Re: [Help-bash] How to understand the end part

[Eric Blake]

On 01/02/2012 07:32 AM, lina wrote:
> Hi,
> 
> $ stringZ=abc123; echo `expr "$stringZ" : '.*' `

Your use of "echo `command`" is wasteful; almost anywhere you ever see
that pattern, you can be more efficient by just executing the command
directly (the only benefit from echoing the output of a command
substitution is whitespace normalization, as well as file name splitting
if the output happens to include glob characters since you didn't
enclose your `` in ""):

stringZ=abc123; expr "$stringZ" : '.*'

> 
> I don't know how to understand the : '.*' part,
> 
> Thanks for any explaination,

info expr

Or read the POSIX specification:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/expr.html

In expr, the ':' operator says to locate a match within the first string
according to the regular expression of the second string; with no
sub-expressions, the result is the number of characters that matched.
And since .* matches everything, that is a common idiom for  using expr
to compute the length of a string.

As long as you are okay assuming POSIX, your above expression is done
more efficiently without using 'expr' in the first place, as:

stringZ=abc123; echo ${#stringZ}

But if you care about portability to old shells like Solaris /bin/sh,
which lack the support for POSIX ${#var}, then expr is the portable
alternative.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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