Shell Programming and Scripting

How do you get the contents of a line directly after a selected one? Also, how about before it? I am using CSH and don't have any GNU products installed.

Input file:

apple
orange
plum

So say I use grep on orange...how do I get plum (this line can be different so I can't just grep plum).

Also, if I want to get the line before orange (apple in this case) how do I do that?

I am thinking this can be done via awk, but I'm not that familar with it.

Two small awk programmes -- first prints the line before 'orange' and the second prints the line after:

Code:

awk ' /orange/ { print last; exit; } { last = $0}' input-file >output-file

awk ' /orange/ { getline; print; exit }'  input-file >output-file

To make sure the search term gives the correct answer when it is on the first or last line, an additional check would be required:

Code:

awk '/orange/ && p{print p}{p=$0}' infile

Code:

awk '/orange/ && getline' infile

Test with plum or apple instead of orange..

Thanks for the response. I understand the first line...but doesn't this only work if the orange is on the second line? I guess what I need is a way to get the line number of the orange line and then add or subtract this line number in order to print the line before or after orange. Can someone help with this?

Awk reads each record from input and applies each block* of the programme to each record. The format of each awk "block" is

Code:

<expression> {<action>}

Action statements are executed when the expression evaluates to true. Because of this, awk will locate 'orange' on any line in the input file and if the remainder of the expression is true, will execute the action, so 'orange' can appear anywhere. Scrutinizer's improvements prevent issues if 'orange' is on the first line when printing the previous record, and on the last line when printing the next record -- things that I overlooked.

A bit more details if needed using Scrutinizer's improvements...

The first expression is /orange/ && p and is true if the record contains 'orange' AND the variable p is not empty. When true, the action is a single statement which prints the value of p.

The second expression is empty which equates to true, and thus it captures the first field of the current record. This will be printed as the previous record if the next record contains 'orange'.

I assumed, maybe incorrectly, that you only wanted the first 'orange' matched, and I added an exit after the print to stop processing.


In the second programme, the expression is /orange/ && getline and the action is empty. The expression will evaluate to true when the current record contains 'orange' and the getline command is successful. The getline command causes awk to read the next record. A null action defaults to 'print $0' so in this case when orange is on the current line, and there is a 'next' line, the next line will w printed.

* There are statements that can be placed in the action portion of a block that cause the remaining blocks to not be applied to the current record, but that is beyond the scope of this small example.


A pretty detailed overview of awk: Awk - A Tutorial and Introduction - by Bruce Barnett