Amazing Command-Line Tools: The Pipe

 Computing, Linux  Add comments
Jul 232022
 

I was following one of those Twitter threads posting their favourite command-line tools (specifically for infosec), and added my own entry – the incomparable tshark. Later it occurred to me that the best command-line tool isn’t really a tool at all as it is built into the shell – the pipe. Many of the command-line tools just wouldn’t be quite the same without it.

For those who aren’t familiar with the command-line, the pipe (“|”) takes the output of one command and feeds it as input to another command. And you can string such pipelines together to add to each other (which can lead to inefficiencies).

For example :-

» ls | wc -l
84

This takes the usual command for listing files and sends the output into the “word count” command to produce a count of the number of files in the current directory. To be more precise, it produces a count of the number of files that ls thinks is in the directory. You can get different results with different variations :-

» echo * | wc -w
89
» ls -a | wc -l
463

If you had a log file containing DHCP requests you could :-

» grep DHCPDISCOVER 2022.07.local0.info.log | head
2022-06-30T23:59:05+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 4D:6D:4F:55:59:B4 (esp32-D04CCC) via 10.72.0.1
2022-07-01T01:30:04+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 4D:6D:4F:55:59:B4 (esp32-D04CCC) via 10.72.0.1
2022-07-01T02:53:33+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from DF:69:AF:DC:79:3E via eth0
2022-07-01T02:53:33+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from DF:69:AF:DC:79:3E via 10.0.0.1
2022-07-01T02:53:39+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from a8:a6:48:92:9d:36 via eth0
2022-07-01T03:01:03+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 4D:6D:4F:55:59:B4 (esp32-D04CCC) via 10.72.0.1
2022-07-01T04:32:02+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 4D:6D:4F:55:59:B4 (esp32-D04CCC) via 10.72.0.1
2022-07-01T04:56:53+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 91:06:27:15:EF:DC via 10.72.0.1
2022-07-01T06:03:01+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 4D:6D:4F:55:59:B4 (esp32-D04CCC) via 10.72.0.1
2022-07-01T07:34:00+00:00 <local0.info> 2001:db8:bad:cafe::b/d-FCB dhcpd: DHCPDISCOVER from 4D:6D:4F:55:59:B4 (esp32-D04CCC) via 10.72.0.1

List out the first few DHCP DISCOVER requests (the astute may notice that I’ve done some obfuscating). We can then pick out a field using awk to list just the MAC addresses :-

» grep DHCPDISCOVER 2022.07.local0.info.log | awk '{print $7}' | head
4D:6D:4F:55:59:B4
4D:6D:4F:55:59:B4
DF:69:AF:DC:79:3E
DF:69:AF:DC:79:3E
a8:a6:48:92:9d:36
4D:6D:4F:55:59:B4
4D:6D:4F:55:59:B4
91:06:27:15:EF:DC
4D:6D:4F:55:59:B4
4D:6D:4F:55:59:B4

We can then remove the “head” command and add a sort and uniq command to produce a full list of all MAC addresses that have performed a DHCP DISCOVER :-

» grep DHCPDISCOVER 2022.07.local0.info.log | awk '{print $7}' | sort | uniq -c
      4 DF:69:AF:DC:79:3E
      3 89:C1:67:B8:9D:6F
      6 F3:55:1E:06:D4:49
      4 F3:55:1E:06:D4:48
     12 4D:6D:4F:55:59:B3
     92 91:06:27:15:EF:DC
     46 85:2C:B4:B3:70:7E
    333 4D:6D:4F:55:59:B4
      2 40:5B:D8:FF:FA:29
     72 FD:D4:00:41:29:BE
      5 36:1E:07:2D:AD:76
     41 44:FD:6E:05:82:21
     81 CC:78:14:BB:E4:3D

We can sort the result into reverse numerical order :-

» grep DHCPDISCOVER 2022.07.local0.info.log | awk '{print $7}' | sort | uniq -c | sort -r -n
    333 4D:6D:4F:55:59:B4
     92 91:06:27:15:EF:DC
     81 CC:78:14:BB:E4:3D
     72 FD:D4:00:41:29:BE
     46 85:2C:B4:B3:70:7E
     41 44:FD:6E:05:82:21
     12 4D:6D:4F:55:59:B3
      6 F3:55:1E:06:D4:49
      5 36:1E:07:2D:AD:76
      4 F3:55:1E:06:D4:48
      4 DF:69:AF:DC:79:3E
      3 89:C1:67:B8:9D:6F
      2 40:5B:D8:FF:FA:29

And if you have access to the relevant script, you can produce terminal graphics (just to keep innumerate managers happy) :-

» grep DHCPDISCOVER 2022.07.local0.info.log | awk '{print $7}' | sort | uniq -c | sort -r -n | awk '{print $2, $1}' | tbar --replace 1 --max 350
4D:6D:4F:55:59:B4 ■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■
91:06:27:15:EF:DC ■■■■■■■■■■■■■■■
CC:78:14:BB:E4:3D ■■■■■■■■■■■■■■
FD:D4:00:41:29:BE ■■■■■■■■■■■■
85:2C:B4:B3:70:7E ■■■■■■■
44:FD:6E:05:82:21 ■■■■■■■
4D:6D:4F:55:59:B3 ■■
F3:55:1E:06:D4:49 ■
36:1E:07:2D:AD:76 
F3:55:1E:06:D4:48 
DF:69:AF:DC:79:3E 
89:C1:67:B8:9D:6F 
40:5B:D8:FF:FA:29

The pipe isn’t so much a tool itself as a mechanism to combine tools into producing interesting results.

It’s Round