Locked Shell Scripting

[SP]

Table of Contents

1. Tour de Shell Scripting ...........................................................................1

Introduction to the Course structure ....................................................1

Adage 1.0: ................................................................................1

What you will need ....................................................................1

What is Shell Scripting ..............................................................2

Introduction to info, man and the whatis database ................................4

info pages .................................................................................4

Man Pages ................................................................................9

The whatis Database ................................................................12

Revising some Basic Commands ......................................................13

The who command ..................................................................14

The w Command .....................................................................16

The "date" command ...............................................................17

The 'echo' command ................................................................20

File Commands .......................................................................23

System Commands ..................................................................29

stdin, stdout, stderr ..........................................................................33

stdin .......................................................................................34

stdout ......................................................................................34

Using stdin and stdout simultaneously .......................................37

Appending to a file ..................................................................37

stderr ......................................................................................38

stdout, stderr and using the ampersand (&) ................................40

Exercises: ................................................................................41

Unamed Pipes .........................................................................41

2. The Shell .............................................................................................45

Introduction ....................................................................................45

What is the login shell? ....................................................................46

Exercises .................................................................................48

The job of the shell ..........................................................................50

Command Interpreter ...............................................................50

Allows for variables to be set ....................................................51

I/O redirection .........................................................................51

Pipelines .................................................................................51

Customising your environment .................................................51

Conclusion: .............................................................................51

3. Regular Expressions .............................................................................53

Introduction ....................................................................................53

What are regular expressions? ..........................................................53

The fullstop .............................................................................54

Let's explore "sed" syntax ........................................................55

Square brackets ( [ ] ), the caret ( ^ ) and the dollar ( $ ) .............56

Using sed and pipes .................................................................59

The splat (asterisk) ( * ) ...........................................................61

The plus operator ( + ) .............................................................63

Returning from detour to our discussion on curly braces… ........66

RE's are greedy for matching patterns .......................................68

Placeholders and word boundaries ............................................70

Word boundaries ( < and > ) - a formal explanation ...................72

The tr command ..............................................................................73

The cut command ............................................................................75

First Example in stages: ...........................................................77

Second Example in stages: .......................................................78

Third example in stages ...........................................................79

Exercises: ................................................................................81

The paste command .........................................................................81

The uniq command ..........................................................................83

The Sort command ..........................................................................84

The grep command ..........................................................................88

grep, egrep and fgrep ...............................................................92

Exercises: ................................................................................93

Challenge sequence: ................................................................93

4. Practically Shell Scripting ....................................................................95

Section Techniques to use when writing, saving and executing Shell

Scripts ............................................................................................95

Detour: File Extension labels ....................................................96

Comments in scripts ................................................................97

Variables .................................................................................98

Shebang or hashpling #! .........................................................100

Exit ......................................................................................102

Null and unset variables .........................................................102

Variable Expansion ................................................................105

Environmental vs shell variables .............................................106

Arithmetic in the shell ............................................................108

Examples ..............................................................................110

Exercises: ..............................................................................112

5. Using Quotation marks in the Shell .....................................................115

Introduction ..................................................................................115

Single Quotes or "ticks" .................................................................115

Exercises: ..............................................................................118

Double Quotes ..............................................................................118

Exercises ...............................................................................121

Backticks ......................................................................................121

Exercises: ..............................................................................124

Shell Arithmetic's with expr and back quotes ...................................125

Another tip when using quotation marks .........................................126

6. So, you want an Argument? ................................................................129

Introduction ..................................................................................129

Positional Parameters 0 and 1 through 9 ..........................................130

vi Shell Scripting

Exercises: ..............................................................................132

Challenge sequence: ..............................................................132

Other arguments used with positional parameters .............................133

$# How many positional arguments have we got ? ...................133

$* - display all positional parameters ......................................133

Using the "shift" command - for more than 9 positional parameters

..............................................................................................134

Exit status of the previous command .......................................136

7. Where to From Here? .........................................................................139

Making Decisions ..........................................................................139

Testing for a true or false condition .........................................139

Different types of tests ...........................................................140

Logical Operators ..........................................................................146

Exercises: ..............................................................................150

Conditions in the shell ...................................................................151

Using the "if" statement .........................................................151

The "if" "then" "else" statement ..............................................152

The "elif" statement ...............................................................153

The "case" statement ..............................................................154

Exercises ...............................................................................156

Challenge sequence: ..............................................................157

Debugging your scripts ..................................................................157

The NULL command .............................................................158

The || and && commands .......................................................159

Exercises: ..............................................................................160

8. Loops ................................................................................................163

Introduction ..................................................................................163

The "for" loop .......................................................................163

while and until loops ..............................................................170

getopts Using arguments and parameters .........................................178

Exercises: ..............................................................................181

9. User input to a script ..........................................................................183

Introduction ..................................................................................183

The read command ........................................................................183

Presenting the output .....................................................................188

The echo command ................................................................188

The printf command ..............................................................191

10. Additional Information .....................................................................197

The shell environmental variables pertaining to scripting .................197

The Source command ....................................................................199

Exercises: ..............................................................................202

the exec command .........................................................................202

Other methods of executing a script or a series of commands ....204

11. Positional parameters & variables re-visited .......................................209

Introduction ..................................................................................209

PARAM:-value .............................................................................210

PARAM:=value .............................................................................211

vii

${param:+value} ...........................................................................212

?${variable%pattern} .....................................................................214

MAGIC%%r*a ..............................................................................215

variable#pattern .............................................................................216

variable:OFFSET:LENGTH ...........................................................217

#variable .......................................................................................219

Re-assigning parameters with set ....................................................220

Explaining the default field separator field - IFS ..............................221

Setting variables as "readonly" .......................................................222

Exercises: ..............................................................................222

Challenge sequences: .............................................................223

12. Bits and pieces - tying up the loose ends ............................................225

The eval command ........................................................................225

Running commands in the background using & ...............................227

Traps and signals ...........................................................................229

Signals ..................................................................................229

Traps ....................................................................................232

Exercises: ..............................................................................236

File descriptors re-visited ...............................................................236

Exercises ...............................................................................241

Here documents .............................................................................242

Exercises ...............................................................................246

Functions ......................................................................................246

Exercises: ..............................................................................249

Challenge sequence ...............................................................249

A. Writing html pages ............................................................................251

B. An introduction to dialog ...................................................................253

C. A Comparisson of bash, tsch and ksh ..................................................255

Index ....................................................................................................257

viii Shell Scripting

List of Figures

1.1. The structure of the info pages .............................................................5

2.1. Parent- and sub-shells ........................................................................47

6.1. Using Shift Command to access parameters ......................................134

10.1. Parent- and sub-shells ....................................................................200

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List of Tables

1.1. Wildcards .........................................................................................23

1.2. Standard Input, Standard Output and Standard Error ............................34

 

 

 

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