the program that checks whether a value was passed for $1, and if not to use the default that I'll provide. But I don't know the syntax for that. Can anyone help me?

Community · Accepted Answer · 2017-05-23 10:26:47Z

7

Use parameter expansion:

var=${1:-default}

From the given link:

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

See an example:

$ echo "$v"

$ t=${v:-hello}
$ echo "$t"
hello
$ v=2
$ t=${v:-hello}
$ echo "$t"
2

And note also that ${var:-value} and ${var-value} are not the same: What is the difference between ${var:-word} and ${var-word}?.

edited May 23, 2017 at 10:26

CommunityBot

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answered Jul 2, 2015 at 10:49

fedorqui

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Comments

Alfe · Accepted Answer · 2015-07-02 10:55:02Z

1

Try:

echo "${1:-"my default"}"

in your script (e. g. foo.sh) to see the effect of the :- modifier.

Notice that in case you pass an empty string to your script, also the default kicks in:

$ ./foo.sh ""
my default

So you cannot distinguish the empty string from a not-given argument this way.

In case you need to have that distinction, you should rely on $# to tell you the number of arguments:

if [ $# = 0 ]
then
  echo "my default"
else
  echo "$1"
fi

=>

$ printf "[%s]\n" "$(./foo.sh "")"
[]

edited Jul 2, 2015 at 10:55

answered Jul 2, 2015 at 10:49

Alfe

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5 Comments

Alfe Over a year ago

Actually, I didn't know about ${a-b} yet and would consider it superior to using $#.

123 Over a year ago

@cdarke so does [ $# -eq 0 ]

Alfe Over a year ago

@cdarke Absolutely correct. In these cases I prefer the first as it results in the same, is probably cheaper, is less to type, etc. But as soon as comparisons like (( $# > 5 )) are involved, the numeric syntax is probably nicer to read.

cdarke Over a year ago

@User112638726 true, old Bourne shell syntax. But = was used, not -eq.

Alfe Over a year ago

I always considered -eq, -lt, etc. a clutch and a second-best solution. Using =, >=, etc. help with their visual to prevent errors, so I prefer them if possible.

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