web/content/drafts/bash.md

title, description, summary, draft, tags, author, showToc

title	description	summary	draft	tags	author	showToc
Shell Scripting [BASH]			true		TrudeEH	true

Run a Bash Script

Example script.sh:

#! /bin/bash

echo "Bash Script"

Execute the script:

chmod u+x script.sh  # Give the script execution permission (to its owner)
./script.sh          # Run the script

Documentation

You will learn more about these commands later

whatis command     # Short description of the command
tldr command       # Examples of how to use the command
man command        # Full documentation for the command
man bash           # Bash documentation; Includes most built-in commands

Strings

• "" Defines a string which supports substitutions ($ and \, for example).
• '' Defines a string, but preserves its actual value (substitutions are treated as regular characters).
• ANSI Escape Sequences apply when using "".

Comments

# comment

Commands

A shell command consists of the command itself, followed by its arguments.

command "arg1" "arg2"

If the first word of a command is a reserved word, bash handles the command, otherwise, it searches for an executable on the system's $PATH, a list of directories where a binary could be located.

Reserved Words

if	then	elif	else	fi	time
for	in	until	while	do	done
case	esac	coproc	select	function
{	}	[]()	!

List of Commands

• command1 ; command2 Execute command2 after command1, sequentially.
• command1 & Execute command1 asynchronously in a subshell.
• command1 && command2 AND: Only execute command2 if command1 returns 0 (success).
• command1 || command2 OR: Only execute command2 if command1 returns a non-zero exit value (failure).

Loops

until

until test-commands; do 
	...
done

Execute the code in ... for as long as test-commands return non-zero.

while

while test-commands; do 
	...
done

Execute ... for as long as test-commands return 0.

for

Expand words and execute ... for each member in the resultant list, with name bound to the current member.

Iterate through List

for item in list; do
	echo $item
done

C-like Loop

for (( i=1; i<=10; i++ )); do  
 echo "Loop number:" $i  
done

Infinite Loop

for (( ; ; )); do
 echo "Press Ctrl+C to stop..."
done

Conditional Constructs

if

if test-commands; then
  ...
elif more-test-commands; then
  ...
else
	...
fi

Execute the first ... if test-commands returns 0, and evaluate the next condition otherwise. This process repeats until else is found, or one of the tests evaluates to a 0.
Once any ... executes, the remaining if construct is skipped.

case

case word in
  p1 | p2 | p3) ... ;;
  p4 | p5 )
	  ... 
	;;
  *) ... ;;
esac

Execute the ... corresponding to the first pattern (pX) that matches the word.
The | operator separates multiple patterns, and each clause can end with ;;, ;& or ;;&. It's common to use * as the default case, since the pattern will always match.

Using ;& instead of ;; would cause the next ... to be executed as well, and ;;& would test the next clause, instead of immediately exiting.

select

PS3="Enter a number: "

select option in entry1 entry2 entry3 entry4 entry5
do
    echo "Selected character: $option"
    echo "Selected number: $REPLY"
done

The select command generates a menu, displaying each entryX in a list. The user is then prompted to select an option (in this case, a number from 1-5), and the resultant $option and $REPLY are then provided as variables.

Output:

1) entry1
2) entry2
3) entry3
4) entry4
5) entry5
Enter a number:

((...))

The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic TODO link to shell arithmetic).

[...](...)

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions.

Combine Expressions

• ( expression ) Returns the value of expression. (Can be used to override precedence).
• ! expression NOT an expression. (true if expression is false).
• exp1 && exp2 AND - true if both expressions are true.
• exp1 || exp2 OR - true if either expressions are true.

Grouping Commands

Bash allows for commands to be grouped as a single unit. That way, if the group is redirected, the output of every command in the list is passed to a single stream.

• ( list ) Create a subshell (variables created inside it can't be accessed outside).
• { list; } No subshell is created.

