bash.md

INDEX

1. Introduction
2. Commenting
3. Conditional Statements
4. case Statement
5. Looping
   1. For Loop
   2. While Loop
   3. Until
   4. break and continue statements
6. Array
7. Command Line Arguments
8. Exit Staus

Introduction

All shell script files start with #!/bin/bash line. This is called a shebang. This tells which interpreter should call to parse the script. Example of a shell script file:

#!/bin/bash

echo "hello, world"

Commenting

Any line begin with # is a comment.

#!/bin/bash

# This is a comment
echo "hello, world"     # This is a comment too

Here documents

Feed text as it is into the standard input including whitespace and quotation marks. Here documents have the following form:

command << token
text
token

For example following script will feed the whole text to cat. As a result whole text will be printed as it is into the shell.

#!/bin/bash

cat << __EOF__
#include <stdio.h>

int main() {
    printf("hello, world!\n");
    return 0;
}
__EOF__

• if <<- is used insted of <<, leading tabs will be ignored.
• Can be used for multiline comment:

#!/bin/bash

<< __TOKEN__
    This will be ignored
    This line too
    And this
__TOKEN__

echo "hello, world!"

Output:

$ ./script.sh
hello, world!
$

Variables

When variable isn't set or have empty string default value can be set.

#!/bin/bash

arg1=${1:-"dfile.txt"}

Conditional Statements

Following example will show conditional statements in shell scripting

#!/bin/bash

# Structure
#
# if [ Expression ]; then
#       do something
# elif [ Expression ]; then
#       do something else
# else
#       do something else
# fi

number=2

if [ $number -lt "2" ]; then
    echo "Less than 2"
elif [ $number -gt "2" ]; then
    echo "Greater than 2"
else
    echo "Equal to 2"
fi

File expressions can be used to do different tests on file/directory

#!/bin/bash

FILE_NAME=$HOME/hello.o

# Remove file if exists
if [ -e "$FILE_NAME" ]; then    # quotation used so that empty variable don't cause error
    rm $FILE_NAME
fi

TODO: String expressions and regular expression matching. [[ ]] and (()).

To see how to write the conditional expressions see man test.

case Statement

case statements has the following form:

case $variable in
    pattern1)
        do something
    ;;

    pattern2)
        do something else
    ;;

    pattern3 | pattern4)
        do third important work
    ;;

    *)
        do another thing
    ;;
esac

• | matches multiple patterns.
• * default action if no pattern matches.

Example:

#!/bin/bash

# A program to do some usefull things with unix tools

TOOL=shell

case $TOOL in
    shell)
        echo "Throw into the sea."
    ;;

    strip)
        echo "Jump into the water."
    ;;

    time)
        echo "Jump in a black hole"
    ;;

    sleep)
        echo "Now do a interstellar travel"
    ;;

    *)
        echo "You are in a limbo"
    ;;
esac

Looping

For Loop

#!/bin/bash

# Structure of the for loop
#
# for $variable in sequence; do
#	something with the variables
# done

for f in $(ls); do
    echo $f
done

While Loop

Executes while a condition is true.

#!/bin/bash

# Structure of while loop
#
# while [ Expression ]; do
#	do something
# done

number=0

while [ $number -lt 10 ]; do
    echo "Number: $number"
    number=$((number + 1))
done

Until

Executes until a condition is true.

number=0

until [ $number -gt 10 ]; do
    echo "Number $number"
    number=$((number + 1))
done

break and continue

Like any other programming languages break and continue statement can be applied in a loop to break the loop or continue the loop on some condition.

Array

Arrays are defined as array_name=(0 "name" 4) where array_name is the name of the array an 0, name are the elements of the array. Array can be subscripted and iterated in following way:

array_name=(0 "name" 4)

# Print first element of the array
echo ${array_name[1]}

# Iterating through the array
# Print all the elements of the array
for elem in ${array_name[@]}; do
    echo $elem
done

Functions

Structure of a function in shell script is following:

func_name () {
    commands
    return
}

# Function call
func_name

Example:

#!/bin/bash

get_hostname () {
    echo $HOSTNAME
    return
}

get_hostname

• Functions must be defined before they are called.
• In shell all variables are global. A local variable can be created using local command.

#!/bin/bash

ID=0

get_hostname () {
    local ID
    ID=1
    echo $HOSTNAME
    echo "local ID before calling function: ${ID}"
    return
}

echo "global ID before calling function: ${ID}"
get_hostname
echo "global ID after calling function: ${ID}"

Command Line Arguments

sample code:

#!/bin/bash

echo "Number of arguments: '$#'"
echo "All arguments(space seperated): '$@'"
echo "Program name: '$0'"
echo "Argument 1: '$1'"
echo "Argument 2: '$2'"

sample output:

Number of arguments: '2'
All arguments(space seperated): 'hello world'
Program name: './cmd.sh'
Argument 1: 'hello'
Argument 2: 'world'

sample code:

echo "Number of arguments given: $#"

number=1
while [ "$1" != "" ]; do
    echo "Argument $number: $1"
    number=$((number + 1))      # number+=1
    shift
done

sample output:

Number of arguments given: '2'
Argument 1: hello
Argument 2: world

Exit Status

touch
echo $?