UNIX for Dummies Questions & Answers

I am new to this forum; actually, this is the first forum I've ever joined of any kind. Looking forward to using it.

I know some basic UNIX, but this is over my head. Can someone tell me what this is doing?

export MY_ROOT=${MY_ROOT:-$(expr "$0" : '\(.*\)/bin/.*')}

Thanks!

First look at the expr command. It is using $0 which is the name of the script. But I'll try a few examples using $x ...

Code:

$ x=/something/bin/script
$ expr "$x" : '\(.*\)/bin/.*'
/something
$ x=/bin/script
$ expr "$x" : '\(.*\)/bin/.*'

$

So it is capturing what if anything precedes /bin in the value of $x (or $0 in your case). It is sitting in a $( ) which captures the output of a command. As in:
echo the password file has $(wc -l < /etc/passwd) lines in it.

So finally we have (in effect)

MYROOT=${MYROOT:-something}

If MYROOT has a value we use that value. Otherwise we use "something".