Chapter 15 Introductory Bash Programming

To introduce the concept of shell programming

To describe how shell programs are executed

To discuss how command line arguments are passed to shell programs

To explain the concept of command substitution

To describe some basic coding principles

To write and discuss some shell scripts

To cover the related commands

Introductory Bash Programming15.1 Introduction

Bash is more than a command interpreter.

It has own programming language.

A shell program is commonly known as shell script.

In order to permit non-sequential execution of the commands in a shell script, Bash also supports program flow control commands (statements) such as if, case, for, while and until.

Introductory Bash Programming15.2 Running a Bash Script

Three ways to run a Bash script: Firstly, you run chmod u+x script_file

If you use the bash shell, then you will run the following command:

./script_file If you use the Linux shell other than the bash shell, you

can run /bin/bash ./script_file

Secondly, you run /bin/bash script_file

Lastly, you run chmod u+x script_file and you can put the following line at the beginning of the script_file and then you run ./script_file

#!/bin/bash

Introductory Bash Programming15.3 Running a Bash Script

Table 15.1 Some Important Writable Bash Environment Variables (continued on next page)

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

Table 15.1 Some Important Writable Bash Environment Variables (continued from previous page)

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

Two types of shell variables are shell environments variables (writable or read-only) and user-defined variables.

Table 15.2 Some Important Read-Only Bash Environment Variables

One can use the set command to display shell environment variables and user defined variables with their values. The env command can be used to display the former ones with their values.

15.3.1 Controlling The Prompt

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

Table 15.3 Some Useful Prompt Characters and their Descriptions

15.3.1 Controlling The Prompt (Cont'd)

Example:

$ PS1='\w $ '~ $ $PS1='\d $ 'Mon May 12 $ $PS1='\h $ 'pardus $ $ PS1='\u@\h \w \$ 'ctis@pardus ~ $$ PS1='bash-\v|\d|\w\$ 'bash-3.1|Mon May 12|~$

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

15.3.2 Variable Declaration Linux users can use the declare and typeset commands to declare variables, initialize them, and set their attributes. Syntax:

declare [± options] [name [=value] ]typeset [± options] [name [=value] ]

Commonly used options / features: -a each 'name' is an array -f each 'name' is a function -i 'name' is an integer -r mark each 'name' read-only ( can not be turned off by

using +x) -x mark each 'name' exported.

Note that using + instead of – turns attributes off.

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

15.3.2 Variable Declaration (cont'd)

Examples: $ declare -i age=42$ declare -rx OS=LINUX

$echo $age 42 $echo $OSLINUX

$declare OS$declare age$echo $age42$echo $OsLINUX

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

15.3.2 Variable Declaration (cont'd)

Examples: In order to display all integer and read-only variables in

your environment: $ declare -ir

Similarly,

$ declare -x

$ declare -i

$ declare

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

15.3.2 Variable Declaration (cont'd) One can change the value of a variable by using the name=value syntax

For an integer variable, if a noninteger value is assigned, then this variable will get a value of zero. $ declare -i age=22

$ echo $age22$ age=”Twenty”$ echo $age0

For generic variables, any type of value (integer or string) can be assigned. $ name=John

$ echo $nameJohn $ name=22 $ echo $name22

Introductory Bash Programming15.3 Running a Bash Script (cont'd)

Introductory Bash Programming 15.3.3 Reading and Writing Shell Variables

Table 15.4 Variable Substitution Operators and Their Descriptions

Example:$ echo $name

$ name=Mutlu$ echo $nameMutlu $ echo $place

$ echo ${name:-Sacit} ${place:-Ankara}Mutlu Ankara$ echo ${name:+Defined} Defined$ echo ${place:+Not Defined} $ echo ${place:=Istanbul} Istanbul$ echo ${name:-Sacit} ${place:-Ankara}Mutlu Istanbul

Introductory Bash Programming 15.3.3 Reading and Writing Shell Variables (cont'd)

The use of single and double quotes, *, and \ in an assignment statement.

Example:$ name=Mutlu $ echo $nameMutlu $ name= Mutlu Ankarabash: Ankara: command not found$ name='Mutlu Ankara'$ echo $nameMutlu Ankara$ touch Ankara{1,2,3}$ name= Ankara*$ echo $nameAnkara1 Ankara2 Ankara3$ echo “$name”Ankara*$ echo "All files starting with Ankara: $name "All files starting with Ankara: Ankara*

Introductory Bash Programming 15.3.3 Reading and Writing Shell Variables (cont'd)

$ echo \$name$name$ echo '$name'$name

Example:

$ command=pwd$ $command

/home/ctis$ command=hello $ $command

bash: hello: command not found

$ command=”ls -l”$ $command................$ $command /etc................

