Index

This index / glossary / quick-reference lists many of the important topics covered in the text. Terms are arranged in approximate sorting order, reshuffled as necessary for enhanced clarity.

Note that commands are indexed in Part 4.

* * *

(caret) Beginning-of-line, in a Regular Expression

~ Tilde

• ~ home directory, corresponds to $HOME

• ~/ Current user's home directory

• ~+ Current working directory

• ~- Previous working directory

= Equals sign

• = Variable assignment operator

• = String comparison operator

  == String comparison operator

• =~ Regular Expression match operator

  Example script

< Left angle bracket

• Is-less-than

  String comparison

  Integer comparison within double parentheses

• Redirection

  < stdin

  << Here document

  <<< Here string

  <> Opening a file for both reading and writing

> Right angle bracket

• Is-greater-than

  String comparison

  Integer comparison, within double parentheses

• Redirection

  > Redirect stdout to a file

  >> Redirect stdout to a file, but append

  i>&j Redirect file descriptor i to file descriptor j

  >&j Redirect stdout to file descriptor j

  >&2 Redirect stdout of a command to stderr

  2>&1 Redirect stderr to stdout

  &> Redirect both stdout and stderr of a command to a file

  :> file Truncate file to zero length

| Pipe, a device for passing the output of a command to another command or to the shell

|| Logical OR test operator

- (dash)

• Prefix to default parameter, in parameter substitution

• Prefix to option flag

• Indicating redirection from stdin or stdout

• -- (double-dash)

  Prefix to long command options

  C-style variable decrement within double parentheses

; (semicolon)

• As command separator

• \; Escaped semicolon, terminates a find command

• ;; Double-semicolon, terminator in a case option

• Required when ...

  do keyword is on the first line of loop

  terminating curly-bracketed code block

: Colon, null command, equivalent to the true Bash builtin

• :> file Truncate file to zero length

! Negation operator, inverts exit status of a test or command

• != not-equal-to String comparison operator

? (question mark)

• Match zero or one characters, in an Extended Regular Expression

• Single-character wild card, in globbing

• In a C-style Trinary operator

// Double forward slash, behavior of cd command toward

. (dot / period)

• . Load a file (into a script), equivalent to source command

• . Match single character, in a Regular Expression

• . Current working directory

  ./ Current working directory

• .. Parent directory

' ... ' (single quotes) strong quoting

" ... " (double quotes) weak quoting

() Parentheses

• ( ... ) Command group; starts a subshell

• ( ... ) Enclose group of Extended Regular Expressions

• >( ... )

  <( ... ) Process substitution

• ... ) Terminates test-condition in case construct

• (( ... )) Double parentheses, in arithmetic expansion

[ Left bracket, test construct

[ ]Brackets

• Array element

• Enclose character set to match in a Regular Expression

• Test construct

[[ ... ]] Double brackets, extended test construct

$ Anchor, in a Regular Expression

$ Prefix to a variable name

$( ... ) Command substitution, setting a variable with output of a command, using parentheses notation

` ... ` Command substitution, using backquotes notation

${ ... } Variable manipulation / evaluation

• ${var} Value of a variable

• ${#var} Length of a variable

• ${#@}

  ${#*} Number of positional parameters

• ${parameter?err_msg} Parameter-unset message

• ${parameter-default}

  ${parameter:-default}

  ${parameter=default}

  ${parameter:=default} Set default parameter

• ${parameter+alt_value}

  ${parameter:+alt_value}

  Alternate value of parameter, if set

• ${!var}

  Indirect referencing of a variable, new notation

• ${!varprefix*}

  ${!varprefix@}

  Match names of all previously declared variables beginning with varprefix

• ${string:position}

  ${string:position:length} Substring extraction

• ${var#Pattern}

  ${var##Pattern} Substring removal

• ${var%Pattern}

  ${var%%Pattern} Substring removal

• ${string/substring/replacement}

  ${string//substring/replacement}

  ${string/#substring/replacement}

  ${string/%substring/replacement} Substring replacement

\ Escape the character following

• \< ... \> Angle brackets, escaped, word boundary in a Regular Expression

• \{ N \} "Curly" brackets, escaped, number of character sets to match in an Extended RE

