Variables

name="Alice"               # assign (no spaces around =)
age=30                     # numbers are strings too
readonly PI=3.14           # read-only variable
declare -r CONST=42        # same as readonly
declare -i num=10          # integer variable (arithmetic forced)
declare -l lower="HELLO"   # auto-lowercase: lower="hello"
declare -u upper="hello"   # auto-uppercase: upper="HELLO"
declare -x PATH            # mark for export (same as export)
declare -p varname         # print declaration of variable
unset varname              # delete variable

echo $name                 # basic expansion
echo ${name}               # braces — required before letters/digits
echo "${name}"             # always quote to prevent word splitting
echo "${name}suffix"       # concatenate suffix
echo "Value: ${age}"       # embed in string

# Undefined vs empty
echo ${undef}              # empty string (no error by default)
echo ${undef:-"default"}   # use default if unset or empty

echo ${name:-"stranger"}   # "stranger" if name is unset/empty
: ${config:="/etc/app.conf"}  # set default silently
echo ${required:?"required is not set"}  # abort if missing
echo ${debug:+"--verbose"}    # only add flag if debug is set

str="Hello, World"

echo ${#str}               # length: 12
echo ${str:7}              # from index 7: "World"
echo ${str:7:5}            # from index 7, length 5: "World"
echo ${str: -5}            # last 5 characters (space before -)
echo ${str: -5:3}          # 3 chars starting 5 from end: "Wor"

file="path/to/file.tar.gz"

echo ${file#*/}            # remove shortest prefix matching */  → "to/file.tar.gz"
echo ${file##*/}           # remove longest  prefix matching */  → "file.tar.gz"
echo ${file%.*}            # remove shortest suffix matching .*  → "path/to/file.tar"
echo ${file%%.*}           # remove longest  suffix matching .*  → "path/to/file"

# Common idiom: extract filename and extension
base="${file##*/}"         # "file.tar.gz"
noext="${base%%.*}"        # "file"
ext="${file##*.}"          # "gz"
dir="${file%/*}"           # "path/to"

str="foo bar foo baz"

echo ${str/foo/FOO}        # replace first occurrence: "FOO bar foo baz"
echo ${str//foo/FOO}       # replace all occurrences:  "FOO bar FOO baz"
echo ${str/#foo/FOO}       # replace if at start:      "FOO bar foo baz"
echo ${str/%baz/BAZ}       # replace if at end:        "foo bar foo BAZ"

# Delete by replacing with empty string
echo ${str/foo/}           # " bar foo baz"
echo ${str//foo/}          # " bar  baz"

str="Hello World"

echo ${str,,}              # all lowercase: "hello world"
echo ${str^^}              # all uppercase: "HELLO WORLD"
echo ${str,}               # first char lowercase: "hello World"
echo ${str^}               # first char uppercase: "Hello World"

# With a pattern
echo ${str,,[ ]}           # lowercase only space chars (rarely useful)

key="name"
name="Alice"
echo ${!key}               # → "Alice" (value of variable named by $key)

# List variables matching a prefix
echo ${!BASH*}             # BASH BASH_VERSION BASH_SOURCE …
declare -p ${!BASH*}       # print declarations for all BASH* vars

declare -i n=5
n+=3                       # n is now 8 (integer context)

(( x = 10 + 5 ))           # arithmetic evaluation
(( x++ ))                  # post-increment
(( x-- ))                  # post-decrement
(( x *= 2 ))               # multiply in-place

x=$(( 10 / 3 ))            # integer division → 3
x=$(( 10 % 3 ))            # modulo → 1
x=$(( 2 ** 8 ))            # exponent → 256

export VAR="value"         # export to child processes
export -n VAR              # un-export (keep variable, remove from env)
printenv                   # print all environment variables
printenv PATH              # print a specific env var
env                        # print env (also used to run with modified env)
env VAR=val cmd            # run cmd with extra env var, without exporting

# Common env vars
HOME, PATH, USER, SHELL, PWD, OLDPWD, HOSTNAME
LANG, LC_ALL, TZ
EDITOR, VISUAL, PAGER
TMPDIR, XDG_DATA_HOME

arr=(one two three)        # indexed array
declare -A map             # associative array
map[key]="value"

echo ${arr[0]}             # first element
echo ${#arr[@]}            # array length

# Variables are global to the function unless declared local
outer="global"

myfunc() {
  local inner="local"      # only visible inside myfunc
  echo $outer              # can read globals
  outer="modified"         # modifies the global
}

myfunc
echo $inner                # empty — gone after function returns
echo $outer                # "modified"

# Subshells get a COPY — changes don't propagate up
(export TEMP="subshell"; echo $TEMP)
echo $TEMP                 # empty in parent

declare -n ref=varname     # ref is an alias for varname
varname="hello"
echo $ref                  # "hello"
ref="world"
echo $varname              # "world"

# Useful for passing variable names to functions
set_value() {
  declare -n _target=$1
  _target="$2"
}
set_value myvar "42"
echo $myvar                # "42"