Strategy for Answering

Exactly what the conda init command does and its consequences are shell-specific. Instead of trying to cover all cases, let's walk through a case, noting along the way that one can replicate this analysis by substituting their shell of interest.

Case Study: conda init zsh

Let's look at zsh as the shell. This is a common shell (default for macOS 10.15+) and very close to bash. Plus, I don't already have it configured.

Probing the Command: Dry Run

Many Conda commands include some form of dry run functionality via a --dry-run, -d flag, which - combined with verbosity flags - enables seeing what this would do without doing them. For the init command, dry run alone will only tell us what files it would modify:

$ conda init -d zsh
no change     /Users/mfansler/miniconda3/condabin/conda
no change     /Users/mfansler/miniconda3/bin/conda
no change     /Users/mfansler/miniconda3/bin/conda-env
no change     /Users/mfansler/miniconda3/bin/activate
no change     /Users/mfansler/miniconda3/bin/deactivate
no change     /Users/mfansler/miniconda3/etc/profile.d/conda.sh
no change     /Users/mfansler/miniconda3/etc/fish/conf.d/conda.fish
no change     /Users/mfansler/miniconda3/shell/condabin/Conda.psm1
no change     /Users/mfansler/miniconda3/shell/condabin/conda-hook.ps1
no change     /Users/mfansler/miniconda3/lib/python3.7/site-packages/xontrib/conda.xsh
no change     /Users/mfansler/miniconda3/etc/profile.d/conda.csh
modified      /Users/mfansler/.zshrc

==> For changes to take effect, close and re-open your current shell. <==

Here we can see that it plans to target the user-level resources file for zsh, /Users/mfansler/.zshrc, but it doesn't tell us how it will modified it. Also, OMG! the UX here is awful, because it in no way reflects the fact that I used the -d flag. But don't worry: as long as the -d flag is there it won't actually change things.

Patch Preview

To see what exactly it will do, add a single verbosity flag (-v) to the command. This will give everything from the previous output, but will now shows us the diff it will use to patch (update) the .zshrc file.

$ conda init -dv zsh

/Users/mfansler/.zshrc
--- 

+++ 

@@ -0,0 +1,16 @@

+
+# >>> conda initialize >>>
+# !! Contents within this block are managed by 'conda init' !!
+__conda_setup="$('/Users/mfansler/miniconda3/bin/conda' 'shell.zsh' 'hook' 2> /dev/null)"
+if [ $? -eq 0 ]; then
+    eval "$__conda_setup"
+else
+    if [ -f "/Users/mfansler/miniconda3/etc/profile.d/conda.sh" ]; then
+        . "/Users/mfansler/miniconda3/etc/profile.d/conda.sh"
+    else
+        export PATH="/Users/mfansler/miniconda3/bin:$PATH"
+    fi
+fi
+unset __conda_setup
+# <<< conda initialize <<<
+

# ...the rest is exactly as above

That is, the plan of action is to add these 16 lines to the .zshrc file. In this case, I don't have an existing .zshrc file, so it plans to add it at line 1. If the file had already existed, it would append these lines.