source ~/.zshrc vs exec zsh

"Reload vs. Replace — how your shell config actually takes effect"

Photo by Jake Walker on Unsplash

These both “reload” your shell config, but they work very differently under the hood.

source ~/.zshrc

This runs the file in the current shell process. No new process is created.

• Your current shell’s PID stays the same

• All existing variables, functions, and aliases are still in memory before the file runs

• The new config is layered on top of the existing state

• Stale values can persist — e.g. if you removed an export from .zshrc, the old value is still set in your session

zsh

echo $$        # e.g. 1234
. ~/.zshrc
echo $$        # still 1234 — same process

The gotcha: it accumulates state. If PATH is appended in .zshrc, sourcing it multiple times will keep duplicating entries in PATH.

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exec zsh

This replaces the current shell process with a brand new zsh instance (via the exec syscall).

• Same PID is reused, but the process image is completely replaced

• Starts with a clean slate — reads .zshenv, .zprofile, .zshrc fresh

• No leftover variables or stale state from the old session

• Any variables that were set manually in your session (not in config files) will be gone

zsh

echo $$        # e.g. 1234
exec zsh
echo $$        # still 1234 — same PID, but brand new shell proces

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“Well begun is half done.”

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