Consider the example recfile acquaintances.rec introduced earlier.
It contains names of people along with their respective ages.
Suppose we want to get a list of the names of all the children.
It would not be easy to do this using
Neither would it, for any reasonably large recfile, be feasible to search
manually for the children.
recsel command provides an easy way to do
such a lookup:
$ recsel -e "Age < 18" -P Name acquaintances.rec Bart Simpson Adrian Mole
Let us look at each of the arguments to
recsel in turn.
Firstly we have
-e which tells
recsel to lookup records
matching the expression
Age < 18 — in other words all those people
whose ages are less than 18.
This is an example of a selection expression.
In this case it is a simple test, but it can be as complex as needed.
Next, there is
-P which tells
recsel to print out the value of
Name field — because we want just the name, not the entire record.
The final argument is the name of the file from whence the records are
to come: acquaintances.rec.
Rather than explicitly storing ages in the recfile, a more realistic example might have the date of birth instead (otherwise it would be necessary to update the people’s ages in the recfile on every birthday).
# Date of Birth %type: Dob date Name: Alfred Nebel Dob: 20 April 2010 Email: email@example.com Name: Bertram Worcester Dob: 3 January 1966 Email: firstname.lastname@example.org Name: Charles Spencer Dob: 4 July 1997 Email: email@example.com Name: Dirk Hogart Dob: 29 June 1945 Email: firstname.lastname@example.org Name: Ernest Wright Dob: 26 April 1978 Email: email@example.com
Now we can achieve a similar result as before, by looking up the names of all those people who were born after a particular date:
$ recfix acquaintances.rec $ recsel -e "Dob >> '31 July 1994'" -p Name acquaintances.rec Name: Alfred Nebel Name: Charles Spencer
>> operator means “later than”, and is used
here to select a date of birth after 31st July 1994.
Note also that this example uses a lower case
-p whereas the preceding example
used the upper case
-P. The difference is that
-p prints the field name
and field value, whereas
-P prints just the value.
recsel accepts more than one
each introducing a selection expression,
in which case the records which satisfy all expressions are selected.
You can provide more than one field label to
-p in order to select
additional fields to be displayed.
For example, if you wanted to send an email to all children 14 to 18
years of age,
and today’s date were 1st August 2012, then you could use the following command to get
the name and email address of all such children:
$ recfix acquaintances.rec $ recsel -e "Dob >> '31 July 1994' && Dob << '01 August 1998'" \ -p Name,Email acquaintances.rec Name: Charles Spencer Email: firstname.lastname@example.org
As you can see, there is only one such child in our record set.
Note that the example command shown above contains both double quotes
The double quotes are interpreted by the shell (e.g.
the single quotes are interpreted by
recsel, defining a
string. (And the backslash is interpreted by the shell, the usual
continuation character so that this manual doesn’t have a too-long line.)