Introduction
1. You can use any editor text to write shell script.
# vi script.sh
2. Write your shell script & save it.
press ESC, then :wq!.
press ENTER.
3. Set execute permission to the script.
# chmod +x [your_script_name]
or
# chmod 755 [your_script_name]
4. Then, you can execute your script.
# ./[your_script_name]
or
# bash [your_script_name]
# sh [your_script_name]
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How to Write a Shell Script?
Important
There are 2 types of variable:
- System Variables (SV) - in capital letters
- User Defined Variables (UDV) - in lower letters
- Syntax :
- UDV variable name must begin with Alphanumeric character or underscore character (_), followed by >= 1 Alphanumeric character.
- No space on either side of the equal sign.
- Variables are case-sensitive.
- Can define NULL variable.
- variable_name=
- variable name=""
Quotes
| " | Double Quotes | "Double Quotes" - Anything enclose in double quotes removed meaning of that characters (except \ and $). |
| ' | Single quotes | 'Single quotes' - Enclosed in single quotes remains unchanged. |
| ` | Back quote |
`Back quote` - To execute command |
Syntax that holds number of arguments specified on command line.
Syntax that refer to all arguments passed to script.
| Mathematical Operator in Shell Script | Meaning | Normal Arithmetical/ Mathematical Statements | But in Shell |
| | | For test statement with if command | For [ expr ] statement with if command |
| -eq | is equal to | 5 == 6 | if test 5 -eq 6 | if [ 5 -eq 6 ] |
| -ne | is not equal to | 5 != 6 | if test 5 -ne 6 | if [ 5 -ne 6 ] |
| -lt | is less than | 5 < 6 | if test 5 -lt 6 | if [ 5 -lt 6 ] |
| -le | is less than or equal to | 5 <= 6 | if test 5 -le 6 | if [ 5 -le 6 ] |
| -gt | is greater than | 5 > 6 | if test 5 -gt 6 | if [ 5 -gt 6 ] |
| -ge | is greater than or equal to | 5 >= 6 | if test 5 -ge 6 | if [ 5 -ge 6 ] |
| Operator | Meaning |
| string1 = string2 | string1 is equal to string2 |
| string1 != string2 | string1 is NOT equal to string2 |
| string1 | string1 is NOT NULL or not defined |
| -n string1 | string1 is NOT NULL and does exist |
| -z string1 | string1 is NULL and does exist |
| Test | Meaning |
| -s file | Non empty file |
| -f file | Is File exist or normal file and not a directory |
| -d dir | Is Directory exist and not a file |
| -w file | Is writeable file |
| -r file | Is read-only file |
| -x file | Is file is executable |
| Operator | Meaning |
| ! expression | Logical NOT |
| expression1 -a expression2 | Logical AND |
| expression1 -o expression2 | Logical OR |
| Standard File | File Descriptors number | Use | Example |
| stdin | 0 | as Standard input | Keyboard |
| stdout | 1 | as Standard output | Screen |
| stderr | 2 | as Standard error | Screen |
2>&1 : is used to redirect both standard output and error to /dev/null
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Related Commands
To print or acees UDV.
To print contains of variable "abc" type
To display text or value of variable
- echo [options] [string, variables...]
Options
- -n Do not output the trailing new line.
- -e Enable interpretation of the following backslash escaped characters in the strings:
- \a alert (bell)
- \b backspace
- \c suppress trailing new line
- \n new line
- \r carriage return
- \t horizontal tab
- \\ backslash
To perform arithmetic operations.
- expr op1 math-operator op2
math-operator
To print output of an arithmetic operations.(Both are back quotes)
- echo `expr op1 math-operator op2`
To determine the exit status of command or shell script.
To get input (data from user) from keyboard and store (data) to variable.
- read variable1, variable2, ..., variableN
To run 2 commands with 1 command line.
To sort contents in a file & redirect the output to new file.
sort [file_name] [new_file_name]
To translate all lower case character to upper case letters.
tr "a-z" "A-Z" [file_name] [new_file_name]
[Remark: All commands line you can use for Shell Script.]
**************************************************************
if condition
then
command1 if condition is true or if exit status
of condition is 0 (zero)
...
...
fi
**************************************************************
if condition
then
if condition
then
.....
..
do this
else
....
..
do this
fi
else
...
.....
do this
fi
**************************************************************
if condition
then
condition is zero (true - 0)
execute all commands up to elif statement
elif condition1
then
condition1 is zero (true - 0)
execute all commands up to elif statement
elif condition2
then
condition2 is zero (true - 0)
execute all commands up to elif statement
else
None of the above condtion,condtion1,condtion2 are true (i.e.
all of the above nonzero or false)
execute all commands up to fi
fi
**************************************************************
for { variable name } in { list }
do
execute one for each item in the list until the list is
not finished (And repeat all statement between do and done)
done
**************************************************************
for (( expr1; expr2; expr3 ))
do
.....
.....
repeat all statements between do and
done until expr2 is TRUE
Done
**************************************************************
while [ condition ]
do
command1
command2
command3
..
....
done
**************************************************************
case $variable-name in
pattern1) command
...
..
command;;
pattern2) command
...
..
command;;
patternN) command
...
..
command;;
*) command
...
..
command;;
esac
**************************************************************
function-name ( )
{
command1
command2
.....
...
commandN
return
}
**************************************************************
**************************************************************
Example 1:
#!/bin/bash
# ask user the enter a value
echo "Enter an order number: [Enter]"
read orderNumber
#postgresql command line to get the information for the entered order number
command=`psql -d [database_name] -U [user_name] -At << EOF
select * from [table_name] where [column_name]='$orderNumber';
EOF`
#print of the output
echo $commands
#if string $command is NOT NULL and does exit
if [ -n $command ]
then
#command1 if condition is true or if exit status
of condition is 0 (zero)...
.....
#end of if condition
fi
#end the shell script
exit 0
**************************************************************
Example 2:
(a) aScript.sh
#!/bin/bash
#if there is no environment path for $APP_HOME
if [-z $APP_HOME]; then #if two command line in a line, it need to be seperated by semicolon
echo "ERROR MESSAGE: ....";
exit 1;
fi
#remove an existed file
if [-e "[file_name]"]
then
rm [file_name]
fi
#To remove the trailing slash
APP_HOME=${APP_HOME%/} ;
(b) mainProgram.sh
#!/bin/bash
# run a script in background
./aScript.sh &
#set date format
NOW=`date + %d-%m-%y`
...
.....