Functions

fname() {
	...
}

function fname {
	...
}

A function can store a block of code (compound command), so it can be reused by calling its name:

fname

Any variables defined inside the function

Arguments

fname() {
	echo $1 $2
}

fname "a" "b"

Scope

var1='A'
var2='B'

fname () {
  local var1='C'
  var2='D'
  echo "var1: $var1, var2: $var2" # C, D
}

echo "$var1, var2: $var2"       # A, B
fname                           # C, D
echo "var1: $var1, var2: $var2" # A, D

Defining a variable inside the function overwrites the global scope. To prevent this, use the local keyword.

return

fname() {
	return 1;
}

fname
echo $? # 1

Use the return command to exit the function and return a value.

Variables (Parameters)

name="Trude"
echo $name # Trude

name+=" EH"
echo $name # Trude EH
echo ${name}

Variables can be of any type, and grow to any needed size.

Special Variables

• $* Expands to every positional parameter: $1$2$3.
• $@ Expands to every positional parameter, separated by spaces: "$1" "$2" "$3".
• $# Number of positional arguments.
• $? Exit status of last command / pipeline.
• $- Current option flags set by set, or by the shell itself.
• $$ Process ID of the shell. In a subshell, it expands to the process ID of the parent shell.
• $! Process ID of the latest job placed into the background.
• $0 Name of the shell or script.

Shell Expansions

Brace Expansion

echo a{d,c,b}e # ade ace abe

Tilde Expansion

• ~ = $HOME
• ~+ = $PWD
• ~- = $OLDPWD

Shell Parameter Expansion

• ${var} Braces are required if the variable is positional and over one digit, or if it is followed by a character that is not part of its name.
• ${!var} Access the value of var, and checks if it is the name of another variable. If so, expands that variable. (Pointer)
• ${var:-word} If var is null or unset, use the word value instead.
• ${var:=word} If var is null or unset, set its value to word. (Good for default values)
• ${var:?word} If var is null or unset, word is written to the standard error, and the shell, if not interactive, exits.
• ${var:+word} Use the word value if var is not unset or null.
• ${var:offset:length} Offset var and return the desired length. (Cut strings)
• ${@:offset:length} Same as before, but with positional arguments.
• ${!word*} Access the value of all variables whose names begin with word. Use @ instead of * to separate the result to separate words.
• ${!var[*]} Expand all indices (keys) in an array. (Not values;@ also works).
• ${#var} Length of a variable's value.
• ${var#word} If word is found in var, return the remaining text after it appears for the first time. Otherwise, print the entire variable.
• ${var##word} If word is found in var, return the remaining text after it appears for the last time. Otherwise, print the entire variable.
• ${var%word} If word is found in var, return the remaining text before it appears for the first time. Otherwise, print the entire variable.
• ${var%%word} If word is found in var, return the remaining text before it appears for the last time. Otherwise, print the entire variable.
• ${var/pattern/word} Read var's value, then replace the first occurrence of pattern with word.
• ${var//pattern/word} Read var's value, then replace all occurrences of pattern with word.
• ${var/#pattern/word} Replace pattern with word, only if pattern is at the beginning of var.
• ${var/%pattern/word} Replace pattern with word, only if pattern is at the end of var.
• ${var^word} Convert the first character that matches word to uppercase.
• ${var^^word} Convert all characters that match word to uppercase.
• ${var,word} Convert the first character that matches word to lowercase.
• ${var,,word} Convert all characters that match word to lowercase.
• ${var@X} Replace X with one of the following operators:
  • U Uppercase.
  • u Uppercase first character.
  • L Lowercase.
  • Q Quote in a format that can be reused as input.
  • E Expand escape sequences.
  • P Expand the value as if it was a prompt string.
  • A Generate the variable declaration command for var.
  • K Output key-value pairs, suitable for generating the source array.
  • a Output the flags corresponding to var's attributes.
  • k Same as K, but separates keys and values using spaces, making it easier to loop through them.

Command Substitution

echo "$(command)"
echo "`command`"

Execute a command and substitute itself with the command's result.

Arithmetic Expansion

echo "$(( expression ))"

Performs an arithmetic expression and substitutes itself with the result.