• \; Semicolon, escaped, terminates a find command

• \$$ Indirect reverencing of a variable, old-style notation

• Escaping a newline, to write a multi-line command

&

• &> Redirect both stdout and stderr of a command to a file

• >&j Redirect stdout to file descriptor j

  >&2 Redirect stdout of a command to stderr

• i>&j Redirect file descriptor i to file descriptor j

  2>&1 Redirect stderr to stdout

• Closing file descriptors

  n<&- Close input file descriptor n

  0<&-, <&- Close stdin

  n>&- Close output file descriptor n

  1>&-, >&- Close stdout

• && Logical AND test operator

• Command & Run job in background

# Hashmark, special symbol beginning a script comment

#! Sha-bang, special string starting a shell script

* Asterisk

• Wild card, in globbing

• Any number of characters in a Regular Expression

• ** Exponentiation, arithmetic operator

% Percent sign

• Modulo, division-remainder arithmetic operation

• Substring removal (pattern matching) operator

+ Plus sign

• Character match, in an extended Regular Expression

• Prefix to alternate parameter, in parameter substitution

• ++ C-style variable increment, within double parentheses

* * *

Shell Variables

$_ Last argument to previous command

$- Flags passed to script, using set

$! Process ID of last background job

$? Exit status of a command

$@ All the positional parameters, as separate words

$* All the positional parameters, as a single word

$$ Process ID of the script

$# Number of arguments passed to a function, or to the script itself

$0 Filename of the script

$1 First argument passed to script

$9 Ninth argument passed to script

Table of shell variables

* * * * * *

-a Logical AND compound comparison test

Advanced Bash Scripting Guide, where to download

Alias

• Removing an alias, using unalias

And list

• To supply default command-line argument

Angle brackets, escaped, \< . . . \> word boundary in a Regular Expression

Anonymous here document, using :

Arithmetic expansion

• variations of

Arithmetic operators

• combination operators, C-style

  += -= *= /= %=

  In certain contexts, += can also function as a string concatenation operator.

Arrays

• Bracket notation

• Concatenating, example script

• Copying

• Declaring

  declare -a array_name

• Embedded arrays

• Empty arrays, empty elements, example script

• Indirect references

• Initialization

  array=( element1 element2 ... elementN)

  Example script

  Using command substitution

• Loading a file into an array

• Multidimensional, simulating

• Nesting and embedding

• Notation and usage

• Number of elements in

  ${#array_name[@]}

  ${#array_name[*]}

• Operations

• Passing an array to a function

• As return value from a function

• Special properties, example script

• String operations, example script

• unset deletes array elements

awk field-oriented text processing language

• rand(), random function

• String manipulation

• Using export to pass a variable to an embedded awk script

* * *

Backquotes, used in command substitution

Base conversion, example script

Bash

• Bad scripting practices

• Basics reviewed, script example

• Command-line options

  Table

• Features that classic Bourne shell lacks

• Internal variables

• Version 2

• Version 3

$BASH_SUBSHELL

Basic commands, external

Batch processing

bc, calculator utility

• In a here document

• Template for calculating a script variable

Bison utility

Bitwise operators

Blocks of code

• Redirection

  Script example: redirecting output of a a code block

Brace expansion

• Extended, {a..z}

Brackets, [ ]

• Array element

• Enclose character set to match in a Regular Expression

• Test construct

Brackets, curly, {}, used in

• Code block

• find

• Extended Regular Expressions

• Positional parameters

• xargs

break loop control command

• Parameter (optional)

Builtins in Bash

• Do not fork a subprocess

* * *

case construct

• Command-line parameters, handling

• Globbing, filtering strings with

cat, concatentate file(s)

