How to trim the result of speedtest-cli to only output the download speed?

That's a job for awk:

speedtest-cli --simple | awk 'NR==2{print$2}'      # just the numeral
speedtest-cli --simple | awk 'NR==2{print$2" "$3}' # numeral and unit

Explanations

• NR==2 – take line 2
• {print$2} – print the second column (space-separated by default)
• {print$2" "$3} – print the second column followed by a space and the third one

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With sed it's a little more complicated:

speedtest-cli --simple | sed '/D/!d;s/.* \(.*\) .*/\1/' # just the numeral
speedtest-cli --simple | sed '/D/!d;s/[^ ]* \(.*\)/\1/' # numeral and unit

Explanations

• /D/!d – search for lines containing D and don't (!) delete them, but every other line
• s/A/B/ – substitute A with B
• .* – take everything
• [^ ]* – take everything that's not (^) a space
• ␣ (space character) – a literal space
• \(…\) - take everything inside and save it as a group
• \1 – get the content of group 1

As speedtest-cli is a python program and library it's fairly easy to make a minimal alternative program that only performs a download test and prints the output.

Open an editor, save as dl-speedtest.py

import speedtest

s = speedtest.Speedtest()
s.get_config()
s.get_best_server()
speed_bps = s.download()
speed_mbps = round(speed_bps / 1000 / 1000, 1)
print(speed_mbps)

run with python dl-speedtest.py

This gives the result in bps, as a floating point number Mbps rounded to one decimal as requested

The minimal version of speedtest-cli for this to work is 1.0.0 I think, you may need to use pip install speedtest-cli --upgrade to upgrade.