Scientific Computing

A Windows short path can be converted to long path and vice versa using the Windows C library as in this standalone, single source file example.

When moving between Linux systems that often default to Bash, and macOS systems that often default to Zsh, one may wish to change the default shell parameters.

For example, to remove duplicate entries in shell history, so that pressing “up” on repeated commands doesn’t make you press “up” repeatedly to get to the last non-duplicated command, set like the following.

Bash: ~/.inputrc: ignore duplicate lines, and omits lines that start with space.

export HISTCONTROL=ignoreboth

Zsh: ~/.zshrc: approximately the equivalent of the above Bash setting.

setopt hist_ignore_dups
setopt hist_ignore_space

Users (or developers!) may not realize that the shell expands glob asterisk * unless enclosed in quotes. This can surprise users unfamiliar with this shell behavior, say when using Python argparse with position-based arguments. Say a user has a single file to process in a directory, and doesn’t want to type the long filename, so they type:

python myScript.py ~/data/*.h5 32

Here we assume myScript.py expects two positional arguments, the first being a filename, and the second being an integer. If more than one “*.h5” file subsequently exists and myScript.py is run, the actual input to Python would be like:

python myScript.py ~/data/file1.h5 ~/data/file2.h5 32

Which causes a Python argparse exception.

To see what the shell is going to expand to, with default keybindings and Bash or Zsh at least, press after typing the command these keys:

CMake ExternalProject works for many types of sub-projects across CMake generators. An implementation detail of Ninja is by default ExternalProject doesn’t print progress until each ExternalProject step is finished. For large external projects that take several minutes to download and build, users could be confused thinking CMake has frozen up. To make ExternalProject show live progress as it does with Makefiles generators, add the USES_TERMINAL_* true arguments to ExternalProject_Add.

ExternalProject_Add(
  BigProject
  ...
  USES_TERMINAL_DOWNLOAD true
  USES_TERMINAL_UPDATE true
  USES_TERMINAL_PATCH true
  USES_TERMINAL_CONFIGURE true
  USES_TERMINAL_BUILD true
  USES_TERMINAL_INSTALL true
  USES_TERMINAL_TEST true
)

“USES_TERMINAL* true” forces ExternalProject steps to run sequentially. For large projects this is ordinarily not significant.

CMake doesn’t print the Generator version by default. Sometimes bugs are related to a specific generator version. Reveal CMake generator version like this snippet.

PyTest can work with Matlab Engine if the Matlab Engine is setup. Use a try-catch to ensure any non-functioning Matlab Engine issue is skipped.

import pytest


def test_me():
    try:
        mateng = pytest.importorskip("matlab.engine")
    except Exception:  # can also get RuntimeError, let's just catch all
        pytest.skip("Matlab engine not available")

    eng = mateng.start_matlab("-nojvm")
    # test code

Python subprocess can be used to run a long-running program, capturing the output to a variable and printing to the screen simultaneously. This gives the user the comfort that the program is working OK and gives program status messages without waiting for the program to finish.

This example demonstrates the “tee” subprocess behavior.

Python subprocess can run inline multi-line Python code. This is useful to use Python as a cross-platform demonstration or for production code where a new Python instance is called.

import subprocess
import sys

# the -u is to ensure unbuffered output so that program prints live
cmd = [sys.executable, "-u", "-c", r"""
import sys
import datetime
import time

for _ in range(5):
    print(datetime.datetime.now())
    time.sleep(0.3)
"""]

subprocess.check_call(cmd)

Matlab command batch “matlab -batch” is useful for running Matlab scripts from the command line. When using “stdout” text output from Matlab, especially if only a single line is expected, there may be extraneous text output from Matlab with regard to licensing. A command example is prereleases like:

matlab -batch "disp(matlabroot)"

outputs to stdout:

    Prerelease License -- for engineering feedback and testing
	purposes only. Not for sale.

/Applications/MATLAB_R2023b.app

A workaround for this in shell scripts is like:

set -e  # stop on error

r=$(matlab -batch "disp(matlabroot)" | tail -n1)

cd ${r}
# and so on

It can be convenient to open a file by launching the default program without first leaving the Terminal. For simplicity, we assume the file is named “file.txt” but it can be any file openable by a program on the computer. This technique works with any file type that has an associated default program on the computer.

• macOS: open file.txt
• Linux: xdg-open file.txt
• Windows: start file.txt