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AWK

A Pattern scanning and processing language made by Aho Weinberger Kernighan and explained by Albert Montijn. AWK. AWK. Usage: awk-instructions on the command line awk 'awk-instructions' filename ... awk-instructions in a script file: awk -f awk-scriptfile ...

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AWK

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

  2. 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 ...

  3. 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

  4. AWK • Variables: • NR recordnumber • NF number of fields • FS field separator • $n contents of field n. n can be an expression. Eg.: ${NF-1} • a,b untyped, 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)

  5. 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

  6. 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]

  7. 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

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

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

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