Get exacly a specific line after a match in a file

Question

I have a log file with this pattern
Time = 100

GAMG:  Solving for p, Initial residual = 0.2174470886, Final residual = 0.01001676933, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.02692910633, Final residual = 0.0004864615367, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.005753534485, Final residual = 0.0001502172182, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.001526301953, Final residual = 5.384448346e-05, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.0004529702305, Final residual = 1.80754637e-05, No Iterations 1
time step continuity errors : sum local = 2.722585271, global = 0.09763545089, cumulative = 0.09763545089
ExecutionTime = 27.35 s  ClockTime = 31 s

Time = 101

GAMG:  Solving for p, Initial residual = 0.04799119366, Final residual = 0.002390404161, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.02803010067, Final residual = 0.0009371130484, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.007210353379, Final residual = 0.0002494798057, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.002410795933, Final residual = 0.000116871876, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.0007431402835, Final residual = 1.649739876e-05, No Iterations 3
GAMG:  Solving for p, Initial residual = 0.0003489407136, Final residual = 1.262095062e-05, No Iterations 1
time step continuity errors : sum local = 0.7142966337, global = -0.1125594528, cumulative = -0.01492400189
ExecutionTime = 41.42 s  ClockTime = 45 s

Time = 102

The line after Time = is always Iterations 1 and I want to get get that line. I have tried these ideas:
With grep, if I write grep -A 2 "Time =" test.txt, it will show
Time = 100

GAMG:  Solving for p, Initial residual = 0.2174470886, Final residual = 0.01001676933, No Iterations 1

and
ExecutionTime = 27.35 s  ClockTime = 31 s

Time = 101

If I use awk like cat test.txt | awk '/Time/{getline;getline;print}' it will show
GAMG:  Solving for p, Initial residual = 0.2174470886, Final residual = 0.01001676933, No Iterations 1
Time = 101
Time = 102

Those Time = 101 and Time = 102 are the two lines after ExecutionTime.
Briefly, I only want to get the first non-empty line after Time = which I know it is always Iterations 1. How can I get that?

AdminBee · Answer

If you want to be absolutely sure that you print the first line ending in No Iterations 1 after a Time = ... line, you could use the following awk command:
awk '/^Time =/ {f=1} /No Iterations 1$/ && f {f=0; print}' test.txt

This awk command has two rules ({ ... }).

The first one is processed if the line matches the regular expression ^Time =, i.e. it starts with the string Time =. In this rule, we simply set a flag f to 1 to indicate that the "start line" of the pattern was found.
The next one is processed if the line matches the regular expression No Iterations 1$, i.e. it ends with the string No Iterations 1, and we have previously found the Time =-line. If both conditions are met, the line is printed, and the flag f reset so that we don't print any of the following lines that end in No Iterations 1.

A more "relaxed" variant, which simply prints the first non-empty line after the Time = ... statement (not checking if it contains the No Iterations 1 pattern) would be:
awk '/^Time =/ {f=1; next} NF>0 && f {f=0; print}' test.txt

This simply looks if the flag f is set, and that NF, the "number of fields" (or columns, if you will) in the line is larger than zero. Note that here, we have to add a next command in the first rule to skip execution to the next line, as otherwise the NF>0-rule would already be processed for the Time =-lines, effectively printing only these.
For your example, both print:
GAMG:  Solving for p, Initial residual = 0.2174470886, Final residual = 0.01001676933, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.04799119366, Final residual = 0.002390404161, No Iterations 1

Get exacly a specific line after a match in a file

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