Pattern matching
To convert a .tsv to a .csv file, you can use the xsv
tool, with the command:
xsv fmt -d '\t' file.tsv > file.csv
We can write the same in our makefile:
file.csv: file.tsv
xsv fmt -d '\t' file.tsv > file.csv
We can make it shorter with automatic variables:
file.csv: file.tsv
xsv fmt -d '\t' $< > $@
However, since we can do this for any .tsv file, we can write a generic rule:
%.csv: %.tsv
xsv fmt -d '\t' $< > $@
The % is a wildcard, working similarly to a shell *: it will try to match
the longest string it can, matching any character.
If the resulting full strings exactly match, the rule applies to that case.
A generic rule must have one and only one
%in the target. There can be one and only one%in each of the requirements.
In this case, any file ending in .csv can be created from a file (in the same
folder) ending in .tsv:
path/to/file.csvfrompath/to/file.tsv-> OK!;file.csvfrompath/to/file.csv-> NO! The files do not share the same stem;file.csvfromfile.tsv.gz-> NO! The files do not share the same suffix;
Here, using automatic variables is not optional. Since we do not know what
%will be replaced with at runtime, we cannot write static filenames. Using%in the body of the rule is not supported.
Make will use the generic rule whenever it has to, but not more than that.
This means that you cannot have make create all possible .csv files from
all .tsv files by just writing the generic rule above.
You need to specifically ask for .csv files as requirements to have make
create them.
Assume that you have the one.tsv and two.tsv files and you want to convert
them in .csv.
You could write this makefile:
default: one.csv two.csv
# The above rule has no body, but it's ok! Make will just do nothing when
# the rule is executed.
%.csv: %.tsv
xsv fmt -d '\t' $< > $@
When you call make, it will use the generic rule twice to create the
prerequisites of default, as if you had written:
default: one.csv two.csv
one.csv: one.tsv
xsv fmt -d '\t' $< > $@
two.csv: two.tsv
xsv fmt -d '\t' $< > $@