The best tool I found for the job is slackdump. The following steps apply to MacOS only, though of course you may find some work on other platforms.

Setup

The easiest way to get started is with Homebrew:

brew install slackdump

To set up authentication, load the Slack web app, and follow the instructions here to extract the token and cookie, which should then be added to a file called secrets.txt in the folder where slackdump will be run:

SLACK_TOKEN=xoxc-<...elided...>
COOKIE=xoxd-<...elided...>

Channel Listing & Splitting

Unfortunately, slackdump doesn’t provide any runtime estimation while it’s exporting, and exports (particularly with uploaded files) can get very large. Slack rate limiting means they can also take rather a long time.

Let’s split the export up into chunks of different types of data.

slackdump list channels

This can take a few minutes, but eventually spits out a file like channels-XYZ.txt with a line for each type of chat you have access to.

First let’s get rid of archived channels. We won’t bother with those.

sed '/  arch  /d' channels-XYZ.txt > channels-live.txt

Now we’ll split it up into DMs, groups chats, public and channels:

awk '$1 ~ /^D/ {print $1}' channels-live.txt > channels-dms.txt
awk '$3 ~ /^Group:/ {print $1}' channels-live.txt > channels-groups.txt
awk '$3 ~ /^#/ {print $1}' channels-live.txt > channels-public.txt
awk '$3 ~ /^🔒/ {print $1}' channels-live.txt > channels-private.txt

Make sure they’re all there before moving on:

➜ wc -l channels-dms.txt channels-groups.txt channels-public.txt channels-private.txt
     209 channels-dms.txt
     424 channels-groups.txt
    1459 channels-public.txt
      53 channels-private.txt
    2145 total
➜ wc -l channels-live.txt
    2146 channels-live.txt

Should you wish to triage any of these first, do it in two stages like this:

awk '$3 ~ /^🔒/' channels-live.txt > channels-private-triage.txt
# Remove any channels/chats which are not required
awk '{print $1}' channels-private-triage.txt > channels-private.txt

Exporting

Run an export for each list:

slackdump export -o dms.zip -avatars -files=true @channels-dms.txt # 26m45s
slackdump export -o groups.zip -avatars -files=true @channels-groups.txt # 7m35s
slackdump export -o private-channels.zip -files=false @channels-private.txt # 1h51m
slackdump export -o public-channels.zip -files=false @channels-public.txt

Note that I decided to only download user avatars for the DM export, and omit uploaded files for channels to save space.

I also provided my own runtimes here, which correspond to the number of channels in the wc output above (except public which I cut down significantly). Clearly the runtime also depends a lot on channel sizes, but it gives a vague idea of the order of magnitude.

Slack rate limiting definitely seems to be the limiting factor, allowing bursts of ~5s followed by 10s waits, once the export gets going.

Monitoring Progress

As mentioned, it’s not easy to track export progress. I noticed that when it starts, Slackdump shows its temporary directory, for example:

INFO temporary directory in use tmpdir=/var/folders/59/xwg_1t6d29l23h48lddxdlsr0000fn/T/slackdump-1846332288

Inside are a bunch of files, one for each channel (plus one or two more). Knowing how many files are in the input channel list, we can put together a makeshift progress indicator as follows:

cd /var/[your dir]
TOTAL=732; while :; do count=$(ls -1 2>/dev/null | grep -vE 'users.json.gz|workspace.json.gz' | wc -l); printf "\r[%-*s] %d/%d (%.1f%%)" 40 "$(printf '#%.0s' $(seq $((40 * count / TOTAL))))" "$count" "$TOTAL" "$((100 * count / TOTAL)).0"; [ "$count" -ge "$TOTAL" ] && break; sleep 1; done; echo

(Set TOTAL to the total number of channels in the input list).

[########################                ] 455/732 (62.0%)

Viewing

As the README describes, Slackdump contains its own built-in viewer:

slackdump view slack-dms.zip

Alternatively I’ve also had success with slack-export-viewer:

python3 -m venv .
source bin/activate
pip3 install slack-export-viewer
source bin/activate
slack-export-viewer -z ./slack-export.zip -p 8000
deactivate

Note: If you get an error like “codec can’t encode character”, it’s this. Extract the zip and point it at the directory instead.