Part IV
Appendixes

A  Command Quick Reference

B  Glossary

C  Quiz Answers

Appendix A
Command Quick Reference

by Frank Watson

This appendix summarizes and reviews the script elements you have covered:

•  Reserved words and built-in shell commands

•  Conditional expressions

•  Arithmetic expressions (available Korn/Bash only)

•  Parameters and variables

•  Parameter substitution

•  Pattern matching

•  I/O

•  Miscellaneous command summaries

•  Regular expression wildcards

You can also find details not discussed earlier that are included here for completeness.

Reserved Words and Built-in Shell Commands

. (period)

executes the following command in the current shell instead of as a child process.

: (colon)

no-op command. Its arguments are processed for variable substitution.

!!

(Bash) re-executes the previous command.

alias

(only Korn/Bash) creates a short name for the command.

bg

(Korn/Bash) starts a suspended job running in background.

break

exits from current for, while, or until loop.

case

executes commands given for first pattern that match expr. Patterns can contain filename expansion wildcards.

case expr in
     pattern1) commands ;;
     pattern2) commands ;;
esac

cd

changes the directory.

continue

skips the rest of the commands in a loop and starts the next iteration of a loop.

do

indicates the start of a block of code, for example, in a for, while, or until loop.

done

indicates the end of a block of code, for example, in a for, while, or until loop.

echo

displays its arguments to standard output. Sometimes this is a built-in shell command replacing the external echo command.

esac

denotes the end of a case statement.

eval

causes the shell to reinterpret the command that follows.

exec

executes the following command which replaces the current process instead of running it as a child process.

exit n

ends the shell script with status code n.

export

marks the following variables, flagging them to be passed to any child processes and called programs. Korn/Bash enable assignment within the export command:

export VAR1=value VAR2=value

false

(Korn/Bash builtin) command that always returns an unsuccessful or logical false result.

fc

(Korn/Bash) displays or edits a command in history list.

fg

(Korn/Bash) brings a background or suspended job to the foreground.

fi

denotes the end of an if statement.

for

executes a block of code multiple times.

     for variable [in list]
     do
          commands
     done

function

(Korn/Bash) keyword to define a function enabling local variables.

getopts

a function called repeatedly in a loop to process the command line arguments.

history

(Korn/Bash) shows the most recent commands run by this user.

if

allows conditional execution.

if test-command
then commands
[elif commands]
[else commands]
fi

integer

(Korn/Bash) specifies an integer variable.

jobs

(Korn/Bash) lists the background and suspended jobs.

kill

sends a signal to a process; often used to terminate a process or to reinitialize a daemon background process.

let

(Korn/Bash) performs integer arithmetic.

newgrp

(Korn) changes your primary group, affecting the group of all new files and directories that you create.

print

(Korn) an alternative to echo.

pwd

prints the present working or current directory.

r

(Korn) re-executes the previous command.

read

waits for one line of standard input and saves each word in the following variables. If there are more words than variables, it saves the remaining words in the last variable.

readonly

marks the following variables to give error if an attempt to assign a new value is made.

return

returns from a function.

select

(Korn/Bash) presents a menu and enables user selection.

set

displays or changes shell options.

shift

discards $1 and shifts all the positional parameters up one to take its place.

test

(Korn/Bash builtin) provides many options to check files, strings, and numeric values. Often denoted by [ (left bracket).

trap

designates code to execute if a specific signal is received, such as:

0     exit from script
1     hangup/disconnect
2     intr key pressed (Ctrl-C or DEL)
3     quit key pressed
15    request to terminate process

type

displays the pathname of the following command or indicates whether it is built-in or an alias.

typeset

(Korn/Bash) sets the type of variable and optionally its initial value.

ulimit

displays or sets the largest file or resource limit.

umask

displays or sets a mask to affect permissions of any new file or directory you create.

unalias

(Korn/Bash) removes an alias.

unset

undefines the variables that follow.

until

(Korn/Bash) loops until the test command is true (successful).

until test-command
do
     commands
done

wait

pauses until all background jobs are complete.

whence

(Korn) similar to the type command.

while

loops while a test command is true (successful).

while test-command
do
     commands
done

Conditional Expressions

These can be used with

if [ test-expression ]
while [ test-expression ]
until [ test-expression ]

File Tests

String Tests

Integer Comparisons

Comparisons stop on first non-digit.

Arithmetic Expressions (Korn/Bash Only)

Follow the general format for variable assignment:

let "VARIABLE=integer_expresson"

To embed integer calculations within a command

$((integer_expression))

Operators Allowed in Korn/Bash Integer Expressions

•  Logical operators return 1 for true and 0 for false

•  This list is from highest to lowest operator precedence

Parameters and Variables

User-Defined Variables

User-defined variable names

•  Must start with letter or _

•  Can contain only letters, digits, or _

•  Are often in capital letters to differentiate from UNIX commands

Korn/Bash 2.x Support Arrays

Note index must be an integer.

