带宽受限场景下，如何查看opam安装软件包的下载大小？

Great question—dealing with bandwidth constraints is always a hassle, so it’s totally sensible to want to know download sizes before pulling packages! OPAM has built-in tools to handle both of your needs:

Checking the download size of a single package #

To get the compressed archive size (the actual amount of data you’ll download) for a specific package, use the opam show command with the --field flag to target the archive-size field:

opam show some-package --field=archive-size

This will output a number representing the size in bytes. If you want more context (like the package version, source URL, etc.), you can omit the --field flag to see the full package details, which includes the archive size near the top.

Estimating total download data for opam install #

When you’re installing a package and its dependencies, use the dry-run mode to simulate the installation without actually downloading anything. This will show you all packages that need to be downloaded, along with their individual sizes.

Run this command to get a verbose breakdown:

opam install -n -v some-package

The -n flag enables dry-run mode, and -v (verbose) will print out the archive size for each package that needs downloading.

If you want a quick total of all required download data, you can pipe the output to a few command-line tools to sum the sizes:

opam install -n -v some-package 2>&1 | grep "archive size" | awk '{sum += $4} END {print "Total download size: " sum " bytes (" sum/1024/1024 " MB)"}'

This will parse the verbose output, extract all archive sizes, and calculate the total in both bytes and megabytes for easier reading.

A quick note: Dry-run mode automatically accounts for packages you already have installed, so it only shows the data you actually need to download—perfect for bandwidth-limited environments!

内容的提问来源于stack exchange，提问作者Olle Härstedt