• Abuse of

• cat scripts

• Less efficient than redirecting stdin

• Piping the output of, to a read

• Uses of

Child processes

Colon, : , equivalent to the true Bash builtin

Colorizing scripts

• Table of color escape sequences

• Template, colored text on colored background

Comma operator, linking commands or operations

Command substitution

• $( ... ), preferred notation

• Backquotes

• Extending the Bash toolset

• Invokes a subshell

• Nesting

• Removes trailing newlines

• Setting variable from loop output

• Word splitting

Comment headers, special purpose

Commenting out blocks of code

• Using an anonymous here document

• Using an if-then construct

Compound comparison operators

continue loop control command

Control characters

• Control-C, break

• Control-D, terminate / log out / erase

• Control-H, rubout

• Control-M, Carriage return

C-style syntax , for handling variables

Curly brackets {}

• in find command

• in an Extended Regular Expression

• in xargs

* * *

Daemons, in UNIX-type OS

dc, calculator utility

dd, data duplicator command

• Conversions

• Copying raw data to/from devices

• File deletion, secure

• Keystrokes, capturing

• Options

• Random access on a data stream

• Swapfiles, initializing

• Thread on www.linuxquestions.org

Debugging scripts

• Tools

• Trapping at exit

• Trapping signals

Decimal number, Bash interprets numbers as

declare builtin

• options

Default parameters

/dev directory

• /dev/null pseudo-device file

• /dev/urandom pseudo-device file, generating pseudorandom numbers with

• /dev/zero, pseudo-device file

dialog, utility for generating dialog boxes in a script

$DIRSTACK directory stack

Disabled commands, in restricted shells

do keyword, begins execution of commands within a loop

done keyword, terminates a loop

DOS batch files, converting to shell scripts

DOS commands, UNIX equivalents of (table)

dot files, "hidden" setup and configuration files

Double brackets [[ ... ]] test construct

Double quotes " ... " weak quoting

Double-spacing a text file, using sed

* * *

-e File exists test

echo

• Feeding commands down a pipe

• Setting a variable using command substitution

• /bin/echo, external echo command

elif, Contraction of else and if

esac, keyword terminating case construct

Environmental variables

-eq , is-equal-to integer comparison test

Escaped characters, special meanings of

$EUID, Effective user ID

eval, Combine and evaluate expression(s), with variable expansion

• Effects of, Example script

• Risk of using

exec command, using in redirection

Exit and Exit status

• exit command

• Exit status (exit code, return status of a command)

  Table, Exit codes with special meanings

  Out of range

  Specified by a function return

  Successful, 0

Export, to make available variables to child processes

• Passing a variable to an embedded awk script

expr, Expression evaluator

• Substring extraction

• Substring index (numerical position in string)

• Substring matching

Extended Regular Expressions

• ? (question mark) Match zero / one characters

• ( ... ) Group of expressions

• \{ N \} "Curly" brackets, escaped, number of character sets to match

• + Character match

* * *

false, returns unsuccessful (1) exit status

File descriptors

• Closing

  n<&- Close input file descriptor n

  0<&-, <&- Close stdin

  n>&- Close output file descriptor n

  1>&-, >&- Close stdout

• File handles in C, similarity to

find

• {} Curly brackets

• \; Escaped semicolon

Filter, feeding output back to same filter

Floating point numbers, Bash does not recognize

fold, a filter to wrap lines of text

Forking a child process

for loops

Functions

• Arguments passed referred to by position

• Capturing the return value of a function using echo

• Definition must precede first call to function

• Exit status

• Local variables

  and recursion

• Passing an array to a function

• Passing pointers to a function

• Recursion

• Redirecting stdin of a function

• return

  Returning an array from a function

  return range limits, workarounds

• shift arguments passed to a function

* * *

getopt, external command for parsing script command-line arguments

• Emulated in a script

getopts, Bash builtin for parsing script command-line arguments

• $OPTIND / $OPTARG

Globbing, filename expansion

-ge , greater-than or equal integer comparison test

-gt , greater-than integer comparison test

$GROUPS, Groups user belongs to

* * *

Hashing, creating lookup keys in a table

• Example script

head, echo to stdout lines at the beginning of a text file

help, gives usage summary of a Bash builtin

Here documents

• Anonymous here documents, using :

  Commenting out blocks of code

  Self-documenting scripts

• bc in a here document

• cat scripts

• Command substitution

• ex scripts

• Function, supplying input to

• Here strings

  Prepending text

  Using read

• Limit string

  Closing limit string may not be indented

  Dash option to limit string, <<-LimitString

• Literal text output, for generating program code

• Parameter substitution

  Disabling parameter substitution

• Passing parameters

• Temporary files

• Using vi non-interactively

$HOME, user's home directory

$HOSTNAME, system host name

* * *

if [ condition ]; then ... test construct

• if-grep, if and grep in combination

  Fixup for if-grep test

$IFS, Internal field separator variable

• Defaults to whitespace

Integer comparison operators

in, keyword preceding [list] in a for loop

Initialization table, /etc/inittab

Interactive script, test for

I/O redirection

Indirect referencing of variables

• New notation, introduced in version 2 of Bash

• Example script

Iteration

* * *

Job IDs, table

jot, Emit a sequence of integers. Equivalent to seq.