Process Substitution

cat <(command) >(command)

cat <(cat /etc/hostname) >(gzip > output.gz)

The <() substitution executes the command asynchronously, stores the result in a temporary file (in /tmp/), and substitutes itself with the temporary file's path.

The >() substitution is also executed asynchronously, and creates a temporary file with the command's output, then passes it to the next program's input.

Neither cat nor gzip are bash commands ('builtins'), but external programs.

Pattern Matching

• * Matches any string.
• ? Matches any single character.
• [...] Matches any of the enclosed characters, and supports ranges. ([a-z])
• ?(pattern-list) Matches zero or one of the given patterns.
• *(pattern-list) Matches zero or more occurrences of the given patterns.
• +(pattern-list) Matches one or more occurrences of the given patterns.
• @(pattern-list) Matches one of the given patterns.
• !(pattern-list) Matches anything except the given patterns.

Redirections

• command > dest Redirect the output of a command to a destination: A file, device, command, list, etc.
• command >> dest Append to the destination instead of overwriting.
• command < file Provide a file as input.
• command 2> dest Redirect the error output of a command.
• command &> dest or command >& dest Redirect both input and output.
• command 2>&1 Redirect Standard Error to Standard Output.
• command > /dev/null Discard the output.

It is also possible to provide strings as input directly:

cat <<< "String!"

cat << EOF
Multi-line
string
EOF

Shell Builtin Commands

This section is an introduction to every command available in bash.
To learn more about some command, run help command.

Bourne Shell Commands

• : arguments Do nothing beyond expanding arguments and performing redirections.
• . file Read and execute commands from file.
• break Exit from a loop. (Adding a number as an argument selects which enclosing loop to exit from.)
• cd directory Change the current directory to directory. If directory is...
  • -> $HOME
  • .. -> Parent directory
  • - -> $OLDPWD
• continue Skip to the next iteration of a loop. (Also supports loop number as an argument.)
• eval arguments Arguments are concatenated together and executed as a single command.
• exec command Replaces the shell without creating a new process, and executes the command.
• exit Exit the shell. (Optionally, add a return status.)
• export name Make a variable available to child processes. export can also handle functions and assignment.
• getopts "optstring" var Parse positional parameters (script arguments).
• hash List the full paths to executables cached by the shell (to accelerate future queries)
• pwd Print the full pathname of the current directory.
• return value Exit a function and return value. If no value is provided, returns the last exit status of the last command executed in the function.
• shift value Shift the positional parameters to the left by value. If no value is provided, shift by 1.
• test or [] Evaluate a condition and return 0 if true, or 1 if false:
• times Print the user and system times used by the shell and its children.
• trap "command" signal Execute a command, if one of the following signals are received: (The SIG prefix is optional)
  • SIGINT Interrupt signal (usually generated by pressing Ctrl+C).
  • SIGTERM Termination signal (Request to terminate).
  • SIGKILL Kill signal (Forceful termination; Cannot be trapped).
  • SIGQUIT Quit signal.
  • SIGUSR1 and SIGUSR2 User-defined signals.
  • SIGCHLD Child process terminated.
  • 0 or EXIT Executes when the shell exits.
  • DEBUG Executes before every simple command.
  • RETURN Executes each time a shell function or a script executed with the . or source builtins finishes executing.
  • ERR Executes whenever any pipeline, list, or command returns a non-zero exit status.
• umask Defines which permissions should be removed from newly created files.
• unset Remove a variable or function name. (Use -f to remove the actual function definition)

Bash Commands

• alias Prints the list of aliases or defines new ones (with alias name=value).
• bind Displays or sets key and function bindings for Readline.
• builtin Executes a shell builtin command, bypassing any function with the same name.
• caller Returns context information of the current active subroutine call.
• command Executes a command, ignoring any shell function with the same name. (If the -v option is supplied, prints a description of command.)
• declare Declares variables with various attributes (typeset is a synonym).
• echo Outputs its arguments to the standard output.
• enable Enables or disables shell builtin commands.
• help Provides helpful information about shell builtins.
• let Evaluates arithmetic expressions.
• local Creates a variable with a scope local to the function.
• logout Exits a login shell with an optional exit status.
• mapfile Reads lines from input into an indexed array (readarray is a synonym).
• printf Prints formatted output to the standard output.
• read Reads a line of input and splits it into words based on IFS.
• readarray Reads lines from input into an indexed array (synonym for mapfile).
• set Set or unset values of shell options and positional parameters
• shopt Change additional shell optional behavior.
• source Executes commands from a file in the current shell environment.
• type Describes how the shell interprets a given command name.
• ulimit Controls resource limits for processes created by the shell.
• unalias Removes defined aliases, with an option to remove all.

