This index / glossary / quick-reference lists many of the important topics covered in the text. Terms are arranged in approximate ASCII sorting order, modified as necessary for enhanced clarity. Note that commands are indexed in Part 4. * * * ^ (caret) Beginning-of-line, in a Regular Expression ^ ^^ Uppercase conversion in parameter substitution ~ 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 ;;& ;& Terminators in a case option (version 4+ of Bash). : Colon :> filename Truncate file to zero length null command, equivalent to the true Bash builtin Used in an anonymous here document Used in an otherwise empty function Used as a function name ! 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 Double-quoting the backslash (\) character , Comma operator , ,, Lowercase conversion in parameter substitution () 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 $[ ... ] Integer expansion (deprecated) ${ ... } 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 ${!#} Final positional parameter. (This is an indirect reference to $#.) ${!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 $' ... ' String expansion, using escaped characters. \ Escape the character following \< ... \> Angle brackets, escaped, word boundary in a Regular Expression \{ N \} Curly brackets, escaped, number of character sets to match in an Extended RE \; Semicolon, escaped, terminates a find command \$$ 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 ** Extended globbing file-match 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 Address database, script example Advanced Bash Scripting Guide, where to download Alias Removing an alias, using unalias Anagramming And list To supply default command-line argument And logical operator && Angle brackets, escaped, \< . . . \> word boundary in a Regular Expression Anonymous here document, using : Archiving rpm tar Arithmetic expansion exit status of variations of Arithmetic operators combination operators, C-style += -= *= /= %= In certain contexts, += can also function as a string concatenation operator. Arrays Associative arrays more efficient than conventional 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 Arrow keys, detecting ASCII Definition Scripts for generating ASCII table awk field-oriented text processing language rand(), random function String manipulation Using export to pass a variable to an embedded awk script * * * Backlight, setting the brightness 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 Version 4 Version 4.1 Version 4.2 .bashrc $BASH_SUBSHELL Basic commands, external Batch files, DOS Batch processing bc, calculator utility In a here document Template for calculating a script variable Bibliography Bison utility Bitwise operators Example script Block devices testing for Blocks of code Iterating / looping Redirection Script example: Redirecting output of a a code block Bootable flash drives, creating Brace expansion Extended, {a..z} Parameterizing With increment and zero-padding (new feature in Bash, version 4) 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 Character devices testing for Checksum Child processes Colon, : , equivalent to the true Bash builtin Colorizing scripts Cycling through the background colors, example script Table of color escape sequences Template, colored text on colored background Comma operator, linking commands or operations Command-line options command_not_found_handle () builtin error-handling function (version 4+ of Bash) 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 Communications and hosts Compound comparison operators Compression utilities bzip2 compress gzip zip continue loop control command Control characters Control-C, break Control-D, terminate / log out / erase Control-G, BEL (beep) Control-H, rubout Control-J, newline Control-M, carriage return Coprocesses cron, scheduling daemon C-style syntax , for handling variables Crossword puzzle solver Cryptography Curly brackets {} in find command in an Extended Regular Expression in xargs * * * Daemons, in UNIX-type OS date 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 Raspberry Pi, script for preparing a bootable SD card Swapfiles, initializing Thread on www.linuxquestions.org Debugging scripts Tools Trapping at exit Trapping signals Decimal number, Bash interprets numbers as declare builtin options case-modification options (version 4+ of Bash) Default parameters /dev directory /dev/null pseudo-device file /dev/urandom pseudo-device file, generating pseudorandom numbers with /dev/zero, pseudo-device file 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 and evaluation of octal/hex constants Double parentheses (( ... )) arithmetic expansion/evaluation construct Double quotes " ... " weak quoting Double-quoting the backslash (\) character 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 else Encrypting files, using openssl esac, keyword terminating case construct Environmental variables -eq , is-equal-to integer comparison test Eratosthenes, Sieve of, algorithm for generating prime numbers Escaped characters, special meanings of Within $' ... ' string expansion Used with Unicode characters /etc/fstab (filesystem mount) file /etc/passwd (user account) file $EUID, Effective user ID eval, Combine and evaluate expression(s), with variable expansion Effects of, Example script Forces reevaluation of arguments And indirect references Risk of using Using eval to convert array elements into a command list Using eval to select among variables Evaluation of octal/hex constants within [[ ... ]] exec command, using in redirection Exercises Exit and Exit status exit command Exit status (exit code, return status of a command) Table, Exit codes with special meanings Anomalous Out of range Pipe exit status Specified by a function return Successful, 0 /usr/include/sysexits.h, system file listing C/C++ standard exit codes 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 * * * factor, decomposes an integer into its prime factors Application: