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Go to the previous, next chapter.
Control statements such as if, while, and so on
control the flow of execution in awk programs. Most of the control statements
in awk are patterned on similar statements in C.
All the control statements start with special keywords such as if and while,
to distinguish them from simple expressions.
Many control statements contain other statements; for example, the if
statement contains another statement which may or may not be executed. The contained
statement is called the body. If you want to include more than one statement in
the body, group them into a single compound statement with curly braces, separating them with newlines or semicolons.
The if-else statement is awk's decision-making
statement. It looks like this:
if (condition) then-body [else else-body]
condition is an expression that controls what the rest of the statement will
do. If condition is true, then-body is executed; otherwise, else-body
is executed (assuming that the else clause is present). The else
part of the statement is optional. The condition is considered false if its value is zero
or the null string, and true otherwise.
Here is an example:
if (x % 2 == 0)
print "x is even"
else
print "x is odd"
In this example, if the expression x % 2 == 0 is true (that is, the value
of x is divisible by 2), then the first print statement is
executed, otherwise the second print statement is performed.
If the else appears on the same line as then-body, and then-body
is not a compound statement (i.e., not surrounded by curly braces),
then a semicolon must separate then-body from else. To illustrate
this, let's rewrite the previous example:
awk '{ if (x % 2 == 0) print "x is even"; else
print "x is odd" }'
If you forget the ;, awk won't be able to parse the
statement, and you will get a syntax error.
We would not actually write this example this way, because a human reader might fail to
see the else if it were not the first thing on its line.
In programming, a loop means a part of a program that is (or at least can
be) executed two or more times in succession.
The while statement is the simplest looping statement in awk.
It repeatedly executes a statement as long as a condition is true. It looks like this:
while (condition)
body
Here body is a statement that we call the body of the loop, and condition
is an expression that controls how long the loop keeps running.
The first thing the while statement does is test condition. If condition
is true, it executes the statement body. (condition is true when the
value is not zero and not a null string.) After body
has been executed, condition is tested again, and if it is still true, body
is executed again. This process repeats until condition is no longer true. If condition
is initially false, the body of the loop is never executed.
This example prints the first three fields of each record, one per line.
awk '{ i = 1
while (i
Here the body of the loop is a compound statement enclosed in braces, containing two statements.
The loop works like this: first, the value of i is set to 1. Then, the while
tests whether i is less than or equal to three. This is the case when i
equals one, so the i-th field is printed.
Then the i++ increments the value of i and the loop repeats. The
loop terminates when i reaches 4.
As you can see, a newline is not required between the condition and the body; but using
one makes the program clearer unless the body is a compound statement or is very simple.
The newline after the open-brace that begins the compound statement is not required
either, but the program would be hard to read without it.
The do loop is a variation of the while looping statement.
The do loop executes the body once, then repeats body
as long as condition is true. It looks like this:
do
body
while (condition)
Even if condition is false at the start, body is executed at
least once (and only once, unless executing body makes condition
true). Contrast this with the corresponding while statement:
while (condition)
body
This statement does not execute body even once if condition is
false to begin with.
Here is an example of a do statement:
awk '{ i = 1
do {
print $0
i++
} while (i
prints each input record ten times. It isn't a very realistic example, since in this
case an ordinary while would do just as well. But this reflects actual
experience; there is only occasionally a real use for a do statement.
The for statement makes it more convenient to count iterations of a loop.
The general form of the for statement looks like this:
for (initialization; condition; increment)
body
This statement starts by executing initialization. Then, as long as condition
is true, it repeatedly executes body and then increment. Typically initialization
sets a variable to either zero or one, increment adds 1 to it, and condition
compares it against the desired number of iterations.
Here is an example of a for statement:
awk '{ for (i = 1; i
This prints the first three fields of each input record, one field per line.
