1. Copyright © 2016-2017 – Curt Hill
Bash Scripting
Fundamentals
Copyright © – Curt Hill

2. Copyright © 2016-2017 – Curt Hill
Introduction
Previously, we have considered the PowerShell scripts and Batch scripts
The original script is the shell script
We will now consider Bash scripts
This includes:
Comments
Variables and types
Subsequent presentations will consider flow of control and functions
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First Line
Since
There are multiple scripting languages
Extensions are largely ignored
How do we know the script language?
The first line has a special format
Starts with an octothorpe and exclamation point
Next is the path to the script interpreter #!/bin/bash
This is known as the shebang
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Debugging
Not needed
This class is too good to make a mistake
For the lesser mortals we can make a script trace what it is doing
Append -x to the shebang and bash will trace it
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Permissions
When we create a script we generally use an editor
Different than a compiler
A compiler will set the execute permission bit but the editor will not
Thus we need a chmod to make it executable
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Comments
The use of comments in a console are somewhat pointless
We can use them there as we will see
However we desperately need these in scripts
Which we will get to
The comment in Bash is an octothorp
Like the // in C++ it finishes the line
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7. Variables and Assignment
Bash variables have approximately the same format as in C
No declaration is needed
Assignment is just the =
Like C but terminated by the line end rather than a semicolon
No blanks in the assignment
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Two Sides
There are two usages of a variable
To assign to a variable use the name
To obtain the value prefix with a $
Consider the following simple script
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Hello World
$ cat hello.sh
#!/bin/bash
greet="Hello World"
second=$greet
echo $greet
echo $second
$ ./hello.sh
Hello World $
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Commentary
The quotes are needed because there is a blank in the phrase
Had it been a simple Hello no quotes would be needed
If the assignment had been second=greet then the second line would have been greet
Rather than the contents of variable greet
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Types
There are no types in variables
Variables may merely hold a string of characters
Strings may be manipulated using a variety of operators
We will look at this first
This makes arithmetic rather interesting
We must use the let
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Strings
Only strings with blanks need to be in quotes
Strings may be concatenated by having them adjacent
There are several other manipulation operators
$ s1=Thisisalargestring
$ s2="This one has blanks"
$ s3=$s1$s2
$ echo $s3
ThisisalargestringThis one has blanks
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String Length
The length of a string may be obtained in a rather curious way: ${#str} where str is a variable
For example: $ echo ${s3} ThisisalargestringThis one has blanks $ echo ${#s3} 37
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String Functions
This unusual notation ${…} is the basis for a whole set of string manipulations
What changes are the operators used inside the braces
Length was preceding the string with the octothorpe
Substring is following with :position:length
The position is a C subscript – starts at zero
Length is optional
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Substring Example
$ letter=abcdefghijklmnopqrstuvwxyz
$ echo ${letter:5:2} fg
$ echo ${letter:5}
fghijklmnopqrstuvwxyz
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Replacement
Replace a portion of a string with something else
Basic form: ${string/pattern/replace}
Where pattern is the string to look for
Replace substitutes if found
Variations
If first slash is doubled all are replaced
If first slash is followed by # must match front
If first slash is followed by % must match end
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Replacement Example
$ echo ${letter/ghi/..}
abcdef..jklmnopqrstuvwxyz
$ echo ${letter/#ab/....}
....cdefghijklmnopqrstuvwxyz
$ echo ${letter/#def/....}
abcdefghijklmnopqrstuvwxyz
$ echo ${letter/%xyz/....}
abcdefghijklmnopqrstuvw....
$ echo ${letter/%pqr/....}
str=
$ echo ${str//123/99}
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Delete a Substring
Similar form: ${str#pattern}
This delete match from front
The % removes match from back
The pattern may also be a limited regular expression
This is true in replacement also
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Example
$ echo $num
133,321.84
$ echo ${num#[0-9]}
33,321.84
$ echo ${num%[0-9]}
133,321.8
$ echo ${num%%\.[0-9]*}
133,321
$ echo ${num/,[0-9]*./xxx}
133xxx84
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The Let
Similar to a normal assignment but the domain is restricted to numbers and arithmetic
Format is: let expr let string
The first is used if no blanks
The second is a string containing an expression with blanks
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Example
$ let x=5
$ let y=2
$ echo $x+$y
5+2
$ let z = $x+$y
-bash: let: =: syntax error: operand expected (error token is "=")
$ let z=$x+$y
$ echo $z 7
$ let "w = x*2 +y++"
$ echo $w 12
$ echo "$w and $y"
12 and 3
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Commentary
Notice that the stropping $ allows the value to display even in quoted strings
The ++ and -- operators have their usual C/C++/Java meaning
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23. Command-line parameters
The command line parameters of a script are predefined variables
Start with $ and followed by a digit
$0 is the script name
$1 is the first word
$2 is the second word
A parameter not present is an empty string
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Example
$ ./test2.sh test.sh
./test2.sh
test.sh
No 2nd parameter
$ cat test2.sh
#!/bin/bash
echo $0
if [ -n "$1" ]
then
echo $1 fi
if [ -n "$2" ]
echo $2
else
echo "No 2nd parameter"
exit 0
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25. Copyright © 2016-2017 – Curt Hill
Finally for Now
Now we know the minimal amount of BASH scripting
Next we consider some flow of control
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