Generating prime numbers false, returns unsuccessful (1) exit status Field, a group of characters that comprises an item of data Files / Archiving 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 File encryption find {} Curly brackets \; Escaped semicolon Filter Using - with file-processing utility as a filter Feeding output of a filter 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 Colon as function name Definition must precede first call to function Exit status Local variables and recursion Passing an array to a function Passing pointers to a function Positional parameters Recursion Redirecting stdin of a function return Multiple return values from a function, example script Returning an array from a function Return range limits, workarounds Shift arguments passed to a function Unusual function names * * * Games and amusements Anagrams Anagrams, again Bingo Number Generator Crossword puzzle solver Crypto-Quotes Dealing a deck of cards Fifteen Puzzle Horse race Knight's Tour Life game Magic Squares Music-playing script Nim Pachinko Perquackey Petals Around the Rose Podcasting Poem Speech generation Towers of Hanoi Graphic version Alternate graphic version getopt, external command for parsing script command-line arguments Emulated in a script getopts, Bash builtin for parsing script command-line arguments $OPTIND / $OPTARG Global variable Globbing, filename expansion Handling filenames correctly Wild cards Will not match dot files Golden Ratio (Phi) -ge , greater-than or equal integer comparison test -gt , greater-than integer comparison test groff, text markup and formatting language Gronsfeld cipher $GROUPS, Groups user belongs to gzip, compression utility * * * 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 Calculating the Golden Ratio Prepending text As the stdin of a loop Using read Limit string ! as a 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 History commands $HOME, user's home directory Homework assignment solver $HOSTNAME, system host name * * * $Id parameter, in rcs (Revision Control System) 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 Inline group, i.e., code block Interactive script, test for I/O redirection Indirect referencing of variables New notation, introduced in version 2 of Bash ( example script) iptables, packet filtering and firewall utility Usage example Example script Iteration * * * Job IDs, table jot, Emit a sequence of integers. Equivalent to seq. Random sequence generation Just another Bash hacker! * * * Keywords error, if missing kill, terminate a process by process ID Options (-l, -9) killall, terminate a process by name killall script in /etc/rc.d/init.d * * * lastpipe shell option -le , less-than or equal integer comparison test let, setting and carrying out arithmetic operations on variables C-style increment and decrement operators 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 Loopback device, mounting a file on a block device 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 Mounting an ISO image -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 man, manual page (lookup) Man page editor (script) mapfile builtin, loads an array with a text file Math commands Meta-meaning Morse code training script 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 network toolkit for TCP and UDP ports -ne, not-equal-to integer comparison test Negation operator, !, reverses the sense of a test netstat, Network statistics Network programming nl, a filter to number lines of text Noclobber, -C option to Bash to prevent overwriting of files NOT logical operator, ! null variable assignment, avoiding * * * -o Logical OR compound comparison test Obfuscation Colon as function name Homework assignment Just another Bash hacker! octal, base-8 numbers od, octal dump $OLDPWD Previous working directory openssl encryption utility 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 Case modification (version 4+ of Bash). 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) Appending directories to $PATH using the += operator. Pathname, a filename that incorporates the complete path of a given file. Parsing pathnames Perl, programming language Combined in the same file with a Bash script Embedded in a Bash script Perquackey-type anagramming game (Quackey script) Petals Around the Rose PID, Process ID, an identification number assigned to a running process. Pipe, | , a device for passing the output of a command to another command or to the shell Avoiding unnecessary commands in a pipe Comments embedded within Exit status of 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 Double-quoting the backslash (\) character eval, risk of using Execute permission lacking for commands within a script Exit status, anomalous Exit status of arithmetic expression not equivalent to an error code 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 let misuse: attempting to set string variables Multiple echo statements in a function whose output is captured 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 (however, this problem can be circumvented) tail -f to grep Preserving whitespace within a variable, unintended consequences suid commands inside a script Undocumented Bash features, danger of Updates to Bash breaking older scripts Uninitialized variables Variable names, inappropriate Variables in a subshell, scope limited Subshell in while-read loop Whitespace, misuse of Pointers and file descriptors and functions and indirect references and variables Portability issues in shell scripting Setting path and umask A test suite script (Bash versus classic Bourne shell) Using whatis Positional parameters $@, as separate words $*, as a single word in functions POSIX, Portable Operating System Interface / UNIX --posix option 1003.2 standard Character classes $PPID, process ID of parent process Precedence, operator Prepending lines at head of a file, script example Prime numbers Generating primes using the factor command Generating primes using the modulo operator Sieve