In the for statement, body stands for any statement, but initialization,
condition and increment are just expressions. You cannot set more
than one variable in the initialization part unless you use a multiple assignment statement such as x = y = 0, which
is possible only if all the initial values are equal. (But you can initialize additional
variables by writing their assignments as separate statements preceding the for
loop.)
The same is true of the increment part; to increment additional variables,
you must write separate statements at the end of the loop. The C compound expression, using C's
comma operator, would be useful in this context, but it is not supported in awk.
Most often, increment is an increment expression, as in the example above.
But this is not required; it can be any expression whatever. For example, this statement
prints all the powers of 2 between 1 and 100:
for (i = 1; i
Any of the three expressions in the parentheses following the for may be
omitted if there is nothing to be done there. Thus, for (;x > 0;)
is equivalent to while (x > 0). If the
condition is omitted, it is treated as true, effectively
yielding an infinite loop (i.e., a loop that will never
terminate).
In most cases, a for loop is an abbreviation for a while
loop, as shown here:
initialization
while (condition) {
body
increment
}
The only exception is when the continue statement (see section The continue Statement statement to a while
statement in this way can change the effect of the continue statement inside
the loop.
There is an alternate version of the for loop, for iterating over all the
indices of an array:
for (i in array)
do something with array[i]
See section Arrays in awk loop.
The awk language has a for statement in addition to a while
statement because often a for loop is both less work to type and more natural
to think of. Counting the number of iterations is very
common in loops. It can be easier to think of this counting as part of looping rather than
as something to do inside the loop.
The next section has more complicated examples of for loops.
The break statement jumps out of the innermost for, while,
or do-while loop that encloses it. The following example finds
the smallest divisor of any integer, and also identifies prime numbers:
awk '# find smallest divisor of num
{ num = $1
for (div = 2; div*div
When the remainder is zero in the first if statement, awk
immediately breaks out of the containing for loop. This means that
awk proceeds immediately to the statement following the loop and continues
processing. (This is very different from the exit statement which stops the
entire awk program. (see also: The exit
Statement) the condition of a for or while could
just as well be replaced with a break inside an if:
awk '# find smallest divisor of num
{ num = $1
for (div = 2; ; div++) {
if (num % div == 0) {
printf "Smallest divisor of %d is %d\n", num, div
break
}
if (div*div > num) {
printf "%d is prime\n", num
break
}
}
}'
The continue statement, like break, is used only inside for,
while, and do-while loops. It skips over the rest
of the loop body, causing the next cycle around the loop to begin immediately. Contrast
this with break, which jumps out of the loop altogether. Here is an example:
# print names that don't contain the string "ignore"
# first, save the text of each line { names[NR] = $0 }
# print what we're interested in END { for (x in names) { if (names[x] ~ /ignore/)
continue print names[x] } }
If one of the input records contains the string ignore,
this example skips the print statement for that record, and continues back to the first
statement in the loop.
This is not a practical example of continue, since it would be just as
easy to write the loop like this:
for (x in names)
if (names[x] !~ /ignore/)
print names[x]
The continue statement in a for loop directs awk
to skip the rest of the body of the loop, and resume execution with the
increment-expression of the for statement. The following program illustrates
this fact:
awk 'BEGIN {
for (x = 0; x
This program prints all the numbers from 0 to 20, except for 5, for which the printf
is skipped. Since the increment x++ is not skipped, x does not
remain stuck at 5. Contrast the for loop above with the while
loop:
awk 'BEGIN {
x = 0
while (x
This program loops forever once x gets to 5.
As described above, the continue statement has no meaning when used
outside the body of a loop. However, although it was never documented, historical
implementations of awk have treated the continue statement
outside of a loop as if it were a next statement (see section The next Statement silently supports this usage.
However, if -W posix has been specified on the command line (see section Invoking awk standard specifies that continue
should only be used inside the body of a loop.
The next statement forces awk to immediately stop processing
the current record and go on to the next record. This means that no further rules are
executed for the current record. The rest of the current rule's
action is not executed either.