* * *

Keywords

kill, terminate a process by process ID

• Options (-l, -9)

killall, terminate a process by name

killall script in /etc/rc.d/init.d

* * *

-le , less-than or equal integer comparison test

let, setting and carrying out arithmetic operations on variables

Limit string, in a here document

$LINENO, variable indicating the line number where it appears in a script

Link, file (using ln command)

• Invoking script with multiple names, using ln

• symbolic links, ln -s

List constructs

• And list

• Or list

Local variables

• and recursion

Localization

Logical operators (&&, ||, etc.)

Logout file, the ~/.bash_logout file

Loops

• break loop control command

• continue loop control command

• C-style loop within double parentheses

  for loop

  while loop

• do (keyword), begins execution of commands within a loop

• done (keyword), terminates a loop

• for loops

  for arg in [list]; do

  Command substitution to generate [list]

  Filename expansion in [list]

  Multiple parameters in each [list] element

  Omitting [list], defaults to positional parameters

  Parameterizing [list]

  Redirection

• in, (keyword) preceding [list] in a for loop

• Nested loops

• Running a loop in the background, script example

• Semicolon required, when do is on first line of loop

  for loop

  while loop

• until loop

  until [ condition-is-true ]; do

• while loop

  while [ condition ]; do

  Function call inside test brackets

  Multiple conditions

  Omitting test brackets

  Redirection

  while read construct

• Which type of loop to use

Loopback devices, in /dev directory

-lt , less-than integer comparison test

* * *

m4, macro processing language

$MACHTYPE, Machine type

Magic number, marker at the head of a file indicating the file type

Makefile, file containing the list of dependencies used by make command

Modulo, arithmetic remainder operator

• Application: Generating prime numbers

Mortgage calculations, example script

* * *

-n String not null test

Named pipe, a temporary FIFO buffer

• Example script

nc, netcat, a toolkit for TCP and UDP ports

-ne, not-equal-to integer comparison test

Negation operator, !, reverses the sense of a test

netstat, Network statistics

nl, a filter to number lines of text

Noclobber, -C option to Bash to prevent overwriting of files

null variable assignment, avoiding

* * *

-o Logical OR compound comparison test

od, octal dump

$OLDPWD Previous working directory

Operator

• Definition of

• Precedence

Options, passed to shell or script on command line or by set command

Or list

Or logical operator, ||

* * *

Parameter substitution

• ${parameter+alt_value}

  ${parameter:+alt_value}

  Alternate value of parameter, if set

• ${parameter-default}

  ${parameter:-default}

  ${parameter=default}

  ${parameter:=default}

  Default parameters

• ${!varprefix*}

  ${!varprefix@}

  Parameter name match

• ${parameter?err_msg}

  Parameter-unset message

• ${parameter}

  Value of parameter

• Script example

• Table of parameter substitution

Parent / child process problem, a child process cannot export variables to a parent process

Parentheses

• Command group

• Enclose group of Extended Regular Expressions

• Double parentheses, in arithmetic expansion

$PATH, the path (location of system binaries)

Perl, programming language

• Combined in the same file with a Bash script

• Embedded in a Bash script

• Using eval with

PID, process ID

Pipe, | , a device for passing the output of a command to another command or to the shell

• Avoiding unnecessary commands in a pipe

• Pipefail, set -o pipefail option to indicate exit status within a pipe

• $PIPESTATUS, exit status of last executed pipe

• Piping output of a command to a script

• Redirecting stdin, rather than using cat in a pipe

Pitfalls

• - (dash) is not redirection operator

• // (double forward slash), behavior of cd command toward

• #!/bin/sh script header disables extended Bash features

• Abuse of cat

• CGI programming, using scripts for

• Closing limit string in a here document, indenting

• DOS-type newlines (\r\n) crash a script

• eval, risk of using

• Execute permission lacking for commands within a script

• Export problem, child process to parent process

• Extended Bash features not available

• Failing to quote variables within test brackets

• GNU command set, in cross-platform scripts

• null variable assignment

• Numerical and string comparison operators not equivalent

  = and -eq not interchangeable

• Omitting terminal semicolon, in a curly-bracketed code block

• Piping

  echo to a loop

  echo to read

  tail -f to grep

• suid commands inside a script

• Undocumented Bash features, danger of

• Uninitialized variables

• Variable names, inappropriate

• Variables in a subshell, scope limited

• Whitespace, misuse of

Portability issues in shell scripting

• Setting path and umask

• Using whatis

Positional parameters

• $@, as separate words

• $*, as a single word

POSIX, Portable Operating System Interface / UNIX

• Character classes

$PPID, process ID of parent process

Precedence, operator

Prepending lines at head of a file, script example

Prime numbers

• Generating primes without using arrays

• Sieve of Eratosthenes

printf, formatted print command

/proc directory

• Running processes, files describing

• Writing to files in /proc, warning

Process

• Process ID (PID)

Process substitution

• To compare contents of directories

• To supply stdin of a command

• Template

Prompt

• $PS1, Main prompt, seen at command line

• $PS2, Secondary prompt

Pseudo-code, as problem-solving method

$PWD, Current working directory

* * *

Question mark, ?