Korn array initialization

      set -A USERVAR value1 value2 value3 ...

Bash array initialization

      USERVAR=(value1 value2 value3 ...)

Built-in Shell Variables

Built-in Commands that Directly Affect Variables

getopts, export, read, readonly, unset

Two Types of Variables

•  Environment variables are passed to any child processes.

•  Local variables are not passed to any child processes.

Shell Variables

CDPATH

contains colon-separated list of directories to facilitate cd command

HOME

Your home directory

IFS

Internal field separator characters

OPTARG

The last cmd line arg processed by getopts (Korn/Bash)

OPTIND

The index of the last cmd line arg processed by getopts (Korn/Bash)

PATH

Contains a colon-separated list of directories to search for commands that are given without any slash

PS1

The primary shell prompt string

PS2

The secondary shell prompt string for continuation lines

PWD

The current directory

RANDOM

Returns a different random number (from 0 to 32,767) each time it is invoked

REPLY

The last input line from read via select command (Korn/Bash)

SECONDS

The numbers of seconds since shell invocation

SHLVL

The number of shells currently nested

UID

The numeric user ID number

Parameter Substitution

Parameter Substitution in Bourne/Korn/Bash

${parameter}

substitutes the contents of the parameter, which can be a variable name or digit indicating a positional parameter.

${parameter:-word}

substitutes the contents of the parameter but if it is empty or undefined, it substitutes the word, which might contain unquoted spaces.

${parameter:=word}

substitutes the contents of the parameter but if it is empty or undefined, it sets parameter equal to the word and substitutes word.

${parameter:?message}

substitutes the contents of the parameter, but if it is empty or undefined, aborts the script and gives the message as a final error. Message might contain unquoted spaces.

${parameter:+word}

if parameter is not empty, it substitutes the word; otherwise it substitutes nothing.

Parameter Substitution Only in Korn/Bash

${#parameter}

substitutes the number of characters in the contents of parameter.

${#array[*]}

substitutes the number of elements in array.

${parameter#pattern}

if the regular expression pattern given is found at start of the contents of parameter, it deletes the matching characters and substitutes the remainder. The smallest possible match is deleted.

${parameter##pattern}

same as above but deletes the largest possible match at the start of parameter.

${parameter%pattern}

same as above but deletes the smallest match at the end of parameter.

${parameter%%pattern}

same as above but deletes the largest match at the end of parameter.

Pattern Matching

Rules for filename expansion:

•  Any word on the command line containing a wildcard is expanded to a list of files which match the pattern word.

•  If no filename matches are found, the pattern word is not substituted.

•  Wildcards cannot match a leading period or a slash.

Pattern Wildcards Available in Bourne/Korn/Bash

Pattern Wildcards Available Only in Korn

?(pattern1|pattern2...)

matches any of the patterns

*(pattern1|pattern2...)

matches zero or more occurrences of the patterns

+(pattern1|pattern2...)

matches one or more occurrences of the patterns

@(pattern1|pattern2...)

matches only one of the patterns

!(pattern1|pattern2...)

matches anything except one of the patterns

I/O

here Document

Provides STDIN to UNIX command from lines that follow until delimiter is found at start of line:

cmd << delimiter
one or more text lines
delimiter

Miscellaneous Command Summaries

Here is some helpful information about several commands often used in shell programming.

echo

display arguments to standard output

grep

display lines that contain the given pattern

printf

display formatted text output, for example:

printf "text %[-]m.nx" arguments

sort

display lines in sorted order

Regular Expression Wildcards

grep, fgrep, egrep, sed, vi, perl, and awk allow regular expression wildcards in search patterns.

Limited Regular Expression Wildcards

All regular expression patterns can include these wildcards:

Extended Regular Expression Wildcards

These are additional regular expression wildcards that are only supported in some commands:

Summary

This appendix provides a quick reference for shell commands and features:

•  Reserved words and built-in shell commands

•  Conditional expressions

•  Arithmetic expressions (Korn/Bash only)

•  Parameters and variables

•  Parameter substitution

•  Pattern matching

•  I/O

•  Miscellaneous command summaries

•  Regular expression wildcards

As you write scripts and become familiar with the concepts, you might find this summary helps you to locate a symbol, a command name, or the correct syntax.

Questions

1.  What section of this summary describes how to append output to a file?

2.  What section of this summary describes how to end a case statement?

3.  What section of this summary enables you to determine whether the jobs command is supported in the Bourne shell?

4.  What section of this summary enables you to determine whether the + sign is a generally supported regular expression wildcard?

5.  What section of this summary enables you to determine which shell variable gives the numeric result code of the last command executed?