Bash Shell Scripting

CS155 ShellScripts

What’s a shell script?

What is a actor’s script?
- It’s a bunch of words that the actor should say.
What is a shell script?
- It’s a bunch of commands that the shell should execute.

Shell Scripts

All shell scripts should begin with a line indicating the program that should interpret them.
#! /bin/bash
If you write a shell script in another language, use its path:
- #! /bin/sh
- #! /bin/tcsh
- #! /usr/bin/perl
- #! /usr/bin/python
For the rest of this lecture, we’re talking about bash only.

Shell Scripts

#! /bin/bash
mkdir 155tmp
touch 155tmp/helloWorld
echo "Lala it’s Saturday" > 155tmp/satdaze
cat 155tmp/satdaze
date

Put your regular terminal commands in to a shell script to run
To run a shell script, make sure it has execute permission, then type in the name of the file at the prompt.

A common pitfall

Do not call your shell script test.
There’s already a command called test, and the shell will run that test, and not your test.
To be safe, run your shell script like this: ./whatever
This will ensure that the shell runs the script that’s in ., your current directory.

Shell Variables

#! /bin/bash
dir=155tmp
mkdir $dir
touch $dir/helloWorld
echo "Lala it’s Saturday" > $dir/satdaze
cat $dir/satdaze
date

To set a non-numeric (string) variable:
varName=value
- Example: name=Jack
To set a number variable:
let varName=expression
- Example: let age=2024-1957
To refer to either type of variable: $varName
To capture the output of a command:
varName=$(command)
- For example: size=$(wc -c <monster)

Examples of setting variables:

% alpha=beta
% echo $alpha
beta
% gamma=2+2
% echo $gamma
2+2
% let delta=2+2
% echo $delta
4
% epsilon=$(date | cut -c5-7)
% echo $epsilon
Nov
% iota=alpha
% echo $iota
alpha
% pi=$alpha
% echo $pi
beta

An Annoying Special Case

% echo $USER
cs155
% echo $USERtmp

% echo ${USER}tmp
cs155tmp
% echo $USER-tmp
cs155-tmp

Unambiguously designate a variable name: ${var}

Special Shell Variables

Variable	Meaning
`$#`	how many arguments were given to the script
`$0`	the name of the program
`$1`	the first argument
`$2`	the second argument
`$*`	All the arguments in a single string, like `"$1 $2 $3 ..."`
`$@`	All the arguments in separate strings, like `"$1" "$2" "$3" ...`
`$?`	`0` if the previous command succeeded

How do you tell if $2 exists?

More About Shell Variables

#! /bin/bash
if [[ $# -ne 1 ]]
then
    echo "Usage: $0 file"
    exit 1
fi
ls -l $1

Run the file zot like this: ./zot
or this: ./zot x
or this: ./zot x y z
./zot hello 543 results in:
- $0 = ./zot
- $1 = hello
- $2 = 543

Basic Math

% echo $UID
2543
% let x="UID*2"
% echo $x
5086

All numeric values in the shell are integers. There is no fractional part.

let name=expr
assign a numerical expression to name
let x="100/(2*3+4)"
for anything complicated, quote the whole darn thing

Control Flow: Conditional Execution

We might want to execute a command if a condition is true (or false).

if [[ expr ]]
then
     command
     command 
fi

if [[ expr1 ]]
then
    command1
elif [[ expr2 ]]
then
    command2
else
    command3
fi

if [[ expr ]]
then
     command1a
     command1b
else
     command2a
     command2b
fi

Logical operations

if [[ $1 -le 0 || $2 -le 0 ]]
then
    echo "All arguments must be positive."
    exit 1
fi

let age=66
sex="M"
if [[ age -ge 21 && $sex = "M" ]]
then
    echo "Voter in 1916!"
fi
if [[ ! ($sex = "M") ]]
then
    echo "Hello, ma’am!"
fi

&& means “and” (both halves must be true)
|| means “or” (one half or the other half or both halves must be true)
! means “not”

Conditional Execution: Examples

let age=21
let cash=10
let looks=4
let drunk=0

if [[ age -ge 21 && (cash -ge 8 || looks -gt 5) && drunk -eq 0 ]]
then
    echo Consume in moderation.
else
    echo No drink for you!
fi

Consume in moderation.

Conditional Execution: Examples

Conditional examples:

% cat scrpt
#! /bin/bash
if [[ $HOSTNAME = "denver" ]]
then
    echo "Mile high!"
elif [[ $HOSTNAME = "lansing" ]]
then
    echo "Welcome to the mitten state!"
else
    echo "Where am I?"
fi
% ls -l scrpt
-rwx------  1 cs155 class 193 Nov 23 17:06 scrpt
% ./scrpt
Where am I?

A $? example

#! /bin/bash
grep "foo" xyz >dummy
if [[ $? -eq 0 ]]
then
    echo "Hooray, we found foo in the file xyz!"
else
    echo "Sorry, foo was not found in the file xyz."
fi
rm dummy                        # be tidy

Why didn’t we rm dummy immediately after creating it? We don’t need its contents—why not get rid of it as soon as possible?

Comparison Operators

if [[ COLUMNS -lt 80 ]]
then
    echo "This terminal is too narrow."
    echo "It’s only $COLUMNS characters wide."
    echo "This program requires at least 80 characters."
fi

A $ is not required on variables when doing numeric comparisons. You still need a $ when comparing strings.

Comparing strings

Here’s how to compare strings (not numbers):

a `=` b	does a equal b ?
a `!=` b	does a not equal b ?
a `<` b	is a less than b ?
a `>` b	is a greater than b ?

Surprisingly, there are no ≤ or ≥ operators.
Remember: spaces around operators!

Comparing numbers

Here’s how to compare numbers:

a `-eq` b	does a equal b ?
a `-ne` b	does a not equal b ?
a `-lt` b	is a less than b ?
a `-gt` b	is a greater than b ?
a `-le` b	is a ≤ b ?
a `-ge` b	is a ≥ b ?

Remember: spaces around operators!

Combining comparisons:

a `&&` b	are both a and b true?
a `\|\|` b	are either a or b true?
`!` a	is a false?

Remember spaces around operators!

Another conditional example

#! /bin/bash

let hour=$(date "+%k")              # 0–23

if [[ hour -lt 12 ]]                # Is it 0–11?
then
    echo "Good morning!"
elif [[ hour -lt 18 ]]              # Is it 12–17?
then
    echo "Good afternoon!"
else                                # Is it 18–23?
    echo "Good evening!"
fi

Yet another conditional example

#! /bin/bash

# $1 is the first argument to this program.
# $2 is the second argument to this program.
# $3 is the third argument to this program.

if [[ $1 -eq $2 && $2 -eq $3 ]]
then
    echo "They are all the same!"
else
    echo "This is no good—they’re different."
fi

File Inquiry Operators

if [[ -d my_file ]]
then
    rmdir my_file
fi
if [[ -e my_file ]]
then
    echo "my_file still exists"
fi

Often, we want commands executed if a file has a certain feature (e.g., if it is readable)
File Inquiry Operators answer questions about files:
- -r : test if file is readable
- -w : test if file is writable
- -x : test if file is executable
- -e : test if file exists
- -d : test if file is a directory
- -s : test if file has non-zero size

Spaces

If the world were consistent, life would be much easier.
In the following examples, spaces look like this.

Spaces are nearly forbidden in a let statement: let a=b+c/2

Spaces are required in an if statement: if [[ $a -eq 42 ]]

CS155: Introduction to Unix

Spring 2018

Shell Scripts

Bash Shell Scripting

What’s a shell script?

Shell Scripts

Shell Scripts

A common pitfall

Shell Variables

Examples of setting variables:

An Annoying Special Case

Special Shell Variables

More About Shell Variables

Basic Math

Control Flow: Conditional Execution

Logical operations

Conditional Execution: Examples

Conditional Execution: Examples

A $? example

Comparison Operators

Comparing strings

Comparing numbers

Combining comparisons:

Another conditional example

Yet another conditional example

File Inquiry Operators

Spaces