• Character match in an Extended Regular Expression

• Single-character wild card, in globbing

• In a C-style Trinary operator

Quoting

• Character string

• Variables

  within test brackets

• Whitespace, using quoting to preserve

* * *

Random numbers

• /dev/urandom

• rand(), random function in awk

• $RANDOM, Bash function that returns a pseudorandom integer

• Random sequence generation, using date command

• Random string, generating

rcs

read, set value of a variable from stdin

• Detecting arrow keys

• Options

• Piping output of cat to read

• "Prepending" text

• Problems piping echo to read

• Redirection from a file to read

• $REPLY, default read variable

• Timed input

• while read construct

Recursion

• Local variables

Redirection

• Code blocks

• exec <filename,

  to reassign file descriptors

• Introductory-level explanation of I/O redirection

• Open a file for both reading and writing

  <>filename

• read input redirected from a file

• stderr to stdout

  2>&1

• stdin / stdout, using -

• stdinof a function

• stdout to a file

  > ... >>

• stdout to file descriptor j

  >&j

• file descriptori to file descriptor j

  i>&j

• stdout of a command to stderr

  >&2

• stdout and stderr of a command to a file

  &>

• tee, redirect to a file output of command(s) partway through a pipe

Reference Cards

• Miscellaneous constructs

• Parameter substitution/expansion

• Special shell variables

• String operations

• Test operators

  Binary comparison

  Files

Regular Expressions

• (caret) Beginning-of-line

• $ (dollar sign) Anchor

• . (dot) Match single character

• * (asterisk) Any number of characters

• [ ] (brackets) Enclose character set to match

• \ (backslash) Escape, interpret following character literally

• \< ... \> (angle brackets, escaped) Word boundary

• Extended REs

  + Character match

  \{ \} Escaped "curly" brackets

  [: :] POSIX character classes

$REPLY, Default value associated with read command

Restricted shell, shell (or script) with certain commands disabled

return, command that terminates a function

run-parts

• Running scripts in sequence, without user intervention

* * *

Scope of a variable, definition

Script options, set at command line

Scripting routines, library of useful definitions and functions

Secondary prompt, $PS2

Security issues

• nmap, network mapper / port scanner

• sudo

• suid commands inside a script

• Viruses, trojans, and worms in scripts

• Writing secure scripts

sed, pattern-based programming language

• Table, basic operators

• Table, examples of operators

select, construct for menu building

• in list omitted

Semicolon required, when do keyword is on first line of loop

• When terminating curly-bracketed code block

seq, Emit a sequence of integers. Equivalent to jot.

set, Change value of internal script variables

Shell script, definition of

Shell wrapper, script embedding a command or utility

shift, reassigning positional parameters

$SHLVL, shell level, depth to which the shell (or script) is nested

shopt, change shell options

Signal, a message sent to a process

Single quotes (' ... ') strong quoting

Socket, a communication node associated with an I/O port

Sorting

• Bubble sort

• Insertion sort

source, execute a script or, within a script, import a file

• Passing positional parameters

Spam, dealing with

• Example script

• Example script

• Example script

• Example script

Special characters

Stack, emulating a push-down, Example script

Startup files, Bash

stdin and stdout

Strings

• =~ String match operator

• Comparison

• Length

  ${#string}

• Manipulation

• Manipulation, using awk

• Protecting strings from expansion and/or reinterpretation, script example

  Unprotecting strings, script example

• Substring extraction

  ${string:position}

  ${string:position:length}

  Using expr

• Substring index (numerical position in string)

• Substring matching, using expr

• Substring removal

  ${var#Pattern}

  ${var##Pattern}

  ${var%Pattern}

  ${var%%Pattern}

• Substring replacement

  ${string/substring/replacement}

  ${string//substring/replacement}

  ${string/#substring/replacement}

  ${string/%substring/replacement}

  Script example

• Table of string/substring manipulation and extraction operators

Strong quoting ' ... '

