CIS 218 – Advanced UNIX (g)awk CIS 218 Advanced UNIX 1
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 CIS 218 Advanced UNIX 2
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! CIS 218 Advanced UNIX 3
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) CIS 218 Advanced UNIX 4
awk Variables (cont.) – $0 (The entire line as read from the target file) – $ n (Where n is the n th 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) CIS 218 Advanced UNIX 5
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) CIS 218 Advanced UNIX 6
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 CIS 218 Advanced UNIX 7
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 ! } CIS 218 Advanced UNIX 8
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 CIS 218 Advanced UNIX 9
Relational Expression patterns Symbol Meaning Symbol Meaning < Less than == equal to <= Less than or ~ contains the RE equal to > !~ Greater than doesn't contain RE >= && Greater than or logical and equal to != not equal to || logical or CIS 218 Advanced UNIX 10
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 CIS 218 Advanced UNIX 11
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 CIS 218 Advanced UNIX 12
# comments • Like most scripting languages # indicates a comment • awk scripts should be well documented • Comments should explain what you are doing and why. CIS 218 Advanced UNIX 13
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 CIS 218 Advanced UNIX 14
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 % CIS 218 Advanced UNIX 15
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 CIS 218 Advanced UNIX 16
printf formatting characters Format Meaning Format Meaning %c single ASCII %G shortest of %E or character %f %d decimal integer %i decimal integer %e scientific notation %o octal number %E SCIENTIFIC %s string NOTATION %f floating point %x hexadecimal (lc) %g shortest of %f or %X HEXADECIMAL %e CIS 218 Advanced UNIX 17
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 CIS 218 Advanced UNIX 18
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. CIS 218 Advanced UNIX 19
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. CIS 218 Advanced UNIX 20
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. CIS 218 Advanced UNIX 21
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. CIS 218 Advanced UNIX 22
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. CIS 218 Advanced UNIX 23
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. CIS 218 Advanced UNIX 24
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 . CIS 218 Advanced UNIX 25
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. CIS 218 Advanced UNIX 26
if conditions A < B A is less than B A <= B A is less than or equal to B A == B A equals B (note 2 =) A > B A is greater than B A >= B A is greater than or equal to B A != B A is not equal to B A ~ /RE/ A contains the regular expression RE CIS 218 Advanced UNIX 27
if • A sample if CIS 218 Advanced UNIX 28
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 CIS 218 Advanced UNIX 29
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 { } CIS 218 Advanced UNIX 30
for loop example CIS 218 Advanced UNIX 31
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 { } CIS 218 Advanced UNIX 32
while loop example CIS 218 Advanced UNIX 33
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 CIS 218 Advanced UNIX 34
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. CIS 218 Advanced UNIX 35
break example CIS 218 Advanced UNIX 36
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