Unix and Software Tools (P51UST) Awk Programming (3)

1. Unix and Software Tools (P51UST) Awk Programming (3) Ruibin Bai (Room AB326) Division of Computer Science The University of Nottingham Ningbo, China P51UST: Unix and Software Tools

2. Contents • Missing things • Reading input from a pipeline • More string functions • System variables • Forcing variable types • Arithmetic functions P51DBS: Database Systems

3. Reading input from a pipe • The UNIX “who am i” command will give the following type of output: • This output can be piped to getline: • “who am i” | getline • Here, $0 will be set to the output of the command, the line will be parsed into fields such that “zlizrb1” will be put in field $1, “pts/32” will be put into $2, etc. The system variable NF will be set $ who am i zlizrb1 pts/21 Apr 17 15:15 (pcname)

4. Reading Input From a Pipe This script pipes the result of the “who am i” command to getline which parses it into fields. The variable “name” is assigned to field number 1 and the Field Separator is assigned to “:” $ awk ′ BEGIN { “who am i” | getline name = $1 FS = “:” } name ~ $1 {print $5} ′ /etc/passwd The script then tests to see whether the first field ($1) in /etc/passwd is the same as that stored in name (the fields in /etc/passwd are separated by a “:”) If so, the 5th field of /etc/passwd is printed (which contains the corresponding user’s full name)

5. Some Important Limitations • There is a limit to the number of pipes and files that the system can have open at any one time • This limit varies from system to system • In most implementations of awk, up to 10 open files is allowed. • Use the close() function! • Some other limits are: • Number of fields per record • Characters per input record • Characters per field • See the awk manual page for more information

6. Using close() with Pipes and Files • Why use close()? • So your program can open as many pipes and files as it needs without exceeding the system limit • It allows your program to run the same command twice • You may need close() to force an output pipe to finish its work • { do something | “sort > myFile” } • END { • close(“sort > myFile”) • while ((getline < “myFile”) > 0) • { do more stuff }

7. Advanced String Functions (1) • gsub(regex,s,str) • Globally substitutes s for each match of the regular expression regex in the string str. Returns the number of substitutions. If a string str is not supplied, it will use $0 P51UST: Unix and SoftwareTools

8. Advanced String Functions (2) • asort(src[,d]) • Supported in gawk • The function sorts the array src based on the element values. • If d is specified, the function will make a copy of src into d and then d is sorted. • Also replaces the indices with values from 1 to the number of elements in the array. P51UST: Unix and SoftwareTools

9. Advanced String Functions (3) • asorti(src[,d]) • Supported in gawk • Like sort(), but the sorting is done based on indeces in the array, not based on the element values. • The value of original indeces will be stored in the array. P51UST: Unix and SoftwareTools

10. asort vs asorti BEGIN{ arr[1]="a“; arr[2]="d" arr[4]="f“; arr[8]="c " asort(arr,arrcpy1) asorti(arr,arrcpy2) print "Original array" for(i in arr) print "arr["i"]=" arr[i] print "Array after using asort" for(i in arrcpy1){ print "arr["i"]=" arrcpy1[i] } print "Array after using asorti" for(i in arrcpy2){ print "arr["i"]=" arrcpy2[i] } } $ awk -f asorti.awk Original array arr[4]=f arr[8]=c arr[1]=a arr[2]=d Array after using asort arr[4]=f arr[1]=a arr[2]=c arr[3]=d Array after using asorti arr[4]=8 arr[1]=1 arr[2]=2 arr[3]=4 P51UST: Unix and SoftwareTools

11. System Variables that are Arrays • There are two system variables that are arrays in Gawk: • ARGV • An array containing the command line arguments given to awk. • The number of elements is stored in another variable called ARGC (not an array) • The array is indexed from 0 (unlike other arrays in awk) • The last element is therefore ARGC-1 • E.g. ARGV[ARGC-1], ARGV[2] • The first element is the name of the command that invoked the script P51UST: Unix and SoftwareTools

12. System Variables that are Arrays (2) • ENVIRON • An array containing environment variables • Each element is the value of the current environment • The index of each element is the name of the environment variable • E.g. ENVIRON[“PATH”], ENVIRON[“SHELL”] P51UST: Unix and SoftwareTools

13. ARGV Example BEGIN { for (x=0; x<ARGC; x++) print ARGV[x] print ARGC } Output $ awk -f parameters.awk 2007 G51UST “Gail Hopkins” students=80 - awk 2007 G51UST Gail Hopkins Students=80 - 6 P51UST: Unix and SoftwareTools

14. Use of Backslash • Backslash can be used: • To continue strings across new lines marian$ awk ‘BEGIN {print ″hello, \ > world″ }’ output hello, world P51UST: Unix and SoftwareTools

15. Forcing Variable Types • In awk, you do not declare variables and give them types • Sometimes you want to force awk to treat a variable as a particular type, e.g. as a number or as a string. • To force a variable, x, to be treated as a number, put in the line: • x=x+0 • To force a variable, x, to be treated as a string, put in the line: • x=x “” P51UST: Unix and SoftwareTools

16. Built in Arithmetic Functions • awk has a number of arithmetic functions that are built in. Some are shown below: • exp(x) • int(x) • sqrt(x) • cos(x) Returns e to the power x Returns a truncated value of x Returns the square root of x Returns the cosine of x P51UST: Unix and SoftwareTools

17. A Summary of Awk Functions P51UST: Unix and SoftwareTools