How to convert multiple .packet or .pcap files to a single csv file.

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For the first point you will have to open a shell prompt. You haven't told us which OS you're using, but you do mention tshark.exe so I'll assume this is Windows. In that case you can use Cmd or PowerShell, I prefer PowerShell. Search for this in the Start Menu. Note that you'll have to provide the full path to tshark, e.g. '"C:\Program Files\Wireshark\tshark.exe"` when calling it.

For the 2nd point all common shells have their "language" which will allow you to make the same call on multiple files. The way in which this is done depends on the shell in use, which comes back to the first point.

For PowerShell I'd use something like:

Get-ChildItem "Dir with files" |  ForEach-Object { path\to\tshark -r (Convert-Path $_) -T tabs -E "separator=," =E "quote=d" | Out-File -Encoding utf8 "$($_.Basename).csv ...