What is the difference between filename=${1:-/etc/hosts} and filename=/etc/hosts?

Question

What is the difference between filename=${1:-/etc/hosts} and filename=/etc/hosts?

For example:

filename=/etc/hosts

if [ -r "$filename" ] && [ -s "$filename" ]; then
    md5sum $filename
else
    echo "$filename cannot be processed"
fi

and

filename=${1:-/etc/hosts}

if [ -r "$filename" ] && [ -s "$filename" ]; then
    md5sum $filename
else
    echo "$filename cannot be processed"
fi

score 10 · Answer 1 · edited Jun 29 '14 at 12:03

10

filename=${1:-/etc/hosts} assigns value /etc/hosts to variable filename if $1 is not set or null.

${parameter:-word}
If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

edited Jun 29 '14 at 12:03

Mat

answered Jun 29 '14 at 11:57

cuonglm

Which is to say that unless first param $1 sent to it, it wiöl default to /etc/hosts – Petter H Jun 29 '14 at 11:59
@PetterH: No, if you set $1 to null, /etc/hosts is assigned to filename. – cuonglm Jun 29 '14 at 12:05
@PetterH: I mean, if you try: ./yourscript "" 2 3, filename is set to /etc/hosts even if you had pass $1 to script. – cuonglm Jun 29 '14 at 12:23
Right you are! I did not concider that. – Petter H Jun 29 '14 at 12:24

1 Answers1