Running cog

Cog is a command-line utility which takes arguments in standard form.

$ cog -h
cog - generate content with inlined Python code.


INFILE is the name of an input file, '-' will read from stdin.
FILELIST is the name of a text file containing file names or
    other @FILELISTs.

    -c          Checksum the output to protect it against accidental change.
    -d          Delete the generator code from the output file.
    -D name=val Define a global string available to your generator code.
    -e          Warn if a file has no cog code in it.
    -I PATH     Add PATH to the list of directories for data files and modules.
    -n ENCODING Use ENCODING when reading and writing files.
    -o OUTNAME  Write the output to OUTNAME.
    -p PROLOGUE Prepend the generator source with PROLOGUE. Useful to insert an
                import line. Example: -p "import math"
    -P          Use print() instead of cog.outl() for code output.
    -r          Replace the input file with the output.
    -s STRING   Suffix all generated output lines with STRING.
    -U          Write the output with Unix newlines (only LF line-endings).
    -w CMD      Use CMD if the output file needs to be made writable.
                    A %s in the CMD will be filled with the filename.
    -x          Excise all the generated output without running the generators.
    -z          The end-output marker can be omitted, and is assumed at eof.
    -v          Print the version of cog and exit.
    --check     Check that the files would not change if run again.
    --markers='START END END-OUTPUT'
                The patterns surrounding cog inline instructions. Should
                include three values separated by spaces, the start, end,
                and end-output markers. Defaults to '[[[cog ]]] [[[end]]]'.
                Control the amount of output. 2 (the default) lists all files,
                1 lists only changed files, 0 lists no files.
    -h          Print this help.

In addition to running cog as a command on the command line, you can also invoke it as a module with the Python interpreter:

$ python -m cogapp [options] [arguments]

Note that the Python module is called “cogapp”.

Input files

Files on the command line are processed as input files. All input files are assumed to be UTF-8 encoded. Using a minus for a filename (-) will read the standard input.

Files can also be listed in a text file named on the command line with an @:

$ cog @files_to_cog.txt

These @-files can be nested, and each line can contain switches as well as a file to process. For example, you can create a file cogfiles.txt:

# These are the files I run through cog
myschema.sql -s " --**cogged**"
readme.txt -s ""

then invoke cog like this:

$ cog -s " //**cogged**" @cogfiles.txt

Now cog will process four files, using C++ syntax for markers on all the C++ files, SQL syntax for the .sql file, and no markers at all on the readme.txt file.

As another example, cogfiles2.txt could be:

template.h -D thefile=data1.xml -o data1.h
template.h -D thefile=data2.xml -o data2.h

with cog invoked like this:

$ cog -D version=3.4.1 @cogfiles2.txt

Cog will process template.h twice, creating both data1.h and data2.h. Both executions would define the variable version as “3.4.1”, but the first run would have thefile equal to “data1.xml” and the second run would have thefile equal to “data2.xml”.

Overwriting files

The -r flag tells cog to write the output back to the input file. If the input file is not writable (for example, because it has not been checked out of a source control system), a command to make the file writable can be provided with -w:

$ cog -r -w "p4 edit %s" @files_to_cog.txt

Setting globals

Global values can be set from the command line with the -D flag. For example, invoking Cog like this:

$ cog -D thefile=fooey.xml mycode.txt

will run Cog over mycode.txt, but first define a global variable called thefile with a value of “fooey.xml”. This variable can then be referenced in your generator code. You can provide multiple -D arguments on the command line, and all will be defined and available.

The value is always interpreted as a Python string, to simplify the problem of quoting. This means that:

$ cog -D NUM_TO_DO=12

will define NUM_TO_DO not as the integer 12, but as the string "12", which are different and not equal values in Python. Use int(NUM_TO_DO) to get the numeric value.

Checksummed output

If cog is run with the -c flag, then generated output is accompanied by a checksum:

--   import cog
--   for i in range(10):
--      cog.out("%d " % i)
0 1 2 3 4 5 6 7 8 9
--[[[end]]] (checksum: bd7715304529f66c4d3493e786bb0f1f)

If the generated code is edited by a misguided developer, the next time cog is run, the checksum won’t match, and cog will stop to avoid overwriting the edited code.

Continuous integration

You can use the --check option to run cog just to check that the files would not change if run again. This is useful in continuous integration to check that your files have been updated properly.

Output line suffixes

To make it easier to identify generated lines when grepping your source files, the -s switch provides a suffix which is appended to every non-blank text line generated by Cog. For example, with this input file (mycode.txt):

cog.outl('Three times:\n')
for i in range(3):
    cog.outl('This is line %d' % i)

invoking cog like this:

$ cog -s " //(generated)" mycode.txt

will produce this output:

cog.outl('Three times:\n')
for i in range(3):
    cog.outl('This is line %d' % i)
Three times: //(generated)

This is line 0 //(generated)
This is line 1 //(generated)
This is line 2 //(generated)


The -n option lets you tell cog what encoding to use when reading and writing files.

The --verbose option lets you control how much cog should chatter about the files it is cogging. --verbose=2 is the default: cog will name every file it considers, and whether it has changed. --verbose=1 will only name the changed files. --verbose=0 won’t mention any files at all.

The --markers option lets you control the syntax of the marker lines. The value must be a string with two spaces in it. The three markers are the three pieces separated by the spaces. The default value for markers is "[[[cog ]]] [[[end]]]".

The -x flag tells cog to delete the old generated output without running the generators. This lets you remove all the generated output from a source file.

The -d flag tells cog to delete the generators from the output file. This lets you generate content in a public file but not have to show the generator to your customers.

The -U flag causes the output file to use pure Unix newlines rather than the platform’s native line endings. You can use this on Windows to produce Unix-style output files.

The -I flag adds a directory to the path used to find Python modules.

The -p option specifies Python text to prepend to embedded generator source, which can keep common imports out of source files.

The -z flag lets you omit the [[[end]]] marker line, and it will be assumed at the end of the file.