Get the capture's file type
2 min |  Ross Jacobs |  July 7, 2019

Table of Contents

Quicklinks: captype manpage | code


Capytpe reads a file and prints the file type. It has no flags and takes one or more files as argument.

Captype Example

$ captype testdir/*
literally_an_empty_file: erf
aliens.png: mime
largeiftrue.pcapng: pcapng
ch36_monitor.pcap: pcapng unknown
captype: "topsecret" is a directory (folder), not a file.

It’s easy to parse this format with awk. awk -F ': ', where $1 is the filename and $2 is the filetype. Any errors will put captype: in place of the filename.

When Your Pcap Extension != Filetype

You may have a file that has a .pcap extension but is actually a .pcapng file. This can easily happen if you save to a file like tshark -w example.pcap without specifying an encoding. tshark will default to pcapng, so you’ll have pcapng data with a pcap extension. While tshark and friends will read the encoding and not the extension, other programs may not be as forgiving.

Example: Use Captype to Correct Filetype

It’s easy to make this mistake as defaulting to pcap/pcapng varies by Wireshark utility. For example, if we save packets without explicitly setting the capture type using tshark’s -F, we’ll have a pcapng file with a pcap extension.

$ tshark -c 100 -w example.pcap
Capturing on 'Wi-Fi: en0'
$ captype example.pcap
example.pcap: pcapng

To automatically fix this problem, you can use this one-liner. If the filetype is different from the extension, the file is moved to the correct extension.

# If captype doesn't know which filetype a file is, it will classify it as "unknown"
# For any captype or awk error condition, mv's 2nd arg collapses to '' and mv will error.
mv -n $file "$(captype $file | awk -F ': ' '{ if ($2 != "unknown") print "'${file%.*}.'"$2}')"

Further Reading

  • Wikipedia: List of file signatures: How to know from the first few bytes “file magic” of a file what its type is.
  • 2016-01, Algis Salys, Pcap and Pcapng: pcap, pcapng, and converting between the two