Complete Guide to Bash Special Syntax, Variables, and Expansions

Overview

This article provides a comprehensive summary of all special syntax, variables, expansion rules, and control structures used in Bash shell scripting.

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Variable Notation

Dynamic or environment-dependent values are denoted as <<variable>>.
All examples below explicitly show their usage.

Variable	Example	Description
<<var>>	name, path, user	Arbitrary variable name
<<default>>	guest, 8080	Default or fallback value
<<pattern>>	*.txt, */	File or string pattern
<<offset>>	0, 3	Starting index of substring (0-based)
<<length>>	4, 10	Length of substring
<<prefix>>	ENV_, HOST	Prefix pattern for variable names
<<index>>	0, 1, 2	Array or associative key index
<<file>>	/tmp/test.txt	File path

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Step 1: Metacharacters

Symbol	Meaning	Example
*	Matches any string	ls *.txt → lists all .txt files
?	Matches any single character	ls ?.sh → matches a.sh, b.sh
[a-z]	Range match	ls [A-Z]* → files starting with uppercase
{a,b}	Brace expansion	mv {hoge,huga}
{1..3}	Sequence expansion	touch file{1..3}.txt
$	Variable expansion	echo $HOME
! (negation)	Logical NOT	[[ ! -f test.txt ]]
! (history)	History expansion	!10 → reruns command #10
~	Home directory	cd ~
;	Command separator	pwd; ls
&&	Execute if previous succeeded	make && echo OK
`		`
&	Run in background	sleep 5 &
`	`	Pipe output
>	Redirect stdout	echo hi > out.txt
>>	Append output	echo hi >> out.txt
<	Redirect stdin	wc -l < file.txt
()	Subshell	(cd /tmp; ls)
{}	Same shell block	{ echo A; echo B; }
\	Escape character	echo \$HOME
' '	Literal (no expansion)	echo '$USER'
" "	Expand variables	echo "$USER"

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Step 2: Special Variables

Variable	Description	Example
$0	Script name	echo $0 → script.sh
$1–$9	Positional args	echo $1
$@	All args as array	for a in "$@"; do echo $a; done
$#	Arg count	echo $#
$?	Last exit status	ls /nope; echo $?
$$	Current PID	echo $$
$!	Last BG job PID	sleep 10 & echo $!
$-	Current shell options	echo $-
$_	Last arg of previous cmd	echo $_
$PPID	Parent PID	echo $PPID
$RANDOM	Random number	echo $RANDOM
$SECONDS	Seconds since shell start	echo $SECONDS
$FUNCNAME	Function name	f(){ echo $FUNCNAME; }; f
$PIPESTATUS[@]	Exit status of pipe cmds	`ls

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Step 3: Parameter Expansion

Syntax	Description	Example
${<<var>>}	Variable expansion	name=user; echo ${name}
${<<var>>:-<<default>>}	Use default if unset	echo ${user:-guest}
${<<var>>:=<<default>>}	Assign default if unset	echo ${port:=8080}
${<<var>>:+alt}	Use alt if set	x=1; echo ${x:+OK}
${<<var>>:?msg}	Error if unset	echo ${config:?missing}
${#<<var>>}	String length	name=user; echo ${#name}
${<<var>>%<<pattern>>}	Remove shortest suffix	path=/a/b/c; echo ${path%/*}
${<<var>>%%<<pattern>>}	Remove longest suffix	echo ${path%%/*}
${<<var>>#<<pattern>>}	Remove shortest prefix	echo ${path#*/}
${<<var>>##<<pattern>>}	Remove longest prefix	echo ${path##*/}
${<<var>>/<<pattern>>/<<repl>>}	Replace first match	echo ${msg/foo/bar}
${<<var>>//<<pattern>>/<<repl>>}	Replace all	echo ${msg// /_}
${<<var>>:<<offset>>}	Substring from offset	s=abcdef; echo ${s:2}
${<<var>>:<<offset>>:<<length>>}	Substring with length	echo ${s:1:3}
${!<<prefix>>*}	Vars with prefix	HOST1=x; HOST2=y; echo ${!HOST*}
${!<<var>>}	Indirect reference	ref=NAME; NAME=user; echo ${!ref}
${<<var>>,} / ${<<var>>^^}	Case conversion	n=abc; echo ${n^^}
${<<var>>@Q}	Quoted form	x='abc'; echo ${x@Q}

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Step 4: Arrays and Associative Arrays

arr=(a b c)
echo ${arr[<<index>>]}     # Access element
echo ${#arr[@]}            # Count
for i in "${arr[@]}"; do echo $i; done

declare -A map
map[<<index>>]=100
map[name]=user
echo ${map[<<index>>]}     # Value
echo ${!map[@]}            # Keys

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Step 5: Arithmetic Expansion

x=5; y=3
echo $((x + y))   # 8
((x *= 2))
echo $x           # 10

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Step 6: Conditions and Comparisons

if [ "$USER" = "root" ]; then echo root; fi
if [ "$x" -lt 10 ]; then echo small; fi
if [ -f /etc/passwd ]; then echo exists; fi
s="hello123"
[[ $s =~ [0-9]+ ]] && echo "has number"

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Step 7: Command Substitution and Subshells

echo "Today: $(date +%Y-%m-%d)"
(cd /tmp; ls)

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Step 8: Redirection and File Descriptors

echo "Hello" > <<file>>
echo "Append" >> <<file>>
wc -l < <<file>>
ls /nope 2> err.log
echo "Error" >&2
exec 3> custom.log
echo "via fd3" >&3
exec 3>&-

File Descriptor Reference

FD	Name	Description	Example
0	stdin	Input	< file.txt
1	stdout	Normal output	echo test > out.txt
2	stderr	Error output	ls /nope 2> err.log
3+	Custom	User-defined streams	exec 3> log.txt

FD3 is a custom descriptor:

exec 3> process.log
echo "start" >&3
exec 3>&-

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Step 9: Traps and Error Handling

set -e
set -u
set -x
set -o pipefail

trap 'echo done' EXIT
trap 'echo error' ERR

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Step 10: Job Control

sleep 10 &
jobs
fg %1
bg %1
disown %1
kill %1

Job Reference (%1)

Example:

sleep 60 &
jobs

Output:

[1]+ Running sleep 60 &

Symbol	Meaning
%1	Job ID 1
%+	Current job
%-	Previous job

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Step 11: Reserved Commands

:        # No-op
true
false
source ~/.bashrc
eval "echo executed"
{ echo A; echo B; }

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Summary

This guide comprehensively covered Bash’s special syntax, expansions, variables, metacharacters, and control structures.
Understanding these concepts enables you to write safe, reusable, and automation-ready shell scripts.