Bash string manipulation cheatsheet.

README.md

Bash string manipulation cheatsheet

Assignment
Assign value to variable if variable is not already set, value is returned. Combine with a : no-op to discard/ignore return value.	${variable="value"} : ${variable="value"}
Removal
Delete shortest match of needle from front of haystack.	${haystack#needle}
Delete longest match of needle from front of haystack.	${haystack##needle}
Delete shortest match of needle from back of haystack.	${haystack%needle}
Delete longest match of needle from back of haystack.	${haystack%%needle}
Replacement
Replace first match of needle with replacement from haystack.	${haystack/needle/replacement}
Replace all matches of needle with replacement from haystack.	${haystack//needle/replacement}
If needle matches start of haystack replace with replacement.	${haystack/#needle/replacement}
If needle matches end of haystack replace with replacement.	${haystack/%needle/replacement}
Substitution
If variable not set, return value, else variable.	${variable-value}
If variable not set or empty, return value, else variable.	${variable:-value}
If variable set, return value, else null string.	${variable+value}
If variable set and not empty, return value, else null string.	${variable:+value}
Extraction
Extract length characters from variable starting at position.	${variable:position:length}
Return string length of variable.	${#variable}
Escaping
Single quotes inside a single quoted string.	echo 'Don'\''t break my escape!'
Indirection
Return value of variable name held in indirect, else value.	indirect="apple" apple="fruit" ${!indirect-value}

Reference

• https://tldp.org/LDP/abs/html/string-manipulation.html
• https://tldp.org/LDP/abs/html/parameter-substitution.html
• https://tldp.org/LDP/abs/html/ivr.html
• Special characters:
  • *: https://www.tldp.org/LDP/abs/html/special-chars.html#ASTERISKREF
  • ?: https://www.tldp.org/LDP/abs/html/special-chars.html#WILDCARDQU
• https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
• https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion

BASH Lookup ${! and $[!NAME*} can be powerful, too. You can use it to make indirect references or calculate

#!/bin/bash

VAR1="abc"
VAR2="def"
REF="VAR2"
def="1234"

$ echo "  \${!VAR*} = '${!VAR*}'"
  ${!VAR*} = 'VAR1 VAR2'
# Lists all variables that start with VAR

$ echo -e "  \${!REF} = '${!REF}'"
  ${!REF} = 'def'
# Because REF is 'VAR2' and the VALUE of VAR2 is 'def'.

$ echo "  \${!VAR1:-NOT FOUND} = '${!VAR1:-NOT FOUND} '"
  ${!VAR1:-NOT FOUND} = 'NOT FOUND '
# because VAR1 is abc which is not defined. However...

$ CALCREF="VAR${1:-2}"; echo -e "  \${CALCREF} = '${CALCREF}':\n  \${!CALCREF:-missing} = '${!CALCREF:-missing}'"
  ${CALCREF} = 'VAR2':
  ${!CALCREF:-missing} = 'def'
# Use the parameter $1 to lookup VAR${1}. If $1 has no value, use '2' so return the value of VAR2. 
# Or if VAR2 not found, return 'missing'
# NOTE that "${!VAR${INDEX}}" will not work - you get `bad substitution`. You have to create the reference variable 
# in one statement and the lookup in the next.
 
$ echo "  \${!VAR2:-NOT FOUND} = '${!VAR2:-NOT FOUND}'"
  ${!VAR2:-NOT FOUND} = '1234'
# because VAR2 = "def" and the value of def is 1234!

Lastly you can do some nesting

$ MYVALUE="${!AREF:-${DEFAULTVAL:-nothing}}";   echo "  \${MYVALUE} = '${MYVALUE}'"
  ${MYVALUE} = 'nothing'

The above means look up AREF (which does not exist. So instead it looks up DEFAULTVAL which also doesn't exist and thus returns nothing.
If AREF contained a valid reference, or DEFAULTVAL had a value those would have been return.

And here is an important point: by always using :- in every variable reference you can safely run a bash script with the '-u' option. -u is useful for finding uses of unassigned variables. In my experience that often happens due to misspellings which can be a booger to find.

BASH Lookup ${! and $[!NAME*} can be powerful, too. You can use it to make indirect references

thanks @nightskyguy - I've actually been needing this for a script recently. Nicely timed. 👍