Shell Variables

Bourne Shell Variables

• CDPATH Search path directories for the cd command.
• HOME Current user's home directory, default for cd.
• IFS Characters used to separate fields during word splitting.
• MAIL File/directory Bash checks for mail if MAILPATH is unset.
• MAILPATH Colon-separated list of files/directories to check for mail.
• OPTARG Value of the last option argument processed by getopts.
• OPTIND Index of the last option argument processed by getopts.
• PATH Colon-separated list of directories searched for commands.
• PS1 Primary prompt string displayed interactively.
• PS2 Secondary prompt string for continued commands.

Bash Variables

• _ Pathname of invoked shell/script, or last argument of previous command.
• BASH Full pathname used to execute the current Bash instance.
• BASHOPTS Colon-separated list of enabled shell options (via shopt).
• BASHPID Process ID of the current Bash process (can differ from $$).
• BASH_ALIASES Associative array mapping alias names to values.
• BASH_ARGC Array of parameter counts for each function/script in the call stack (requires extdebug).
• BASH_ARGV Array of all parameters in the current call stack (requires extdebug).
• BASH_ARGV0 Name of the shell or script ($0); assigning to it also sets $0.
• BASH_CMDS Associative array mapping hashed commands to their full paths.
• BASH_COMMAND Command currently being executed or about to be executed.
• BASH_COMPAT Sets the shell's compatibility level (e.g., 4.2 or 42).
• BASH_ENV Path to a script read before executing a non-interactive script.
• BASH_EXECUTION_STRING Command argument passed via the -c invocation option.
• BASH_LINENO Array of line numbers where functions in FUNCNAME were invoked.
• BASH_LOADABLES_PATH Search path for dynamically loadable builtins (enable -f).
• BASH_REMATCH Array holding results from regex matching (=~) in [...](...).
• BASH_SOURCE Array of source filenames where functions in FUNCNAME are defined.
• BASH_SUBSHELL Incremented for each subshell level; initial value is 0.
• BASH_VERSINFO Readonly array detailing the Bash version components.
• BASH_VERSION String containing the full version number of Bash.
• BASH_XTRACEFD File descriptor where trace output (set -x) is sent.
• CHILD_MAX Maximum number of exited child statuses the shell remembers.
• COLUMNS Terminal width, used by select and set on SIGWINCH if checkwinsize is on.
• COMP_CWORD Index in COMP_WORDS array of the word containing the cursor.
• COMP_LINE The current command line being completed.
• COMP_POINT Index of the cursor position within the current command line (COMP_LINE).
• COMP_TYPE Integer indicating the type of completion being attempted (TAB, ?, !, @, %).
• COMP_KEY The key (or sequence) that invoked the completion function.
• COMP_WORDBREAKS Characters Readline uses to delimit words for completion.
• COMP_WORDS Array of individual words in the current command line for completion.
• COMPREPLY Array where completion functions place possible matches.
• COPROC Array holding file descriptors for an unnamed coprocess.
• DIRSTACK Array containing the current directory stack (dirs).
• EMACS If set to t at startup, indicates running in Emacs, possibly disabling line editing.
• ENV Script executed when an interactive shell starts in POSIX mode.
• EPOCHREALTIME Seconds since the Unix Epoch, with microsecond precision (float).
• EPOCHSECONDS Seconds since the Unix Epoch (integer).
• EUID Numeric effective user ID of the current user (readonly).
• EXECIGNORE Patterns of filenames to ignore during PATH command lookup.
• FCEDIT Default editor used by fc -e.
• FIGNORE Suffixes to ignore during filename completion.
• FUNCNAME Array of function names currently on the execution call stack.
• FUNCNEST Maximum function nesting level; exceeding it aborts the command.
• GLOBIGNORE Patterns of filenames to ignore during pathname expansion (globbing).
