Page Listing:

• The Shell
• Variables
• Displaying Strings
• User Input variables - assignment
• Filename matching
• Command Assignment
• Parameter Substitution
• Quoting
• Handling numeric expressions
• Operators - Comparisons
• Exit Status
• Test command
• File testing
• Numerical testing
• Combined Tests
• if-then-else
• if-then-elif
• Case
• For
• While-until
• break-continue
• true-false
• Arguments
• shift
• set
• the $@ variable
• Functions
• return
• redirection
• Standard error
• File descriptors
• Word Count
• Which

The shell

The shell selection for a script can be performed with options. These are as follows: ( - turns on + turns off) :

-v Print lines before they are executed.

-x Print the interpreted command before it is executed.

-f Suppress normal filename expansion.

Shell Script selection must be in the first line of the script - Example:

#!/bin/sh -xv

sets -xv

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Variables

• assign a value to a variable

var=value

• To echo the contents of a variable

echo $var

• To reference the variable

$var

• To change the variable

read var

If the variable is contained within text then the variable name should be included within braces {}

echo ${variable}s

To include blank spaces in a variable definition, use ""

the_variable_name="The variable content"

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Displaying Strings

The Echo command automatically adds a new line at the end. To supress this include a: \c

echo "This is some information: \c"

Set reserved special characters can be used within variable assignment and echo statements. These are as follows:

\b	backspace
\c	no new line
\n	new line
\r	carriage return
\t	tab
\0nnn	The ASCII character represented by the octal number nn

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User Input - variables assignment

The read command handles its input as follows:

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---

Filename matching

To list all normal files in the shell's working directory.

echo *

tar_files=*.tar

ab_files=*.[ab] files ending in .a or .b (not .ab)

echo [A-Z]* files starting with a capital letter

NOTE: If a filename pattern does not match any files then the variable would be set to the actual pattern itself!

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Command Assignment

To assign the output of a command to a variable, we use command substitution ``

todays_date=`date`

date normally sends its output to stdout. Enclosing the date command within grave accents(`) sends it to the variable

echo "The date today is `date`"

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Parameter Substitution

${variable} returns the value of variable. It is possible to return a substituted value depending on the original value.

Substitution Expressions

Substitute Expression	Behaviour
${variable:-substitute}	If variable exists and is not null, return its value, otherwise return substitute
${variable:=substitute}	If variable exists and is not null, return its value, otherwise set it to substitute and then return its value
${variable:+substitute}	If variable exists and is not null return substitute, otherwise return null
${variable:?message}	If variable does not exist or is null print message and terminate the script

examples:

echo "Enter date[RETURN=`date`] \c"; read dt

echo "date entered : $dt"

echo "substitution returns : ${dt:=`date` }"

echo "dt is now set to $dt"

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Quoting ""

quote types :

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Handling numeric expressions(integers)

to mathematically evaluate an expression we have to use expr:

echo "1 + 1"

1+1

echo `expr 1 + 1`

2

expr evaluates the expression before the assignment

shell metacharacters i.e. * need to be escaped

expr 1 * 2 expr: syntax error

expr 1 \* 2

The following numeric are avaliable

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Operators - Comparisons

expr operator can be used on integers and strings.

expr "abc" \>= "aaa" - will return true.

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Exit Status

All commands, when executed, return an exit status. The exit status indicates if the command was successful of not.

the exit code is held in the variable $?

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Test command

test $a = $b

[ $a = $b ]

return exit status codes:
      0 if true
      1 if false

Test commands for strings:

The following are true if ...

-z string if length of string is zero

-n string if length of string is non-zero

string1 = string2 equal

string1 != string2 not equal

str str is not null

NOTE: arithmetic expresssions are NOT handled by the test command

When testing strings always put variables in "   "
This will test the condition even if the variable is empty

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File testing

the following tests can be used with test on files:

example:

test -d /home/myusername

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Numerical testing

• -eq
• -ne
• -gt
• -ge
• -lt
• -le

where:

l = less

g=greater

e,eq=equal

n=not

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Combined tests

the following operators are also available

• ! Unary negation operator
• -a Binary and operator
• -o Binary or operator (higher precidence than -0)

examples

test ! -f /tmp true if /tmp does not exist

test -w /tmp -o -w /usr/tmp

True if /tmp is writable or /usr/tmp is writable, if checking /tmp is writable then the check for /usr/tmp is not executed.

