awk print a match only if last record before other match

Question

I want to use awk for printing a match of some pattern, only if it is the last line of the file before some constant text.

For example, i want to print the value of "c" only if it is the last line before some constant text:

cat file.txt:
a=1
b=2
c=3
...constant text...

print 3.

cat file.txt:
a=1
c=3
b=2
...constant text...

do not print anything.

How do I do that..? How can awk know if it's the last number of the file?

Answer 1

I want to use awk for printing a match of some pattern, only if it is the last line of the file before some constant text.

In that case, try this:

awk -F= '/constant text/ && name=="c"{print value} {name=$1;value=$2}' file

Examples

$ cat file1 
a=1
b=2
c=3
...constant text...
$ awk -F= '/constant text/ && name=="c"{print value} {name=$1;value=$2}' file1
3
$ cat file2
a=1
c=3
b=2
...constant text...
$ awk -F= '/constant text/ && name=="c"{print value} {name=$1;value=$2}' file2
$ 

How it works

• -F=

  This tells awk to use = as the field separator. In this way, we can separate the name and value for fields like a=1 or c=3.

• /constant text/ && name=="c"{print value}

  If we reach a line with constant text and name (as last set on the previous line) is c, then print value.

• name=$1;value=$2

  For the current line, save the first field as name and the second field as value.

Answer 2

If you don't mind using grep it has an option to print lines before match.

Option is : -B num, --before-context=num

In your case :

cat file.txt|grep -B 1 "constant text"|grep "c="|cut -f2 -d'='

Explanation :

1. Print just the line before constant text.
2. Filter all c values from output.
3. Extract second field with '=' delimiter.

Answer 3

Some googling found me the best answer so far, though I already got an answer: Using tac makes it much simpler...

tac file | awk 'NR==2{if ( $1=="c") print $2}'

The line number is one after the "constant text". If the "constant text" is one line - NR==2. If it is two lines - NR==3. Haven't decided yet what format the constant text should be...