What does this command mean: awk -F: '{print $4}'?

awk -F: '{print $4}'

• awk - this is the interpreter for the AWK Programming Language. The AWK language is useful for manipulation of data files, text retrieval and processing
• -F <value> - tells awk what field separator to use. In your case, -F: means that the separator is : (colon).
• '{print $4}' means print the fourth field (the fields being separated by :).

Example:

Let's say that there's a file called test, and it contains the following:

Hello:my:name:is:Alaa

If we execute the command awk -F: '{print $4}' test, the output will be:

is

Because is is the fourth field.

      field1   field3  field5
       -----    ----    ----
       |   |    |  |    |  |
       Hello:my:name:is:Alaa
             ||      ||
             --      --
           field2  field4

• You set the field separator with ...

  -F
  

  so that is ":" in this example.

• You print the text that is between the 3th and 4th separator with ...

  '{print $4}'
  

• And this explains it better:

  echo "154:266:377:454:533" | awk -F: '{print $4}'
  454