Good Habit #9: Match certain fields in output, not just lines

A tool like awk is preferable to grep when you want to match the pattern in only a specific field in the lines of output and not just anywhere in the lines.

The following simplified example shows how to list only those files modified in December:

Listing 1. Example of bad habit #9: Using grep to find patterns in specific fields

~/tmp $ ls -l /tmp/a/b/c | grep Dec -rw-r--r-- 7 joe joe 12043 Jan 27 20:36 December_Report.pdf -rw-r--r-- 1 root root 238 Dec 03 08:19 README -rw-r--r-- 3 joe joe 5096 Dec 14 14:26 archive.tar ~/tmp $

In this example, grep filters the lines, outputting all files with Dec in their modification dates as well as in their names. Therefore, a file such as December_Report.pdf is matched, even if it has not been modified since January.

This probably is not what you want. To match a pattern in a particular field, it is better to use awk, where a relational operator matches the exact field, as in the following example:

Listing 2. Example of good habit #9: Using awk to find patterns in specific fields

~/tmp $ ls -l | awk '$6 == "Dec"'
-rw-r--r--  3 joe joe   5096 Dec 14 14:26 archive.tar
-rw-r--r--  1 root root  238 Dec 03 08:19 README
~/tmp $

 Check the awk man pages for more details about how to use awk.