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

Shell Scripts - Usefulness
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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
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Create custom filters to dynamically alter text
Generate a composite file combining data from a set of files
Simplify sequences of commands

Test
 Evaluate an expression
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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

More Test
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[ $i -eq 2 ], [ $i -ne 1 ]
– integer equality, inequality test
[ $i -lt 20 ], [ $i -le 4 ]
– integer L ess T han, L ess than or E qual test
[ $i -gt 0 ], [ $i -ge 5 ]
– integer G reater T han, G reater than or E qual test
[ ! -d $file ]
– NOT a directory = true

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

Cut
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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>
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– 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

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

expr – evaluates math expressions
 expr 2 + 2
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Outputs 4
White space around “+” matters
 Other operations:
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+ - * / % 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.

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

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)

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

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

Shell Scripts - Quotations
 Back-quotes
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Executes quoted command
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 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

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

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

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

Shell Scripts - Loops
 While-loops while test-cmds do commands done

Shell Scripts - Loops
 For-loops for x in list do commands done

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

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

Shell Scripts - Loops
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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

Shell Scripts - Exercise
 Print out the greatest of the variables $1, $2 and $3 (These represent the first 3 commandline arguments passed to the script)