Control Structures

You've already seen various examples requiring conditional expressions. This chapter will revisit the if-else control structure and the ternary operator. Then you will see some examples with explicit loops (recall that awk is already looping over input records). Followed by keywords that control loop flow. Most of the syntax is very similar to the C language.

info The example_files directory has all the files used in the examples.


Mostly, when you need to use if control structure, you can get away with using the condX{actionX} blocks instead. But sometimes, you need additional condition checking within such action blocks. Or, you might need it inside loops. The syntax is if(cond){action} where the braces are optional if you need only one statement. if can be optionally followed by multiple else if conditions and a final else condition. These can also be nested as needed.

# print all lines starting with 'b'
# additionally, if the last column is > 0, then print some more text
$ awk '/^b/{print; if($NF>0) print "------"}' table.txt
brown bread mat hair 42
blue cake mug shirt -7

# same as above, but uses the 'else' condition as well
$ awk '/^b/{print; if($NF>0) print "------"; else print "======"}' table.txt
brown bread mat hair 42
blue cake mug shirt -7

The ternary operator often reduces the need for single statement if-else control structures.

# same as: awk '{if(NR%3) ORS="-" ; else ORS=RS} 1'
$ seq 6 | awk '{ORS = NR%3 ? "-" : RS} 1'

# note that parentheses is necessary for print in this case
$ awk '/^b/{print; print($NF>0 ? "------" : "======")}' table.txt
brown bread mat hair 42
blue cake mug shirt -7

info See also stackoverflow: finding min and max value of a column and gawk manual: switch.


for loops are handy when you are working with arrays. Also for processing input fields, since $N syntax allows passing an expression instead of just fixed values.

$ awk 'BEGIN{for(i=2; i<7; i+=2) print i}'

# looping each field
$ awk -v OFS=, '{for(i=1; i<=NF; i++) if($i ~ /^[bm]/) $i="["$i"]"} 1' table.txt

Here's an example of looping over a dynamically constructed array.

$ cat marks.txt
Dept    Name    Marks
ECE     Raj     53
ECE     Joel    72
EEE     Moi     68
CSE     Surya   81
EEE     Tia     59
ECE     Om      92
CSE     Amy     67

# average marks for each department
$ awk 'NR>1{d[$1]+=$3; c[$1]++} END{for(k in d) print k, d[k]/c[k]}' marks.txt
ECE 72.3333
EEE 63.5
CSE 74

You can use break and continue to alter the normal flow of loops. break will cause the current loop to quit immediately without processing the remaining statements and iterations. continue will skip the remaining statements in the loop and start the next iteration.

$ awk -v OFS=, '{for(i=1; i<=NF; i++) if($i ~ /b/){NF=i; break}} 1' table.txt

info See also stackoverflow: find missing numbers from sequential list.

awk supports the while and do-while loop mechanisms as well.

$ awk 'BEGIN{i=6; while(i>0){print i; i-=2}}'

# recursive substitution
$ echo 'titillate' | awk '{while(gsub(/til/, "")) print}'
$ echo 'titillate' | awk '{do{print} while(gsub(/til/, ""))}'


next is similar to the continue statement but it acts on the default loop that goes through the input records. It doesn't affect BEGIN or END blocks as they are outside the record looping. When next is executed, rest of the statements will be skipped and next input record will be fetched for processing.

$ awk '/\<par/{print "%% " $0; next} {print /s/ ? "X" : "Y"}' anchors.txt
%% sub par
%% cart part tart mart

You'll see more examples with next in the coming chapters.


You saw the use of exit earlier to quit early and avoid unnecessary processing of records. If an argument isn't passed, awk considers the command to have finished normally and the exit status will indicate success. You can pass a number argument for other cases.

$ seq 3542 4623452 | awk 'NR==2452{print; exit}'
$ echo $?

$ awk '/^br/{print "invalid data"; exit 1}' table.txt
invalid data
$ echo $?

# any remaining files to be processed are also skipped
$ awk 'FNR==2{print; exit}' table.txt greeting.txt
blue cake mug shirt -7

If exit is used in BEGIN or normal blocks, any code in the END block will still be executed. For more details and corner cases, see gawk manual: exit.

# first print is executed
# on seeing exit, rest of BEGIN and normal blocks are skipped
# code in the END block is then executed
$ awk 'BEGIN{print "hi"; exit; print "hello"}
       END{print "bye"}' table.txt


This chapter covered some of the control flow structures provided by awk. These features makes awk flexible and easier to use compared to sed.

Next chapter will discuss some of the built-in functions.


info The exercises directory has all the files used in this section.

1) The input file nums.txt contains a single column of numbers. Change positive numbers to negative and vice versa. Solution should use the sub function and shouldn't explicitly use the if-else control structure or the ternary operator.

$ cat nums.txt

$ awk ##### add your solution here

2) For the input file table.txt, change the field separator from space to the , character. Also, any field not containing digit characters should be surrounded by double quotes.

$ awk ##### add your solution here

3) For each input line of the file secrets.txt, remove all characters except the last character of each field. Assume space as the input field separator.

$ cat secrets.txt
stag area row tick
deaf chi rate tall glad
Bi tac toe - 42

$ awk ##### add your solution here

4) For the input file sample.txt, emulate the q and Q commands of sed as shown below.

# sed '/are/q' sample.txt will print till the line containing 'are'
$ awk ##### add your solution here
Hello World

Good day
How are you

# sed '/are/Q' sample.txt is similar to the 'q' command,
# but the matching line won't be part of the output
$ awk ##### add your solution here
Hello World

Good day

5) For the input file addr.txt:

  • if a line contains e
    • delete all occurrences of e
    • surround all consecutive repeated characters with {}
    • assume that the input will not have more than two consecutive repeats
  • if a line doesn't contain e but contains u
    • surround all lowercase vowels in that line with []
$ awk ##### add your solution here
H{ll}o World
How ar you
This gam is g{oo}d
T[o]d[a]y [i]s s[u]nny
You ar fu{nn}y

6) The goal is to print found you if the input file contains you and not found otherwise. However, both the print statements are executed in the awk code shown below. Change it to work as expected.

$ awk '/you/{print "found you"; exit} END{print "not found"}' addr.txt
found you
not found