Creating Scripts

---

A script is a program written in an interpreted language, typically associated with a shell or other program whose primary purpose is something other than as an interpreted language.

A shell script is a script associated with a shell used for automating shell commands.

• is a plain text file

Creating a Shell Script

Begins with a shebang, which is a line that identifies the shell used to run the script.

• aka hashbang, hashpling, or pound bang
• e.g., #!/bin/bash
  • first two characters tell the Linux kernel that this is a script
  • pathname tells which program to run it
• # is a comment
• /bin/sh is typically a symbolic link that points to some other shell
  • usually /bin/bash
• must make file executable when done
  • chmod +x scriptname
• to run a script in current directory:
  • ./scriptname
  • otherwise will search the PATH
• can move the script to directory in the PATH to run script from anywhere

Arguments

A variable is a placeholder in a script for a value that will be determined when the script runs.

• can be
  • passed as parameters to a script
    • called parameters or arguments
    • represented by a $ followed by a number starting at 0
    • $0 is the command name itself, $1 is the first argument, and so on
    • e.g., for script myuser: useradd -m $1
      • mkuser username
  • generated internally
  • or extracted from a script’s environment
• can assign the output of a command to a variable by setting it in backticks
  • e.g., ip=`route -n | grep UG | tr -s " " | cut -f 2 -d " "`
• can read variables from standard input using read
  • read response reads input that is accessed with $response
  • useful for interactive scripts
  • e.g.,

echo -n "Enter a username: "
read name
user add -m $name

An environment is a set of variables that any program can access.

• includes the current directory and the search path for running programs
• environment variable is a special kind of variable
  • uses Bash’s export command to make the value of the variable accessible to programs launched from the shell or shell script
  • by convention, environment variables are all uppercase
  • non-environment shell script variables are lowercase or mixed case

$? is a special variable that holds the exit status (or return value) of the most recently executed command.

• exit status is an integer value that you can check to determine whether or not the command completed correctly
  • returns 0 if program/command terminates normally
  • returns a different value to specify errors
• can display with echo or use it in a conditional expression to perform error handling

Conditional Expressions

A conditional expression enables a script to perform one of several actions contingent on some condition.

• multiple kinds

IF Conditional

• if allows system to take one of two actions depending on whether some condition is true
• syntax if [ condition ]
• fi marks the end of an if block
• condition appears in brackets []
  • e.g.,
    • [ -f file]
      • is true if file exists and is a regular file
    • [ -s file ]
      • is true if file exists and has a size greater than 0
    • [ string1 == string2 ]
      • is true if the two strings have the same values
• can use test as an alternate way to specify a condition
  • e.g., if test -s /tmp/tempstuff
• can use a commands return value as a condition
  • e.g., if [ command ]
    • executes only if the command completes successfully (no errors)
• can expand if with else clause

if [ conditional-expression ]
	then
		command
	else
		other-commands
fi

Case Conditional

A case conditional is useful for situations where more than two outcomes are possible

• better than nesting multiple if/then/else clauses
• e.g.,

case word in
	pattern1) command(s) ;;
	pattern2) command(s) ;;
	...
esac

• a word is likely to be a variable
• each pattern is a possible value of that variable
  • can be expanded like filenames using wildcards and expansion rules
  • each must end with ;;
  • can use * as the final pattern to set a default condition
• ends with esac

Loops

Loops are structures that tell the script to perform the same task repeatedly until some condition is met.

For Loop

• uses the for keyword

Example

for d in `ls *.wav` ; do
	aplay $d
done

• for loop runs once for each item generated by ls *.wav
• each file name is assigned to $d variable

seq command generates a list of numbers starting from its first argument to its last argument.

• e.g., seq 1 10 generates 10 lines, each with a number 1 and 10
• can use in a for loop:

for x in `seq 1 10` ; do
	echo $x
done

While Loop

A while loop executes for as long as its condition is true.

while [ condition ]
do
	commands
done

Until Loop

An until loop executes for as long its condition is false (until it is true).

Functions

A function is a part of a script that performs a specific subtask and that can be called by name from other parts of the script.

• defined by
  • placing parentheses after the function name
  • enclosing the lines within curly braces

myfunction() {
	commands
}

• keyword function can optionally precede the function name

Setting an Exit Value

• typically, a script’s exit status is the same as the last command it ran
• can control the exit value by using exit command
  • immediately terminates the script
  • can pass a number 0-255 to return a specified exit value
    • 0 denotes success
    • 1 denotes general error