sh/bash scripting

SEIE, Soochow University

Week 09, 2015-2016-1

Scripts requirs an interpreter
File: the first line starts with #!
```
#!/bin/bash
ls | more
```

Shell interpreters

/bin/sh
/bin/bash
/bin/tcsh
/bin/csh
/bin/ksh
/usr/bin/perl
/usr/bin/python

vim testsh.sh

#!/bin/sh
echo "HELLO"
echo "WORLD"

bash testsh.sh

chmod +x testsh.sh
./testsh.sh

Interactive scripts

# starts comments

#!/bin/sh
# Interactive reading of variables
echo "ENTER YOUR NAME"
read sname
# Display of variable values
echo $sname

Return Values

0 for success; none-zero for failure.
the last retune value in environment variable $?
return by exit statement in shell.

Syntax

Special character in bash

Character	Description
`#`	comment, except: `\#` and `#!`
`\`	continuation on to the next line
`;`	following is a new command
`$`	indicates a variable

Comments

# This line will not get executed.

Long Lines

scp abc@server1.linux.com:\
/var/ftp/pub/userdata/custdata/read \
abc@server3.linux.co.in:\
/opt/oradba/master/abc/

What is a line?

Multiple Commands on a Single Line

make ; make install ; make clean

make && make install && make clean

Abort if one fails

cat file1 || cat file2 || cat file3

If something succeeds, then stop executing any further steps.

Functions

function_name () {
   command...
}

#!/bin/sh
display() {
    echo "This is the message from the function"
    echo "The parameter passed from calling process is" $1
}
display "Bob"
display "Rambo"
display "Bond"

Built-in Shell Commands

type, man bash or help

Command Substitution

enclosing inner command with `
or $( )
launch a new shell, standard out inserted in the place

echo cd /lib/modules/$(uname -r)/
echo cd /lib/modules/`uname -r`/
cd /lib/modules/$(uname -r)/
$(echo cd /lib/modules/$(uname -r)/)

Environment Variables

Referencing or using: prefix with $. e.g. echo $PATH
When setting, no $ prefix

MYCOLOR=blue; echo $MYCOLOR
To make usable by child processes (sub-shells)

export VAR=value or VAR=value ; export VAR

copied to child process without copy back.

Script Parameters

./script.sh /tmp
./script.sh 100 200

Parameter	Meaning
`$0`	Script name
`$1`	First parameter
`$2, $3`...	Second, third parameter, ...
`$*`	All parameters in a string
`$#`	Number of arguments
`$@`	All parameters in a set

#!/bin/sh
echo "The name of this program is: $0"
echo "The first argument passed from the command line is: $1"
echo "The second argument passed from the command line is: $2"
echo "The third argument passed from the command line is: $3"
echo "All of the arguments passed from the command line are : $*"
echo
echo "All done with $0"

I/O Redirection

>, >>, <
free > /tmp/free.out
free >> /tmp/free.out
wc -l < /temp/free.out

Constructs

The if Statement

if TEST-COMMANDS; then CONSEQUENT-COMMANDS; fi

if condition
then
       statements
else
       statements
fi

if [ -f /etc/passwd ] ; then
    echo "/etc/passwd exists."
fi

[[ -f /etc/passwd ]] or (( -f /etc/passwd ))

Testing for Files

Condition	Meaning
`-e file`	Check if the file exists.
`-d file`	Check if the file is a directory.
`-f file`	Check if the file is a regular file (not symbolic link, device node, directory, etc.)
`-s file`	Check if the file is of non-zero size.
`-g file`	Check if the file has sgid set.
`-u file`	Check if the file has suid set.
`-r file`	Check if the file is readable.
`-w file`	Check if the file is writable.
`-x file`	Check if the file is executable.

man 1 test

String Testing

if [ string1 == string2 ] ; then
   ACTION
fi

Numerical Tests

Operator	Meaning
`-eq`	Equal to
`-ne`	Not equal to
`-gt`	Greater than
`-lt`	Less than
`-ge`	Greater than or equal to
`-le`	Less than or equal to

#!/bin/sh
# Prompt for a user name...
echo "Please enter your age:"
read AGE
if [ "$AGE" -lt 20 ] || [ "$AGE" -ge 50 ] ; then
    echo "Sorry, you are out of the age range."
elif [ "$AGE" -ge 20 ] && [ "$AGE" -lt 30 ] ; then
    echo "You are in your 20s"
elif [ "$AGE" -ge 30 ] && [ "$AGE" -lt 40 ] ; then
    echo "You are in your 30s"
elif [ "$AGE" -ge 40 ] && [ "$AGE" -lt 50 ] ; then
    echo "You are in your 40s"
fi

Arithmetic Expressions

$((...))

echo $((x+1))
built-in shell command let

let x=( 1 + 2 ); echo $x
var=$((...))

x=$((1+2)); echo $x

String Operators

Operator	Meaning
`[[ string1 > string2 ]]`	Compares the sorting order
`[[ string1 == string2 ]]`	Compares the characters
`myLen1=${#string1}`	Saves the length of string1 in the variable myLen1.

Parts of a String

name="bagend.hobbiton.com"; echo $name
loco=${name:0:6}; echo $loco
moto=${name#*.}; echo $moto

Boolean Expressions

evaluate to either TRUE or FALSE

Operator	Operation
`&&`	AND
`\|\|`	OR
`!`	NOT

Case Statement

case expression in
   pattern1) execute commands;;
   pattern2) execute commands;;
   pattern3) execute commands;;
   pattern4) execute commands;;
   * )       execute some default commands or nothing ;;
esac

#!/bin/sh
# Prompt user to enter a character
echo "Please enter a letter:"
read charac
case "$charac" in
    "a"|"A") echo "You have typed a vowel!" ;;
    "e"|"E") echo "You have typed a vowel!" ;;
    "i"|"I") echo "You have typed a vowel!" ;;
    "o"|"O") echo "You have typed a vowel!" ;;
    "u"|"U") echo "You have typed a vowel!" ;;
    *)       echo "You have typed a consonant!" ;;
esac
exit 0

Looping Constructs

`for` Loop

for variable-name in list
do
    execute one iteration for each item in the
            list until the list is finished
done

#!/bin/sh
#
sum=0
for i in 1 2 3 4
do
    sum=$(($sum+$i))
done
echo "The sum of $i numbers is: $sum"

`while` Loop

#!/bin/sh
#
echo "Enter the number"
read no
fact=1
i=1
while [ $i -le $no ]
do
    fact=$(($fact * $i))
    i=$(($i + 1))
done
echo "The factorial of $no is $fact"

`until` Loop

#!/bin/sh
echo "odd NUMBER under 10"
mn=1
mx=10
until [ $mn -gt $mx ]
do
    echo "$mn"
    mn=$(($mn + 2))
done

Debugging

Debug Mode

run in debug mode bash -x script_file

traces, prefixes commands with +, displays them.

Partial debugging

set -x # turns on debugging
...
set +x # turns off debugging

Redirecting Errors

File stream	File Descriptor
stdin	0
stdout	1
stderr	2

bash sample.sh 2> error.txt

See

Bash Programming

UNIX Shell Scripting Basics