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if-then-else

if condition

then

command

fi

if condition

then

command

else

command

fi

example:

if [ -w /tmp ]

then

echo "/tmp is writable"

else

echo "/tmp not writable"

fi

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if-then-elif

(elif - else if)

used the same way as if-then-else but allows multiple testing of condition statements

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Case

case string in

pattern1)

cmds ;;

pattern2)

cmds ;;

pattern3 | pattern4)

cmds ;;

*)

cmds ;;

esac

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For

for variable in word_list

do

cmds

done

Where word_list could be the following :

* - all files in current directory

1 2 3 4 5 6 - simple list

`fgrep -l 193 *.log`

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While-until

while statements executes loop whilst the tested command returns true

until statements executes loop whilst the tested condition is false

while cond-cmd

do

cmds

done

until cond-cmd

do

cmds

done

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break-continue

The command break and continue are used to skip sections of a while and until loop.

break exits from the enclosing for or while loop

continue resumes the next iteration of the enclosing for or while loop

while cond-cmd

do

if cond then

break

fi

cmds

done

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true-false

true always exits with 0

false always exits with 1

these are useful for infinite loops

while true

do

cmds

done

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Arguments

Scripts can be envoked with arguments. These arguments are assigned in the following way

e.g.

script par1 par2 par3

$0 = script

$1 = para1

$2 = para2

$3 = para3

$# = number of arguments (not including $0)

$* lists all arguments except $0

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Shift

A limitation on using $n to print out the n'th argument is we cannot print out the $10 argument and above as n can only be a single digit!

to access the 10th or later argument we have to use shift

shift will cause the command line arguments to be left shifted so the $1 returns the value of $2

Does not affect $0

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Set

set will set the scripts personal parameters

For example:
set hello there
    sets $1 to "hello"
    and $2 to "there"

set `date` will set the following

$1=day of the week

$2=month

$3=day

$4=time

$5=TZ

$6=year

set is very useful for extracting tokens from lines!

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The $@ variable

If we execute a script with the following parameters:

script "file a" "file b"

$* = file a file b (looks like 4 files)

"$*" = "file a file b" (looks like 1 file)

to get around this problem we use $@

$@ = file a file b (looks like 4 files)

but ...

"$@" generates "file a" and "file b"

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Functions

A function is a number of shell commands that are grouped together under a 'function name'. Call the function to execute the group of commands. Functions help to create meaningful scripts that are easier to de-bug.

function_name()

{

command

command

}

function_name [args]

functions emulate scripts within scripts. $* may hold different values to the parent executing script. An Exit codes can be produced by using return

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Return

return causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed

function()

{

command

command

return n ( n is a numeric value usually 1 for true )

}

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Redirection

The following operators can be used for re direction

file1 < file2
file 1 will receive its input from file 2

file1 > file2
file1's output will be redirected to file2, creating file2 if it does not exist, and overwriting any existing contents of file2

file1 >> file2
file1's output will be appended to the end of file2, if file2 does not exist it will be created and written to.

file2 << string
will use the input from stdin for file1 until string is met.

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Standard Error

cat filename > file1

If file filename does not exits or there is an error then redirection to file1 will fail, and instead the content will be displayed on the screen (stderr)

Redirecting stderr

To ensure that error messages go to file1 we need to use the following command

cat filename > file1 2>&1

The operator 2>&1 instructs the shell to redirect stderr to stdout (&1)

we could redirect the errors to a separate file ...

cat filename > file1 2>error_file

To ignore all errors redirect to /dev/null

cat filename > file1 2>/dev/null

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File descriptors

Their uses:

2>file redirect stderr to file

2>&1 merge stderr with stdout

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Word Count

wc -l counts the number of words

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Which

There may be multiple instances of the same command within an operating system environment. Use the "which " command to determine the command you are using.

which echo

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