Unexpected output file template formatting with dollar underscore in filename

[gradha]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2020.09.20
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

$ youtube-dl -v -o 'dash.a$_.mkv' -f 394 -- -80miaG2wF0 
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'-o', u'dash.a$_.mkv', u'-f', u'394', u'--', u'-80miaG2wF0']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2020.09.20
[debug] Python version 2.7.15 (CPython) - Darwin-18.7.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 4.2.1-tessus, ffprobe 3.2.2, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] -80miaG2wF0: Downloading webpage
[debug] Invoking downloader on u'https://r3---sn-h5nhv8pa-cjoe.googlevideo.com/videoplayback?expire=1602645682&ei=URqGX6S-O4n_xwLnxr2YBA&ip=93.115.152.239&id=o-AEYwO0tJwLAmByb2CXPOfsUj2DliKbuzaWqk2gfWo4CG&itag=394&aitags=133%2C134%2C135%2C136%2C137%2C160%2C242%2C243%2C244%2C247%2C248%2C278%2C298%2C299%2C302%2C303%2C394%2C395%2C396%2C397%2C398%2C399&source=youtube&requiressl=yes&mh=IJ&mm=31%2C29&mn=sn-h5nhv8pa-cjoe%2Csn-h5q7kne6&ms=au%2Crdu&mv=m&mvi=3&pcm2cms=yes&pl=24&initcwndbps=1581250&vprv=1&mime=video%2Fmp4&gir=yes&clen=10529263&dur=1199.866&lmt=1602132954559368&mt=1602623914&fvip=3&keepalive=yes&fexp=23915654&beids=9466587&c=WEB&txp=5531432&sparams=expire%2Cei%2Cip%2Cid%2Caitags%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRAIgSUW50H8x9_ChacWZo9bbpH2wRPMPtLwavFNkfV2PheECIA_X3bhSEETRIZbS19HaOMy-fMkUFeZMDa5govuwdOzV&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpcm2cms%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRAIgHxA3HG_ppwx7X5B3Q-HJbCXiWVWl-khXbpUsyMGBNMUCIFnH1_KhXW1tnm2kV-4kiaz8K8PLXs9rbvdki9pop_fT&ratebypass=yes'
[download] Destination: dash.a/Users/gradha/bin/youtube-dl.mkv
[download] 100% of 10.04MiB in 00:02

Description

For some reason when using the -o template option containing the $_ substring, this substring is expanded to the path where youtube-dl script lives. Changing the parameter to dash.a$ _.mkv works fine and will create a file named dash.a$ _.mkv.

I have checked the documented output template options and I see nothing about $_ being any escape sequence.

I have also checked python string formatting operations and don't see any $_ formatting either.

If instead of running the command I prefix the command with echo to see what the bash interpreter understands, I get the expected output including the $_ substring not being replaced:

$ echo youtube-dl -v -o 'dash.a$_.mkv' -f 394 -- -80miaG2wF0 
youtube-dl -v -o dash.a$_.mkv -f 394 -- -80miaG2wF0

If I try to use python3 instead the bug happens as well, just with a different path being replaced:

$ python3 ~/bin/youtube-dl -v -o 'dash.a$_.mkv' -f 394 -- -80miaG2wF0 
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', '-o', 'dash.a$_.mkv', '-f', '394', '--', '-80miaG2wF0']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2020.09.20
[debug] Python version 3.7.2 (CPython) - Darwin-18.7.0-x86_64-i386-64bit
[debug] exe versions: ffmpeg 4.2.1-tessus, ffprobe 3.2.2, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] -80miaG2wF0: Downloading webpage
[debug] Invoking downloader on 'https://r3---sn-h5nhv8pa-cjoe.googlevideo.com/videoplayback?expire=1602646178&ei=QhyGX9SENcyrW8yIstgN&ip=93.115.152.239&id=o-AMEz5JOYsxyHyOIyOMtwEg058va7jXfm3mCc_iqfnpyt&itag=394&aitags=133%2C134%2C135%2C136%2C137%2C160%2C242%2C243%2C244%2C247%2C248%2C278%2C298%2C299%2C302%2C303%2C394%2C395%2C396%2C397%2C398%2C399&source=youtube&requiressl=yes&mh=IJ&mm=31%2C29&mn=sn-h5nhv8pa-cjoe%2Csn-h5q7kne6&ms=au%2Crdu&mv=m&mvi=3&pcm2cms=yes&pl=24&initcwndbps=1585000&vprv=1&mime=video%2Fmp4&gir=yes&clen=10529263&dur=1199.866&lmt=1602132954559368&mt=1602624396&fvip=3&keepalive=yes&fexp=23915654&c=WEB&txp=5531432&sparams=expire%2Cei%2Cip%2Cid%2Caitags%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRgIhALMcpCM74NRxMRRljsoQ7SmPSKLiIoC1S29aCCU2QmVPAiEA3RQ7GKyUuca1Xdd8C_vJd6WJrBOVwZcxnjqp09Byl4M%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpcm2cms%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRAIgGISHkhr6taHJBUs9nTJLRWEcCgtgRLlKyRaB8I3FvMUCIGYXSaitIGW4PMEDZSm-YskubVyOHqQS1fEofHa4L9Gt&ratebypass=yes'
[download] Destination: dash.a/Library/Frameworks/Python.framework/Versions/3.7/bin/python3.mkv
[download]  97.0% of 10.04MiB at  4.25MiB/s ETA 00:00^C
ERROR: Interrupted by user

Adding dollar signs does not change the effect: the last $_ will still be replaced. However, adding underscores seems to not trigger the substitution, so a name like dash.a$__.mkv works as expected.

Where does the substitution come from?

[dstftw]

From your shell.

[gradha]

Is Python buggy then since I'm passing shell = False?:

$ ./test.py 
/Users/gradha/bin/youtube-dl really.a$_.mkv -f 394 -- -80miaG2wF0
[youtube] -80miaG2wF0: Downloading webpage
[download] Destination: really.a./test.py.mkv
[download] 100% of 10.04MiB in 00:02
$ cat test.py 
#!/usr/bin/env python3

import os
import subprocess

binary = os.path.expanduser("~/bin/youtube-dl")
subprocess.run(
    ["/bin/echo", binary, "really.a$_.mkv", "-f", "394", "--", "-80miaG2wF0"],
    shell = False)
subprocess.run(
    [binary, "-o", "really.a$_.mkv", "-f", "394", "--", "-80miaG2wF0"],
    shell = False)