• GROUPS Array containing the group IDs the current user belongs to.
• histchars Characters controlling history expansion (!), quick substitution (^), and comments (#).
• HISTCMD History number (index) of the current command in the history list.
• HISTCONTROL Controls how commands are saved (ignorespace, ignoredups, ignoreboth, erasedups).
• HISTFILE File where command history is saved (default ~/.bash_history).
• HISTFILESIZE Maximum number of lines stored in the history file.
• HISTIGNORE Patterns matching command lines that should not be saved in history.
• HISTSIZE Maximum number of commands remembered in the current shell's history list.
• HISTTIMEFORMAT strftime format for displaying timestamps with history entries.
• HOSTFILE File (like /etc/hosts) used for hostname completion.
• HOSTNAME Name of the current host.
• HOSTTYPE String describing the system architecture (e.g., x86_64-linux-gnu).
• IGNOREEOF Number of consecutive EOF characters (Ctrl+D) needed to exit an interactive shell.
• INPUTRC Readline initialization file (overrides ~/.inputrc).
• INSIDE_EMACS Similar to EMACS, indicates running within an Emacs shell buffer.
• LANG Default locale category setting.
• LC_ALL Overrides LANG and all other LC_ locale settings.
• LC_COLLATE Locale for string sorting and pattern matching ranges/classes.
• LC_CTYPE Locale for character interpretation and classification in patterns.
• LC_MESSAGES Locale for translating double-quoted strings starting with $.
• LC_NUMERIC Locale category for number formatting.
• LC_TIME Locale category for date and time formatting.
• LINENO Current line number within the script or function being executed.
• LINES Terminal height, used by select and set on SIGWINCH if checkwinsize is on.
• MACHTYPE System type string in cpu-company-system format (similar to HOSTTYPE).
• MAILCHECK How often (seconds) Bash checks for mail (default 60).
• MAPFILE Default array variable used by mapfile if no name is given.
• OLDPWD Previous working directory, set by cd.
• OPTERR If set to 1, getopts displays error messages.
• OSTYPE String describing the operating system (e.g., linux-gnu).
• PIPESTATUS Array containing exit statuses of commands in the last foreground pipeline.
• POSIXLY_CORRECT Enables POSIX mode if set at startup or during execution.
• PPID Process ID of the shell's parent process (readonly).
• PROMPT_COMMAND Command(s) executed just before displaying the primary prompt (PS1).
• PROMPT_DIRTRIM Number of trailing directory components shown in \w and \W prompts.
• PS0 Prompt string displayed after reading a command but before executing it (interactive shells).
• PS3 Prompt string used by the select command.
• PS4 Prompt string prefixed to commands echoed during execution tracing (set -x).
• PWD Current working directory, set by cd.
• RANDOM Returns a random integer between 0 and 32767; assigning seeds the generator.
• READLINE_ARGUMENT Numeric argument passed to a Readline command bound via bind -x.
• READLINE_LINE Contents of the Readline line buffer (for bind -x).
• READLINE_MARK Position of the mark (saved point) in the Readline buffer (for bind -x).
• READLINE_POINT Position of the insertion point in the Readline buffer (for bind -x).
• REPLY Default variable used by read if no name is given.
• SECONDS Number of seconds since the shell was started; assigning resets the counter.
• SHELL Full pathname to the current user's login shell.
• SHLVL Shell nesting level, incremented for each new Bash instance started.
• SRANDOM Returns a 32-bit pseudo-random number (cannot be seeded).
• TIMEFORMAT Format string controlling output of the time keyword.
• TMOUT Default timeout (seconds) for read and select; idle timeout for interactive shells.
• TMPDIR Directory used for creating temporary files.
• UID Numeric real user ID of the current user (readonly).

