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CMPSC 60: Week 6 Discussion

CMPSC 60: Week 6 Discussion. Originally Created By: Jason Wither Updated and Modified By: Ryan Dixon. University of California Santa Barbara. Shell Scripts - Usefulness. Mass file modification Run a program on a set of files Rename a set of files Convert image files from Bitmap to JPEG

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CMPSC 60: Week 6 Discussion

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  1. CMPSC 60: Week 6 Discussion Originally Created By: Jason Wither Updated and Modified By: Ryan Dixon University of California Santa Barbara

  2. Shell Scripts - Usefulness • Mass file modification • Run a program on a set of files • Rename a set of files • Convert image files from Bitmap to JPEG • Backup critical files • Tailor command input and output • Create custom filters to dynamically alter text • Generate a composite file combining data from a set of files • Simplify sequences of commands

  3. Test • Evaluate an expression • When the expression is true, exits with 0 • When false, exits with a non-zero value • “test -e filename” == “[ -e filename ]” • if filename exists, exits with 0 • “test $var1 = $var2”, “[ $var1 != $var2 ]” • string equality, inequality test

  4. More Test • [ $i -eq 2 ], [ $i -ne 1 ] • integer equality, inequality test • [ $i -lt 20 ], [ $i -le 4 ] • integer Less Than, Less than or Equal test • [ $i -gt 0 ], [ $i -ge 5 ] • integer Greater Than, Greater than or Equal test • [ ! -d $file ] • NOT a directory = true

  5. Test Example • An example: for i in `ls` do if [ -d $i ] then echo $i fi done

  6. Cut • Cut out selected bytes, columns, or fields • cut -b10-15 <file> • prints bytes 10-15 of each line in <file> • cut -c1-10,20-22 <file> • prints characters 1-10 and 20-22 of each line in <file> • cut -f2 <file> • prints the second field of each line in <file> • by default, fields are separated by tabs • the delimiter can be changed from tabs using the -d option

  7. Head / Tail • Prints the first / last lines of a file • head <file> • Prints the first 10 lines of <file> • tail -15 <file> • Prints the last 15 lines of <file> • tail -2 <file> | head -1 • Prints out only the second to last line of file

  8. expr – evaluates math expressions • expr 2 + 2 • Outputs 4 • White space around “+” matters • Other operations: • + - * / % and more • Note that some operators may need to be escaped to be called from the command line. • expr 2 * 2 # Results in an error since “*” is a wild card. • expr 2 \* 2 # Correct expression, asterisk is escaped.

  9. Shell Scripts - How they start • Begins with #! (Sha-Bang) • #! is followed by the interpreter to use • #!/bin/sh (what we’ll be doing) • #!/usr/bin/perl • etc… • BTW, white-space matters…

  10. Shell Scripts - Variables • Case-sensitive • Created as needed, simply assign to a name • var=3 or var=“Your favorite band” • Access by pre-pending with a $ • echo $var (prints out the value of var)

  11. Shell Scripts - Variable Manipulation • Working with numbers • var=$t + 2 • var=`expr $t+2` • var = `expr $t + 2` • var=`expr $t + 2` (WRONG!!!!!!) (WRONG!!!!!!) (WRONG!!!!!!) Correct Solution

  12. Shell Scripts - Quotations • Double-quotes • Variables within are resolved • If var=“This is cool” • echo “$var stuff” • Outputs “This is cool stuff” • Single-quotes • String is treated literally • Echo ‘$var stuff’ • Outputs “$var stuff”

  13. Shell Scripts - Quotations • Back-quotes • Executes quoted command • echo “Today’s date is `date`” • Prints “Today’s date is Wed May 19…” • var=`ls -l` • var now contains the output of the ls -l command

  14. Shell Scripts - Conditionals • If-statements if test-cmds then commands else commands fi • Else portion is optional

  15. Shell Scripts - Conditionals • Multiple If-statements in a row if test-cmds then commands elif test-cmds then commands else commands fi

  16. Shell Scripts - Conditionals • Examples if [ $var -gt 0 ] then echo ‘$var is greater than zero’ else echo ‘$var is not greater than zero’ fi

  17. Shell Scripts - Loops • While-loops while test-cmds do commands done

  18. Shell Scripts - Loops • For-loops for x in list do commands done

  19. Shell Scripts - Loops • Example: for x in 1 2 3 4 5 do echo “$x” done • Outputs: 1 2 3 4 5

  20. Shell Scripts - Loops • Example: x=1 while [ $x -le 3 ] do echo “$x” x=`expr $x + 1` done • Outputs: 1 2 3

  21. Shell Scripts - Loops • Example: for x in `ls` do if [ -d $x ] then chmod 750 $x else chmod 640 $x fi done • Sets all directories with rwxr-x--- and all other files with rw-r----- permissions

  22. Shell Scripts - Exercise • Print out the greatest of the variables $1, $2 and $3 (These represent the first 3 command-line arguments passed to the script)

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