Nextflow: add unique ID, hash, or row number to tuple - nextflow

Question

ch_files = Channel.fromPath("myfiles/*.csv")
ch_parameters = Channel.from(['A','B, 'C', 'D'])
ch_samplesize = Channel.from([4, 16, 128])
process makeGrid {
input:
path input_file from ch_files
each parameter from ch_parameters
each samplesize from ch_samplesize
output:
tuple path(input_file), parameter, samplesize, path("config_file.ini") into settings_grid
"""
echo "parameter=$parameter;sampleSize=$samplesize" > config_file.ini
"""
}
gives me a number_of_files * 4 * 3 grid of settings files, so I can run some script for each combination of parameters and input files.
How do I add some ID to each line of this grid? A row ID would be OK, but I would even prefer some unique 6-digit alphanumeric code without a "meaning" because the order in the table doesn't matter. I could extract out the last part of the working folder which is seemingly unique per process; but I don't think it is ideal to rely on sed and $PWD for this, and I didn't see it provided as a runtime metadata variable provider. (plus it's a bit long but OK). In a former setup I had a job ID from the LSF cluster system for this purpose, but I want this to be portable.
Every combination is not guaranteed to be unique (e.g. having parameter 'A' twice in the input channel should be valid).
To be clear, I would like this output
file1.csv A 4 pathto/config.ini 1ac5r
file1.csv A 16 pathto/config.ini 7zfge
file1.csv A 128 pathto/config.ini ztgg4
file2.csv A 4 pathto/config.ini 123js
etc.

Answer 1

Given the input declaration, which uses the each qualifier as an input repeater, it will be difficult to append some unique id to the grid without some refactoring to use either the combine or cross operators. If the inputs are just files or simple values (like in your example code), refactoring doesn't make much sense.
To get a unique code, the simple options are:
Like you mentioned, there's no way, unfortunately, to access the unique task hash without some hack to parse $PWD. Although, it might be possible to use BASH parameter substitution to avoid sed/awk/cut (assuming BASH is your shell of course...) you could try using: "${PWD##*/}"
You might instead prefer using ${task.index}, which is a unique index within the same task. Although the task index is not guaranteed to be unique across executions, it should be sufficient in most cases. It can also be formatted for example:
process example {
...
script:
def idx = String.format("%06d", task.index)
"""
echo "${idx}"
"""
}
Alternatively, create your own UUID. You might be able to take the first N characters but this will of course decrease the likelihood of the IDs being unique (not that there was any guarantee of that anyway). This might not really matter though for a small finite set of inputs:
process example {
...
script:
def uuid = UUID.randomUUID().toString()
"""
echo "${uuid}"
echo "${uuid.take(6)}"
echo "${uuid.takeBefore('-')}"
"""
}