Parse Arguments

while getopts "ab:c" opt; do
  case "$opt" in
    a)
      echo "Option -a was specified"
      ;;
    b)
      value="$OPTARG"
      echo "Option -b was specified with value: $value"
      ;;
    c)
      echo "Option -c was specified"
      ;;
    \?)
      echo "Invalid option: -$OPTIND" >&2
      exit 1
      ;;
    :)
      echo "Option -$OPTIND requires an argument." >&2
      exit 1
      ;;
  esac
done

shift $((OPTIND - 1))

echo "Remaining arguments: $@"

To enable silent error reporting, add : at the beginning of the getopts' optstring.

Bash Startup Files

Shell Type	Files
Login	/etc/profile, ~/.bash_profile, ~/.bash_login, ~/.profile
Interactive	~/.bashrc
Non-Interactive	$BASH_ENV
sh	/etc/profile, ~/.profile

Detect if Running in Interactive Shell

case "$-" in
*i*)	echo This shell is interactive ;;
*)	echo This shell is not interactive ;;
esac

Conditional Expressions

Conditional Expressions are used by the [bash](.md), [] and test commands.

• -a file True if file exists.
• -b file True if file exists and is a block special file.
• -c file True if file exists and is a character special file.
• -d file True if file exists and is a directory.
• -e file True if file exists. (Equivalent to -a in this context)
• -f file True if file exists and is a regular file.
• -g file True if file exists and its set-group-id bit is set.
• -h file True if file exists and is a symbolic link.
• -k file True if file exists and its "sticky" bit is set.
• -p file True if file exists and is a named pipe (FIFO).
• -r file True if file exists and is readable.
• -s file True if file exists and has a size greater than zero.
• -t fd True if file descriptor fd is open and refers to a terminal.
• -u file True if file exists and its set-user-id bit is set.
• -w file True if file exists and is writable.
• -x file True if file exists and is executable.
• -G file True if file exists and is owned by the effective group id.
• -L file True if file exists and is a symbolic link.
• -N file True if file exists and has been modified since it was last read.
• -O file True if file exists and is owned by the effective user id.
• -S file True if file exists and is a socket.
• file1 -ef file2 True if file1 and file2 refer to the same device and inode numbers.
• file1 -nt file2 True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
• file1 -ot file2 True if file1 is older than file2, or if file2 exists and file1 does not.
• -o optname True if the shell option optname is enabled.
• -v varname True if the shell variable varname is set (has been assigned a value).
• -R varname True if the shell variable varname is set and is a name reference.
• -z string True if the length of string is zero.
• -n string or string True if the length of string is non-zero.
• string1 == string2 or string1 = string2 True if the strings are equal. Use = for the test command.
• string1 != string2 True if the strings are not equal.
• string1 < string2 True if string1 sorts before string2 lexicographically.
• string1 > string2 True if string1 sorts after string2 lexicographically.
• arg1 OP arg2
  • OP is one of '-eq', '-ne', '-lt', '-le', '-gt', or '-ge'. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively.

Shell Arithmetic

Arithmetic is performed using (()), let and declare -i.

• id++ id-- Variable post-increment and post-decrement
• ++id --id Variable pre-increment and pre-decrement
• - + Unary minus and plus
• ! ~ Logical and bitwise negation
• ** Exponentiation
• * / % Multiplication, division, remainder
• + - Addition, subtraction
• << >> Left and right bitwise shifts
• <= >= < > Comparison
• == != Equality and inequality
• & Bitwise AND
• ^ Bitwise exclusive OR
• | Bitwise OR
• && Logical AND
• || Logical OR
• expr ? expr : expr Conditional operator
• = *= /= %= += -= <<= >>= &= ^= |= Assignment
• expr1 , expr2 Comma

Arrays

Indexed Arrays

declare -a array1

array2=(1 2 "four" 8 16) # Multiple types are supported

Associative Arrays

declare -A name
name=(value1 value2 ...)

6.7 Arrays
https://www.gnu.org/software/bash/manual/bash.html#Bash-Features