Linux – What constitutes a ‘field’ for the cut command

cutlinux

For example, the cut command can take a parameter -f, which according to man

select only these fields; also print any line that contains no
delimiter character, unless the -s option is specified

In this context, what is a field?

Best Answer

The term "field" is often times associated with tools such as cut and awk. A field would be similar to a columns worth of data, if you take the data and separate it using a specific character. Typically the character used to do this is a Space.

However as is the case with most tools, it's configurable. For example:

  • awk = awk -F"," ... - would separate by commas (i.e. ,).
  • cut = cut -d"," ... - would separate by commas (i.e. ,).

Examples

This first one shows how awk automatically will split on spaces.

$ echo "The rain in Spain." | awk '{print $1" "$4}'
The Spain.

This one shows how cut will split on spaces too.

$ echo "The rain in Spain." | cut -d" " -f1,4
The Spain.

Here we have a CSV list of column data that we're using cut to return columns 1 & 4.

$ echo "col1,col2,col3,co4" | cut -d"," -f1,4
col1,co4

Awk too can do this:

$ echo "col1,col2,col3,co4" | awk -F"," '{print $1","$4}'
col1,co4

Awk is also a little more adept at dealing with a variety of separation characters. Here it's dealing with Tabs along with Spaces where they're inter-mixed at the same time:

$ echo -e "The\t rain\t\t in Spain." | awk '{print $1" "$4}'
The Spain.

What about the -s switch to cut?

With respect to this switch, it's simply telling cut to not print any lines which do not contain the delimiter character specified via the -d switch.

Example

Say we had this file.

$ cat sample.txt 
This is a space string.
This is a space   and   tab string.
Thisstringcontainsneither.

NOTE: There are spaces and tabs in the 2nd string above.

Now when we process these strings using cut with and without the -s switch:

$ cut -d" " -f1-6 sample.txt 
This is a space string.
This is a space  
Thisstringcontainsneither.

$ cut -d" " -f1-6 -s sample.txt 
This is a space string.
This is a space

In the 2nd example you can see that the -s switch has omitted any strings from the output that do not contain the delimiter, Space.

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