bash-substitution.md

Bash parameter expansion

# More info
info bash Basic 'Shell Expansions' 'Shell Parameter'

• Bash parameter expansion
• Patten matching

Defaults / empty tests

syntax	if $var	result
${var-default}	undefined	"default"
${var:-default}	empty	"default"
${var=default}	undefined	var="default"
${var:=default}	empty	var="default"
${var?"error"}	undefined	error
${var:?"error"}	empty	error
${var+default}	defined	"default"
${var:+default}	empty	"default"

# get default if empty
echo "${var:-default}"  # 'default', var=''

# set default if empty
echo "${var:=default}"  # 'default', var='default'

# error if empty
echo "${var:?'not set'}"  # bash: var: not set

# set default if empty
echo "${var:+default}"  # '', var=''
var=nonempty
echo "${var:+default}"  # 'default', var='nonempty'

Substring expansion

syntax	from	result
${var#pattern}	front	remove shortest 'pattern'
${var##pattern}	front	remove longest 'pattern'
${var%pattern}	back	remove shortest 'pattern'
${var%%pattern}	back	remove longest 'pattern'
${var/pat/sub}	any	replace first 'pat' with 'sub'
${var//pat/sub}	any	replace all 'pat' with 'sub'
${var:offset}	any	strip offset chars from start
${var:offset:len}	any	take len chars from offset

# remove suffix
file="index.html"
echo "${file%.html}.txt"  # index.txt

path="/path/to/file.ext"
echo "${path%/*}"  # /path/to

# remove prefix
file="index.html"
echo "backup${file#index}"  # backup.html

path="/path/to/file.ext"
echo "${path##*/}"  # file.ext

# replace substring
text="foo bar foo"
echo "${text/foo/baz}"  # baz bar foo

text="foo bar foo"
echo "${text//foo/baz}"  # baz bar baz

# replace everything with placeholder
text="secret"
echo "${text//?/*}"  # ******

# index
text=1234567890
echo "${text:4}"  # 567890
echo "${text: -4}"  # 7890

echo "${text:4:2}"  # 56
echo "${text:2:-2}"  # 345678
echo "${text: -2:2}"  # 90
echo "${text: -2:-2}"  #

echo "${text::4}"   # 1234
echo "${text::-4}"  # 123456

list defined vars

echo "${!SHELL*}"  # SHELL SHELLOPTS

upper/lower

syntax		result
${var,}	lower	first char
${var,pattern}	lower	first char if it matches pattern
${var,,}	lower	all chars
${var,,pattern}	lower	all chars that matches pattern
${var^}	upper	first char
${var^pattern}	upper	first char if it matches pattern
${var^^}	upper	all chars
${var^^pattern}	upper	all chars that matches pattern

text='hello world'
echo ${text^}   # Hello world
echo ${text^^}  # HELLO WORLD
echo ${text^^l} # heLLo worLd