cis 218 advanced unix1 cis 218 – advanced unix (g)awk

CIS 218 Advanced UNIX 1

CIS 218 – Advanced UNIX

(g)awk


Overview

• awk is a programming language

• Awk uses syntax based on grep and sed for handling numbers and text

• awk provides field level addressability. And within a field (word) using substring commands

• awk works field by field


awk command syntax

• There are two ways to execute an awk program/script:– awk [-F field-separator] ‘program’ target-file– awk [-F field-separator] -f program.file target

• From our discussion of sed, and Refrigerator Rule No. 5, I would hope you are firmly committed to the second form!


awk Variables

• There are a number of awk variables that are very useful– FS (The field separator, defaults to white space)– OFS (Output field separator, can be critical)– NR (Number of records, a sequential counter)– NF (Number of fields in the current record)– FILENAME (Name of the current target file)


awk Variables (cont.)

– $0 (The entire line as read from the target file)– $n (Where n is the nth field in the record. This

is how we get field level addressability in awk)

• nawk, gawk, etc give us more variables, the most significant two are:– ARGC (the count of the command line

arguments)– ARGV (an array of the command line

arguments)


Parts of a program

• All programs are composed of one or more of the following three constructs:– sequence (a series of instructions, one

following the next, executed sequentially)– selection (the ability of the code to decide

which instructions to execute, conditional execution)

– iteration (adding looping so that selected code will be repeated over an over)


awk Program Format

• Awk programs are composed of pattern {action} pairs (actions must be

enclosed in French braces {} )– a pattern without a corresponding action takes

the default action, print $0– an action without a corresponding pattern is

applied to every line– each input line is submitted to every

pattern/action pair


awk Program Format (cont.)• Placement of the open French brace is critical

– pattern { both patterns are action 1 executed for

lines action 2 matching

the pattern }– pattern lines matching the pattern

{action 1 are printed, and both

action 2 actions are

performed on } every line!


Patterns

• In an awk program, the pattern is the selection tool that decides what actions are applied to which lines.

• Patterns can be:– relational expressions– regular expressions– magic patterns


Relational Expression patternsSymbol Meaning Symbol Meaning

< Less than == equal to

<= Less than or equal to

~ contains the RE

> Greater than !~ doesn't contain RE

>= Greater than or equal to

&& logical and

!= not equal to || logical or


Regular Expression patterns

• Must be enclosed in slashes /RE/• Anchors apply to the entire line if they are

used as the only pattern• Remember, you can use regular expressions

in relational patterns with ~ and !~ to apply them to fields

• Both true regular expressions and fixed patterns can be used as REs in awk


Pre/Post Processing• There are two in awk:

– BEGIN {the action associated is performed before the target file is opened}

– END {the action associated is performed after the target file is successfully closed}

• Both are coded in UPPER CASE


# comments

• Like most scripting languages # indicates a comment

• awk scripts should be well documented

• Comments should explain what you are doing and why.


print

• The print command is the simplistic output tool for awk. Basically and “echo”/

• You can direct print to send its data to a file with the > operator

• Generally print is used for simple output or debugging output


printf

• Similar in concept to the “C” language command. The format of a printf command is:

printf (“formatting string”,variables)

• The formatting characters correspond to the variables one for one in both lists.

• Each formatting character is prefixed by %


printf (cont.)

• The formatting specifiers contain then following characters:– - indicates that the data should be left justifed– n indicates the minimum width of the field– .n indicates the maximum width of the field

“%-5s”indicates a string field, left

justified, of width 5 bytes


printf formatting characters

Format Meaning Format Meaning

%c single ASCII character

%G shortest of %E or %f

%d decimal integer %i decimal integer

%e scientific notation %o octal number

%E SCIENTIFIC NOTATION

%s string

%f floating point %x hexadecimal (lc)

%g shortest of %f or %e

%X HEXADECIMAL


printf spacing characters

• There are two characters available to change the spacing of your text:– \n inserts a newline character. You must use

this if you want your output to occur on successive lines.

– \t inserts a tab character


getline

• getline is used to read from the keyboard• It can also capture the results of a command

but this form is seldom used• Read from the keyboard using

getline variable < “/dev/tty” • If you don’t supply a variable, awk will use

$0, so in most cases you want to use a variable.


rand() srand()

• The rand() function generates pseudo-random numbers in the range 0 - 1.

• Given the same seed, it will always generate the same series of numbers.

• srand() is used to supply a new seed to rand().

• If you don’t supply srand() a value, it uses the current time as the seed.


system()

• The system() function allows you to execute system commands within an awk script.

• You must enclose the system command in quotation marks.

• You cannot capture the output from the system() function within the script but you can capture the return code.


length()

• The length([argument]) function returns the length of the argument in bytes.

• If you give length() a number, it will return the number of digits in the number.

• If you don’t give length() an argument, it will use $0 by default.


index()

• The index(string,target) function returns the position of the first occurrence of the target within the string.

• The index() function is often used to set the boundary for the substr() function.


substr()

• The substr(string,start[,length]) function will return the part of the string beginning with start and continuing for length bytes.

• If you don’t give it a length, it will return all the bytes between the start and the end of the string.


split()

• You will use split(string, array[, separator]) to divide a string into parts using separator to parse them, storing the resultant parts in the array.

• If you don’t code a separator, the function will use the field separator to parse the string.


if• Besides using patterns, if gives us another

way to perform selection• The format of an if statement is

if (condition){verb(s)}[else

{ verb(s)}]

• If you have more than one verb, they must be enclosed in French braces.


if conditions

A < B A is less than BA <= B A is less than or equal to BA == B A equals B (note 2 =)A > B A is greater than B

A >= B A is greater than or equal to BA != B A is not equal to B

A ~ /RE/ A contains the regularexpression RE


if

• A sample if


exit

• The input file is closed

• Control is transferred to the action associated with the END magic pattern if there is one

• Generally used as a bailout in case of catastrophic errors


for loop• This is a counted loop

• executes until the counter reaches the target value

• Increment (count up) or decrement (count down)

• also works with the elements of an array

• multiple verbs must be enclosed in { }


for loop example


while loop

• The while loop is an example of conditional execution

• The loop cycles as long as the condition specified is true

• A while loop always checks to see if it should execute

• multiple verbs must be enclosed in { }


while loop example


do/while

• Even though it has a while in it, this is an example of until logic.

• Until logic is shunned by conscientious coders.

• ‘nuff said


break

• Used to exit from a loop

• Control is passed to the line following the end of the loop

• Causes an exit from the loop but NOT the awk script. If you want to bail out of the whole script, use the exit command.


break example


continue

• Causes awk to skip the rest of the body of the loop for the current value

• In a for loop the counter is incremented, and the next cycle of the loop is started

• In a while loop, the next iteration of the loop starts


continue example


next

• Causes the script to start over

• takes the next element from standard input or the target file

• Like exit, this command effects the whole script


next example

cis 218 advanced unix1 cis 218 – advanced unix (g)awk

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