Unix Shell Scripts

What are scripts ? Text files in certain format that are run by another program Examples: –Perl –Javascript –Shell scripts (we learn c-shell scripts)

How they differ from C programs ? C programs are compiled into machine code (executables) Executables are not portable, but they run faster (no translation during runtime)

What to use ? Use scripts for lightweight operations: –Simple manipulations on files –Sending mails –Network communication –Rapid development Use c programs for: –CPU bound computations –Complex data structures –Critical time applications –Embedded applications

Some useful c-shell commands head / tail – returns first / last lines of a file echo – print command sort – used to sort files in lexicographic order cat – concatenate files and print grep – find regular expressions in files find – find files by criteria wc – word count on files diff - find differences between files basename / dirname – extract file / directory name from full path touch – change file timestamp mail – sending mail whereis – locate program

Redirection and pipes prog redirection file > : redirect stdout >> : append stdout >& : redirect stdout and stderr >>& : append stdout and stderr < : redirect stdin prog1 pipe prog2 | : redirect stdout of prog1 to stdin of prog2 |& : same, with stdout and stderr

How to write a c-shell script Edit the code file (no mandatory extension) Make sure the first line in the file is: #!/bin/csh –f (for c-shell scripts) #!/bin/awk –f (for awk scripts) Add executable permission: chmod +x filename

#!/bin/csh –f ################ #simple_example # ################ # We can define variables and print their values set course = soft1 echo $course # We can do the same for arrays set names = ( Danny Dina Eyal Ayelet Ori Neta ) echo $names echo $#names # size of array echo $names[2] # the second element echo $names[2-] # starting from the second element echo $names[-2] # until the second element echo $names[2-3] # elements 2,3

# at the beginning of the line - treat as an arithmetic expression and not # as a num = 17 echo num -= 3 echo num *= 14 echo $num # if we want to assign the value of a command to a variable set chars = `wc -l./simple_example` echo $chars # accessing program parameters echo The program name is : $0, the first parameter is $1 and the second is $2 echo The number of parameters \(not including program name\) is $#argv

Loops foreach identifier (set)... end while (condition)... end

Conditional statements if condition then... endif switch (value) case value1: breaksw... default: endsw

Testing files attributes if -op file_name -r : read access -w : write access -x : execute access -e : existence -o : ownership -f : plain file -d : directory -l : link

#!/bin/csh –f ######## # sum # ######## if $#argv == 0 then echo Usage: $0 num1 [num2 num3...] exit 1 sum = 0 foreach sum += $number end echo The sum is : average = $sum / remainder = $sum % $#argv echo The avergae is: $average\($remainder\)

#!/bin/csh -f ############ # sort_files # ############ if $#argv == 0 then echo USAGE: $0 file_names_to_sort echo This command writes on the original files \!\!\! exit 1 endif foreach file($argv) sort $file > $file.tmp mv $file.tmp $file end

#!/bin/csh -f # Biggest_file # INPUT: Directory name # OUTPUT: The file with the biggest number of characters in the given directory if $#argv == 0 then echo USAGE: $0 directory_name exit 1 endif if -d $1 max = 0 foreach file($1/*) if (-r $file && -f $file) then set wc_out = `wc -c $file` if ($wc_out[1] > $max) then set biggest_file = max = $wc_out[1] endif else if !(-r $file) then echo $file unreadable endif end echo The biggest file is $biggest_file echo The number of characters is $max else echo $1 is not a directory endif

#!/bin/csh -f # Modulo3 # INPUT: sequence of integer numbers separated by \n terminated by 0 # OUTPUT: prints the value of each number modulo 3 set num = $< while ($num != modulo = $num % 3 switch ($modulo) case 0: echo 0 breaksw case 1: echo 1 breaksw case 2: echo 2 breaksw default: endsw set num = $< end

Retrieving returned values Returned values (by return, exit) are stored in status environment variable The script: #!/bin/csh -f./prog if $status != 0 then echo "Error occured!" endif runs prog (from current dir) and reports whether an error occurred.

awk Useful utility for file manipulation For details see :

#!/bin/awk -f BEGIN { stud_num = total = high_stud_num = total_high = 0} NF != 3 { print "error in line", FNR, ":", $0 next } { ++stud_num total += $3 if ($3 > 80){ total_high += $3 ++high_stud_num } END { print "Number of students: ", stud_num print "Avergae grade is: ", total / stud_num print "Average of high grades is: ", total_high / high_stud_num}

Running with input : Ron Yael Ran Yoav Tal Output is : error in line 5 : Tal Number of students: 4 Avergae grade is: Average of high grades is: