Linux Scripts

Introduction

The Bash shell allows users to interact with the computer through a terminal window, organizing commands that tie various programs together. First developed by Brian Fox in 1987, the Bash shell seeks to incorporate numerous features from earlier shells for the Unix operating system. In particular, the Bash shell draws upon the Bourne shell (developed by Ken Thompson and then Stephen Bourne around 1977), the Korn shell (developed by David Korn in the early 1980s), and the C shell (developed by Bill Joy, based on Thompson's earlier shells).

Within a terminal window, you already have used several commands within the Bash shell. For example, Linux lab 1 and Linux lab 2 discussed Bash commands, such as:

• password
• sleep, history, cat
• pwd, cd, ls, ls -l, which, whereis
• mkdir, rmdir, rm, cp, mv, more, head, pushd, popd
• man
• lpr, a2ps, lpq, lprm
• chmod

In this lab, we consider several additional commands, and we explore how various capabilities can be packaged to accomplish various tasks. The approach is not unlike programming in C (or Scheme), although the programming elements often are entire programs.

Some Additional Commands

Bash includes a huge range of commands. Here are a few more useful capabilities:

Command	Description	Example
cal	display a calendar for a given year	cal 2010
date	print the current time and date in various formats many options documented with man date	date +'%I:%M %p on %A, %B %d, %Y'
diff	display differences in lines between two files	diff file1 file2
echo	print specified text	echo hello world!
env	print the environmental variables currently set	env
grep	scan a file or other input for a specified text	env | grep home
hostname	return name of current workstation	hostname
sort	sort lines of a file	sort -n -k 12 (sort by numbers in column 12)
users	simple list of users	users
who	print list of users currently logged in	who and who -a
whoami	print the username of the person logged in	whoami
ypcat password	print user information from the password file	ypcat password | grep walker

More information on each of these commands may be found with the man facility (e.g., man date). Beyond this basic documentation included here an in previous labs, many resources are available on line. Here are a few places to begin:

• The Bash home page from the GNU Project provides a complete online manual for Bash.
• The Bash Guide for Beginners from The Linux Documentation Project provides a good introduction to the Bash shell.
• At this writing, a Google search of "bash shell introduction" produced about 2,5000,000 hits, so there is much easily-accessible material to read on this subject!

Additional Notes

Arithmetic Expressions Require (( ))

The variables in a bash script are by default interpreted as text. To get bash to interpret a value as a number, in order to do arithmetic operations including arithmetic comparisons, you must wrap the expression in double-parentheses. For example, to increment a variable i by one, use the expression((i++)) .

Conditionals and Loops

In addition to basic commands, the Bash shell includes capabilities for conditionals (if) and loops (for, while). The long history of Bash is particularly evident in the syntax allowed for if expressions. Bash allows interprets each syntax properly — but do not try to mix and match the different versions. The following examples illustrate acceptable Bash syntax, following two different ancestors.

Task	Bourne shell style	Korn shell style
numeric test a > 0	if [ $a -gt \0 ]; then     echo positive fi	if (( $a > 0 )); then     echo positive fi
numeric test a ≤ 0	if [ $a -le 0 ]; then     echo non-positive else     echo positive fi	if (( $a <= 0 )); then     echo non-positive else     echo positive fi
numeric test a < 0 and a == 0	if [ $a -lt 0 ]; then     echo negative elif [ $a -eq 0 ]; then     echo zero else     echo positive fi	if (( $a <= 0 )); then     echo non-positive elif (( $a == 0 )); then     echo zero else     echo positive fi
numeric test 0 ≤ a &le 10	if [ 0 -le $a -a $a -le 10 ]; then     echo between 0 and 10 else     echo not between 0 and 10 fi	if (( (0 <= $a) && ($a <= 10) )); then     echo between 0 and 10 else     echo not between 0 and 10 fi

For reference, all of these code segments are available in the Bash shell bash-example-1

Comparison Operators

Comparison operators are available in both the Bourne and Korn environments, although the syntax is different:

Korn shell:      ==   !=    <   <=    >   >=   &&   ||
Bourne shell:   -eq  -ne  -lt  -le  -gt  -ge   -a   -o

String comparison is possible with the symbols

  ==  !=  <  <=  >  >=  &&  ||