1P51UST: Unix and Software Tools

Unix and Software Tools (P51UST)

More Shell Programming

Ruibin Bai (Room AB326)

Division of Computer Science

The University of Nottingham Ningbo, China

2

Introduction

• More input/output– echo

– printf

• Flow control

• Arithmetic

P51UST: Unix and Software Tools

echo

• Displays a line of text to standard output. echo [options] [string]

• Options may be different in different shells

• Common options:-n suppress the trailing line (we’ve used this

already)

-e enable interpretation of backslash escapes

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echo

• If -e option is used, the following formatting characters can be used

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Option Function

\b Backspace

\n New line

\t Horizontal tab

\v Vertical tab

\a Alert (bell)

\c Suppress trailing newline

Printf

• A powerful string formatting and printing command

• Similar to printf in C, it interprets % and / metacharacters.

printf format[arguments]

where format: %[flags][width][.precision]specifieroFlags -: left-justify

+: Forces to proceed the result with a plus or minus sign

oWidth: minimum number of characters to be printed

oPrecision: the number of digits to be printed after the decimal point

oSpecifier: is the most significant one and defines the type and the interpretation of the value of the corresponding argument

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Printf – specifier table

• Examples:#!/bin/sh

name=“Ruibin”

printf “Hello %-10s, average=%+7d\n” $name 30

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Specifier

Output Example

s String of characters sample1

d or i Signed decimal integer 342

f Decimal floating point 342.65

e (or E) Scientific notation (mantise/exponent) using e (or E) character

3.4265e+23.4265E+2

% A % followed by another % character will write % to stdout

%

Flow control

• Flow control are used to control the order in which execution happens.

• Common structures are conditionals (if-then-else) and loops (for loops)

• Keywords should appear at the start of a line

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Conditionals

if some conditionthencommand1

command2[else

command3...]fi

Example:

if [ $# -gt 1 ]

then

echo “Multi-arguments…”

fi

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Representing Conditions

1. Using square bracket (like the previous example)– Careful with the space near [ and ]

• Using keyword test– For example the previous example can also be

written as follows

if test $# -gt 1

then

echo “Multi-arguments…”

fi

9P51UST: Unix and Software Tools

More About test

• The test command can be used to evaluate three different general types of conditions– File conditions

– String comparisons and conditions

– Integer comparisons

• For example:

if test –f $1 then echo –e “$1 is a regular file\n”fi

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Test File Conditions

• Some common tests on files/directories

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Condition Meaning (if True)

-d file File exists and is a directory

-f file File exists and is a regular file.

-r file File exists and is readable

-w file File exists and may be written to

-x file File exists and is executable

-s file File exists and has a size greater than zero.

String Comparisons

• String tests

• N.B: equality test vs assignment – white space before and after “=”

12P51UST: Unix and Software Tools

Condition Meaning (if True)

-n str1 Str1 has a length greater than 0

-z str1 str2 is a length of 0 bytes.

s1 = s2 s1 is equal to s2

s1 != s2 s1 is not equal to s2

Numerical Comparison

• In sh, all mathematical comparisons are integer comparisons.

• sh does not natively deal with floating-point numbers.

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Condition Meaning (if True)

n1 -eq n2 n1 is equal in value to n1

n1 -le n2 Less than or equal

n1 -lt n2 Less than

n1 -gt n2 Greater than

n1 -ge n2 Greater than or equal

n1 -ne n2 Not equal

loops

• for loop

for variable [in list]do command1 command2 ...done

• while loop

while conditiondo

command1 command2 ...done

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Loops - examples

#!/bin/shfor file in *.shdo if test ! -r $file then echo “$file is not

readable.” else echo “$file is

readable” fidone

#!/bin/shvalue=$1while test $value -gt 0do echo “\$value= $value” value=`expr $value - 1`done

15P51UST: Unix and Software Tools

Arithmetic

• Bourne shell does integer math. Therefore, suppose I want to divide 10 by 3, the result is 3.

• The command is expr

expr v1 operator v2

where operator can be + add- subtract* multiply/ divide

• Examples:

Correct format

expr 10 / 3

expr 3 + 3

But not

expr 10/3

expr 10/ 3

expr 10 /3

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Command Substitution

• To store the math results into a variable, we need command substitution.

• Use backquotes “`” enclose the command.

• Also remember to protect meta-character “*”

• Example:v1=`expr 25 \* 6`v2=`expr $v1 / 3`v3=`expr $v1 ‘*’ $v2`v4=`expr $v1 + $v3`$echo $v1, $v2, $v3, $v4

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Comparisons using expr

• Exampleequal=`expr $1 = “ruibin”`

flag=`expr $1 > 30`

Other operators:

!= not equal

< less

>= greater than or equal to

<= less than or equal to

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Debugging in Shell Programming

• To debug the shell script, you need to invoke shell using the following format

sh [options] [arguments]

where options

-v(verbose) : show each line of the script

-x(executed): display each command it executed, preceded by a plus sign (+).

19P51UST: Unix and Software Tools

Your .bash_profile is a shell script

# set up personal bin directoriesPATH=$HOME/bin:$PATH:EDITOR=emacsexport PATH TERM EDITORDEFTERM=vt100ASKTERM=false

~/.bash_profile is used by each user to enter information that is specific to his or her own use. It is executed only one when users log in.

However, ~/.bashrc contains information that is specific to your bash shells. It is executed each time you open a new bash shell.

20P51UST: Unix and Software Tools