Contrast this with the effect of the getline function
(see section Explicit Input with getline to
read the next record immediately, but it does not alter the flow of control in any way. So
the rest of the current action executes with a new input
record.
At the highest level, awk program execution is a loop that reads an input
record and then tests each rule's pattern against it. If you think of this loop as a for
statement whose body contains the rules, then the next statement is analogous
to a continue statement: it skips to the end of the body of this implicit
loop, and executes the increment (which reads another record).
For example, if your awk program works only on records with four fields,
and you don't want it to fail when given bad input, you might use this rule near the beginning of the program:
NF != 4 {
printf("line %d skipped: doesn't have 4 fields", FNR) > "/dev/stderr"
next
}
so that the following rules will not see the bad record. The error message is
redirected to the standard error output stream, as error messages should be. See section Standard I/O Streams.
According to the POSIX standard, the behavior is undefined if the next
statement is used in a BEGIN or END rule. gawk will treat it as a syntax error.
If the next statement causes the end of the input to be reached, then the
code in the END rules, if any, will be executed. See section BEGIN and END Special Patterns
Statement
The next file statement is similar to the next statement.
However, instead of abandoning processing of the current record, the next file
statement instructs awk to stop processing the current data file.
Upon execution of the next file statement, FILENAME is
updated to the name of the next data file listed on the command line, FNR is
reset to 1, and processing starts over with the first rule
in the progam. See section Built-in Variables.
If the next file statement causes the end of the input to be reached, then
the code in the END rules, if any, will be executed. See section BEGIN and END Special Patterns
statement is a gawk extension; it is not (currently) available in any other awk
implementation. You can simulate its behavior by creating a library file named nextfile.awk,
with the following contents. (This sample program uses user-defined functions, a feature
that has not been presented yet. See section User-defined
Functions, for more information.)
# nextfile --- function to skip remaining records in current file
# this should be read in before the "main" awk program
function nextfile() { _abandon_ = FILENAME; next }
_abandon_ == FILENAME && FNR > 1 { next } _abandon_ == FILENAME &&
FNR == 1 { _abandon_ = "" }
record. Since this file is read before the main awk program, the rules
that follows the function definition will be executed
before the rules in the main program. The first rule
continues to skip records as long as the name of the input file has not changed, and this
is not the first record in the file. This rule is
sufficient most of the time. But what if the same data file is named twice in a
row on the command line? This rule would not process the
data file the second time. The second rule catches this
case: If the data file name is what was being skipped, but FNR is 1, then
this is the second time the file is being processed, and it should not be skipped.
The next file statement would be useful if you have many data files to
process, and due to the nature of the data, you expect that you would not want to process
every record in the file. In order to move on to the next data file, you would have to
continue scanning the unwanted records (as described above). The next file
statement accomplishes this much more efficiently.
The exit statement causes awk to immediately stop executing
the current rule and to stop processing input; any
remaining input is ignored.
If an exit statement is executed from a BEGIN rule the program stops processing everything immediately.
No input records are read. However, if an END rule
is present, it is executed (see section BEGIN and END
Special Patterns is used as part of an END rule,
it causes the program to stop immediately.
An exit statement that is part of an ordinary rule (that is, not part of a BEGIN or END
rule) stops the execution of any further automatic
rules, but the END rule is executed if
there is one. If you do not want the END rule
to do its job in this case, you can set a variable to nonzero before the exit
statement, and check that variable in the END rule.
If an argument is supplied to exit, its value is used as the exit status
code for the awk process. If no argument is supplied, exit
returns status zero (success).
For example, let's say you've discovered an error condition you really don't know how
to handle. Conventionally, programs report this by exiting with a nonzero status. Your awk
program can do this using an exit statement with a nonzero argument. Here's
an example of this:
BEGIN {
if (("date" | getline date_now) "/dev/stderr"
exit 4
}
}
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