of Eratosthenes, example script printf, formatted print command /proc directory Running processes, files describing Writing to files in /proc, warning Process Child process Parent process Process ID (PID) Process substitution To compare contents of directories To supply stdin of a command Template while-read loop without a subshell Programmable completion (tab expansion) Prompt $PS1, Main prompt, seen at command line $PS2, Secondary prompt Pseudo-code, as problem-solving method $PWD, Current working directory * * * Quackey, a Perquackey-type anagramming game (script) Question mark, ? Character match in an Extended Regular Expression Single-character wild card, in globbing In a C-style Trinary (ternary) 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 sequence generation, using jot Random string, generating Raspberry Pi (single-board computer) Script for preparing a bootable SD card 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 readline library Recursion Demonstration of Factorial Fibonacci sequence Local variables Script calling itself recursively Towers of Hanoi 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 Semaphore 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 set -u, Abort script with error message if attempting to use an undeclared variable. 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 Simulations Brownian motion Galton board Horserace Life, game of PI, approximating by firing cannonballs Pushdown stack 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 Definition Emulating a push-down stack, example script Standard Deviation, example script Startup files, Bash stdin and stdout Stopwatch, example script Strings =~ String match operator Comparison Length ${#string} Manipulation Manipulation, using awk Null string, testing for Protecting strings from expansion and/or reinterpretation, script example Unprotecting strings, script example strchr(), equivalent of strlen(), equivalent of strings command, find printable strings in a binary or data file 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, but ... ... can be accessed outside the subshell? 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 * * * Tab completion Table lookup, script example tail, echo to stdout lines at the (tail) end of a text file tar, archiving utility tee, redirect to a file output of command(s) partway through a pipe Terminals setserial setterm stty tput 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 Text and text file processing Time / Date 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 Associative array more efficient than a numerically-indexed array Capturing the return value of a function, using echo CGI programming, using scripts for Comment blocks Using anonymous here documents Using if-then constructs Comment headers, special purpose C-style syntax , for manipulating variables Double-spacing a text file Filenames prefixed with a dash, removing 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 $PATH, appending to, using the += operator. Prepending lines at head of a file Progress bar template Pseudo-code rcs Redirecting a test to /dev/null to suppress output Running scripts in sequence without user intervention, using run-parts Script as embedded command Script portability Setting path and umask Using whatis Setting script variable to a block of embedded sed or awk code Speeding up script execution by disabling unicode Subshell variable, accessing outside the subshell Testing a variable to see if it contains only digits Testing whether a command exists, using type Tracking script usage while-read loop without a subshell Widgets, invoking from a script $TMOUT, Timeout interval Token, a symbol that may expand to a keyword or command tput, terminal-control command 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 (ternary) operator, C-style, var>10?88:99 in double-parentheses construct in let construct true, returns successful (0) exit status typeset builtin options * * * $UID, User ID number unalias, to remove an alias uname, output system information Unicode, encoding standard for representing letters and symbols Disabling unicode to optimize script Uninitialized variables uniq, filter to remove duplicate lines from a sorted file unset, delete a shell variable until loop until [ condition-is-true ]; do * * * Variables Array operations on Assignment Script example Script example Script example Bash internal variables Block of sed or awk code, setting a variable to C-style increment/decrement/trinary operations Change value of internal script variables using set declare, to modify the properties of variables Deleting a shell variable using unset Environmental Expansion / Substring replacement operators Indirect referencing eval variable1=\$$variable2 Newer notation ${!variable} Integer Integer / string (variables are untyped) Length ${#var} Lvalue Manipulating and expanding Name and value of a variable, distinguishing between Null string, testing for 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 Typing, restricting the properties of a variable Undeclared, error message Uninitialized Unquoted variable, splitting Unsetting Untyped * * * wait, suspend script execution To remedy script hang Weak quoting " ... " while loop while [ condition ]; do C-style syntax Calling a function within test brackets Multiple conditions Omitting test brackets while read construct Avoiding a subshell 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 [:space:], POSIX character class who, information about logged on users w whoami logname Widgets Wild card characters Asterisk * In [list] constructs Question mark ? Will not match dot files Word splitting Definition Resulting from command substitution Wrapper, shell * * * xargs, Filter for grouping arguments Curly brackets Limiting arguments passed Options Processes arguments one at a time Whitespace, handling * * * yes Emulation * * * -z String is null Zombie, a process that has terminated, but not yet been killed by its parent