Stylesheet for writing scripts

Subshell

• Command list within parentheses

• Variables, $BASH_SUBSHELL and $SHLVL

• Variables in a subshell, scope limited

su Substitute user, log on as a different user or as root

suid (set user id) file flag

• suid commands inside a script, not advisable

Symbolic links

Swapfiles

* * *

tail, echo to stdout lines at the (tail) end of a text file

tee, redirect to a file output of command(s) partway through a pipe

Terminals

• setserial

• setterm

• stty

• wall

test command

• Bash builtin

• external command, /usr/bin/test (equivalent to /usr/bin/[)

Test constructs

Test operators

• -a Logical AND compound comparison

• -e File exists

• -eq is-equal-to (integer comparison)

• -f File is a regular file

• -ge greater-than or equal (integer comparison)

• -gt greater-than (integer comparison)

• -le less-than or equal (integer comparison)

• -lt less-than (integer comparison)

• -n not-zero-length (string comparison)

• -ne not-equal-to (integer comparison)

• -o Logical OR compound comparison

• -u suid flag set, file test

• -z is-zero-length (string comparison)

• = is-equal-to (string comparison)

  == is-equal-to (string comparison)

• < less-than (string comparison)

• < less-than, (integer comparison, within double parentheses)

• <= less-than-or-equal, (integer comparison, within double parentheses)

• > greater-than (string comparison)

• > greater-than, (integer comparison, within double parentheses)

• >= greater-than-or-equal, (integer comparison, within double parentheses)

• || Logical OR

• && Logical AND

• ! Negation operator, inverts exit status of a test

  != not-equal-to (string comparison)

• Tables of test operators

  Binary comparison

  File

Timed input

• Using read -t

• Using stty

• Using timing loop

• Using $TMOUT

Tips and hints for Bash scripts

• Array, as return value from a function

• Comment blocks

  Using anonymous here documents

  Using if-then constructs

• Comment headers, special purpose

• C-style syntax , for handling variables

• Double-spacing a text file

• Filter, feeding output back to same filter

• Function return value workarounds

• if-grep test fixup

• Library of useful definitions and functions

• null variable assignment, avoiding

• Passing an array to a function

• Prepending lines at head of a file

• Pseudo-code

• Script as embedded command

• rcs

• Running scripts in sequence without user intervention, using run-parts

• Script portability

  Setting path and umask

  Using whatis

• Setting script variable to a block of embedded sed or awk code

• Testing a variable to see if it contains only digits

• Tracking script usage

• Widgets, invoking from a script

$TMOUT, Timeout interval

tr, character translation filter

• DOS to Unix text file conversion

• Options

• Soundex, example script

• Variants

Trap, specifying an action upon receipt of a signal

Trinary operator, C-style

true, returns successful (0) exit status

typeset builtin

• options

* * *

$UID, User ID number

unalias, to remove an alias

uname, output system information

Uninitialized variables

uniq, filter to remove duplicate lines from a sorted file

unset, delete a shell variable

until loop

until [ condition-is-true ]; do

* * *

Variables

• Array operations on

• Assignment

  Script example

  Script example

  Script example

• Bash internal variables

• Block of sed or awk code, setting a variable to

• C-style increment/decrement/trinary operations

• Change value of internal script variables using set

• declare, to restrict the properties of variables

• Deleting a shell variable using unset

• Environmental

• Expansion / Substring replacement operators

• Indirect referencing

  eval variable1=\$$variable2

  Newer notation

  ${!variable}

• Length

  ${#var}

• lvalue

• Manipulating and expanding

• Name and value of a variable, distinguishing between

• null variable assignment, avoiding

• Quoting

  within test brackets

  to preserve whitespace

• rvalue

• Setting to null value

• In subshell not visible to parent shell

• Testing a variable if it contains only digits

• Undeclared, error message

• Uninitialized

• Unsetting

• Untyped

* * *

wait, suspend script execution

• To remedy script hang

Weak quoting " ... "

while loop

while [ condition ]; do

• while read construct

Whitespace, spaces, tabs, and newline characters

• $IFS defaults to

• Inappropriate use of

• Preceding closing limit string in a here document, error

• Preceding script comments

• Quoting, to preserve whitespace within strings or variables

Widgets

* * *

xargs, Filter for grouping arguments

• Curly brackets

• Limiting arguments passed

• Options

• Whitespace, handling

* * *

yes

* * *

-z String is null

Zombie, a process that has terminated, but not yet been killed by its parent