Introductory Bash Programming 15.3.3 Reading and Writing Shell Variables (cont'd)

Syntax: $(command)

Example:

$ command=pwd

$ echo “The value of command is: $command.”The value of command is: pwd

$ command=(pwd)

$ echo “The value of command is: $command.”The value of command is: /home/ctis/Desktop,

$ echo “The date and time is $(date).”The date and time is Wed May 14 13:00 EST 2008

Introductory Bash Programming 15.3.4 Command Substitution

Syntax: declare –x [name-list]typeset –x [name-list]export [name-list]

Note that you can download the most of scripts given in the following examples from

http://www.bilkent.edu.tr/~hmurat/ctis156/scripts.tar.gzExample:

$ cat display_nameecho $nameexit 0

$ name=“Mutlu Ankara”$ bash display_name$ declare –x name=“Mutlu Ankara”$ bash display_nameMutlu Ankara$ echo $?0

Introductory Bash Programming 15.3.5 Exporting Environment

Example:

$ cat export_name#! /bin/bashdeclare –x name=“Mutlu Ankara”./display_change_namebash display_nameexit 0

$ cat display_change_name#! /bin/bashecho $namename=“Blue Rose”echo $nameexit 0

$ ./export_nameMutlu AnkaraBlue RoseMutlu Ankara$

Introductory Bash Programming 15.3.5 Exporting Environment (Cont’d)

Syntax unset [name-list]

$ declare name=Mutlu place=Ankara

$ echo $name $place Mutlu Ankara

$ unset name

$ echo “$name”

$ echo “$place”Ankara------------------------------------------------------------------------------$ unset name place $ declare name=Mutlu ; echo “$name” Mutlu $ name= ; echo “$name”

Introductory Bash Programming 15.3.6 Resetting Variables

Syntax declare -r [name-list] typeset -r [name-list] readonly [name-list]

Example: $ declare –r name=Mutlu place=Ankara

$ echo $name $place Mutlu Ankara

$ name=Guler place=Bolubash: name: readonly variablebash: place: readonly variable

$ readonly$ unset name bash: unset: name: cannot unset: readonly variable

Introductory Bash Programming 15.3.7 Creating Read-Only User-Defined Variables

Syntax read [options] [name-list]

Example: $ cat read_demo #!/bin/bash echo “Enter input: “ read line echo “You entered: $line” echo “Enter another line: “ read word1 word2 word3 echo “The first word is: $word1” echo “The second word is: $word2” echo “The rest of the line is: $word3”

Introductory Bash Programming 15.3.8 Reading From Standard Input

Syntax read [options] [name-list]

Example: $ cat cmdargs_demos #!/bin/bash echo The command name is $0. echo “The num. of command line arg. passed as

parameters are $#.” echo “These values: $1 $2 $3 $4 $5 $6 $7 $8 $9.” echo “Another way: $@.” echo “Yet Another way: $*.” exit 0$ ./cmdargs_demo .....

Introductory Bash Programming 15.4 Passing Arguments To Shell Scripts

Syntax shift [N]

Example: $ cat shift_demo #!/bin/bash echo The command program name is $0. echo The arguments are $@. echo The first three arg. are $1 $2 $3. shift echo The command program name is $0. echo The arguments are $@. echo The first three arg. are $1 $2 $3. exit 0

Introductory Bash Programming 15.4 Passing Arguments To Shell Scripts (Cont'd)

Syntax set [options] [argument-list]

Example:

$ date Wed May 14 03:18:04 EEST 2008

$ set $(date) $ echo $@ Wed May 14 03:18:04 EEST 2008

$ echo $1, $2 and $4

Introductory Bash Programming 15.4 Passing Arguments To Shell Scripts (Cont'd)

Example:

$ cat set-1#!/bin/bashfilename=output.txt

set $(ls)echo $@echo echo $1 $2 echo $1 $2 > $filename shift 1echo $1 $2echo $1 $2 >> $filename

Introductory Bash Programming 15.4 Passing Arguments To Shell Scripts (Cont'd)

Introductory Bash Programming 15.6 Program Control Flow Commands 15.6.1 The if-then-elif-else-fi Statement

Figure 15.1  Semantics of the if-then-fi statement

The test command is used to evaluate the expression. Syntax

test [expression] [[expression]]

Example: Let's display the contents of the if-1 and run it.

Introductory Bash Programming 15.6 Program Control Flow Commands 15.6.1 The if-then-elif-else-fi Statement (cont'd)

Table 15.5 Some Useful Operators for the test Command (continued on next page)

Table 15.5 Some Useful Operators for the test Command (continued from previous page)

Table 15.5 Some Useful Operators for the test Command (continued from previous page)

Figure 15.2  Semantics of the if-then-else-fi statement

Figure 15.3  Semantics of the if-then-elif-else-fi statement

Example: Let's display the contents of the if-2 and if-3 and run them.

Figure 15.4  Semantics of the for statement

Example: Let's display the contents of the for-1 and for-2 and run them.

Figure 15.5  Semantics of the while statement

Example: Let's display the contents of the while-1 and while-demo run them.

Figure 15.6  Semantics of the until statement

Example: Let's display the contents of the until-1 and run it.

Figure 15.7  Semantics of the break and continue commands

Figure 15.8  Semantics of the case statement

Example: Let's display the contents of the case-demoand run it.