Shell Parameter Exapnsion
In this lesson, we'll see how shell parameter expansions can be used to simply expand a variable's valuable and also provide a default value to a variable, if not set. Note that there are many more possibilities with shell parameter expansions, so check bash's documentation to view them all.
It is same when you doing:
echo $USER
## or
echo ${USER}
${}
is called shell parameter expansion.
It is useful when you want to print as such:
echo $USER_$(date '+%Y')
Expected result was JOHN_2021
. But it just print John
.
That is because it doesn't know $USER_
.
To fix the issue, we can do:
${USER}_($date '+%Y')
Then we get the correct result.
Default value
echo ${str:-'default'}
It prints default
because $str
doesn't exist.
Example
Count files under dir:
nano count-files.sh
count-files.sh:
dir=${1:-$PWD} ## default to current dir
find "$dir" -type f -maxdepth 1 | wc -l