AWK A Pattern scanning and processing language made by Aho Weinberger Kernighan and explained by Albert Montijn

AWK ● Usage: – awk-instructions on the command line ● awk 'awk-instructions' filename... – awk-instructions in a script file: ● awk -f awk-scriptfile... – awk-option: set field-separator ● awk -F:... – awk-option: set variable ● awk -v var=value...

AWK ● Statements in an AWK program are built of lines with patterns and actions: pattern { action } pattern { action ; action ;...} pattern (missing action) { action(s) } (missing pattern) ● Patterns: /regular expression/ $1 == “abc” && NF > 3 || $3 ~ /regex/ BEGIN END

AWK ● Variables: ● NRrecordnumber ● NFnumber of fields ● FSfield separator ● $ncontents of field n. n can be an expression.Eg.: ${NF-1} ● a,buntyped, number or string reference implies existence ● Functies: ● length(X), index(s,t) ● substr(s,i,n) ● sub(r,s,t), gsub(r,s,t) ● match(s,r), split(s,a,r) ● print, (s)printf(fmt,expr-list)

AWK ● Operators in Increasing Precedence ● Assignment:=, +=, -=, *=, /=, %=, ^= ● Logical:||, &&, ~, !~ ● Relational: =, > ● Concatenation: blank ● Add/Subtract:+, - ● Multiply/divide/mod:*, /, % ● Unary plus, minus, not, exponent (^ or **) ● Increment, decrement ● Field:$ expr

AWK ● Arrays ● Associative arrays (hash): index can be any value (integer or string) ● Referencing creates entry: if (arr[“x”] != “”) print “arr[x] exists” ● Control flow statements ● if ( expr ) statement [ else statement ] ● if ( subscript in array )... ● while ( expr ) statement ● for ( init_expr ; test_expr ; increment_expr ) statement ● for ( subscript in array ) statement ● do statement while ( expr ) ● break, continue, next, exit [ expr ], return [ expr ]

AWK ● Reguliere expressies (in AWK) ● c matches non metacharacter c ● \c matches literal character c ●.matches any character but newline ● ^matches beginning of line or string ● $matches end of line or string ● [abc...] character class matches any of abc... ● [^...] negated class matches any but... ● r1|r2 matches r1 or r2 ● r1r2 concatenation: matches r1, then r2 ● r+ matches one or more r's ● r* matches zero or more r's ● (r) grouping: mathes r

AWK ● Functions – Definition func[tion] funcname(parameters..., local_variables){... [return expr] } – Usage [var = ] funcname(actual params)

AWK Geef antwoord op vragen over /etc/passwd 1.Wat is het aantal regels? 2.Wat is de tiende regel? 3.Wat is het laatste veld van elke regel? 4.Wat is het eerste veld van de laatste regel? 5.Geef alle regels waarvan het derde veld > 20 6.Geef alle regels met een regelnummer 7.Geef alle regels zonder het tweede veld 8.Verwissel alle eerste en laatste velden 9.Geef alle regels met eerste veld korter dan 5 10.Welke regels bevatten /etc in voorlaatste veld? 11.Wat is het totaal aantal velden in de file? 12.Wat is het hoogste waarde van het derde veld?