BASH shell scripting tip: Set default values for variable

A shell variable may be assigned to by a statement using following syntax:
var=value

If value is not given, the variable is assigned the null string. In shell program it is quite useful to provide default value for variables. For example consider rsync.sh script:
#!/bin/bash
RSRC=$1
LOCAL=$2
rsync -avz -e 'ssh ' user@myserver:$RSRC $LOCAL

This script can be run as follows:
$ ./rsync.sh /var/www .
$ ./rsync.sh /home/vivek /home/vivek


It will sync remote /home/vivek directory with local /home/vivek directory. But if you need to supply default values for a variable you can write as follows:

#!/bin/bash
RSRC=$1
LOCAL=$2
: ${RSRC:="/var/www"}
: ${LOCAL:="/disk2/backup/remote/hot"}
rsync -avz -e 'ssh ' user@myserver:$RSRC $LOCAL

: ${RSRC:="/var/www"} ==> this means if the variable RSRC is not already set, set the variable to /var/www. You can also write same statement with following code:

if [ -z "$RSRC" ]
then
   RSRC="/var/www"
fi

You can also execute a command and set the value to returned value (output). For example if the variable NOW is not already set, execute command date and set the variable to the todays date using date +"%m-%d-%Y":

#!/bin/bash
NOW=$1
....
.....
: ${NOW:=$(date +"%m-%d-%Y")}
....
..

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