Parameter expansion

One core functionality of Bash is to manage parameters. A parameter is an entity that stores values and is referenced by a name, a number or a special symbol.

• parameters referenced by a name are called variables (this also applies to arrays)
• parameters referenced by a number are called positional parameters and reflect the arguments given to a shell
• parameters referenced by a special symbol are auto-set parameters that have different special meanings and uses

Parameter expansion is the procedure to get the value from the referenced entity, like expanding a variable to print its value. On expansion time you can do very nasty things with the parameter or its value. These things are described here.

If you saw some parameter expansion syntax somewhere, and need to check what it can be, try the overview section below!

Arrays can be special cases for parameter expansion, every applicable description mentions arrays below. Please also see the article about arrays.

For a more technical view what a parameter is and which types exist, see the dictionary entry for "parameter".

Looking for a specific syntax you saw, without knowing the name?

• Simple usage
  • $PARAMETER
  • ${PARAMETER}
• Indirection
  • ${!PARAMETER}
• Case modification
  • ${PARAMETER^}
  • ${PARAMETER^^}
  • ${PARAMETER,}
  • ${PARAMETER,,}
  • ${PARAMETER~}
  • ${PARAMETER~~}
• Variable name expansion
  • ${!PREFIX*}
  • ${!PREFIX@}
• Substring removal (also for filename manipulation!)
  • ${PARAMETER#PATTERN}
  • ${PARAMETER##PATTERN}
  • ${PARAMETER%PATTERN}
  • ${PARAMETER%%PATTERN}
• Search and replace
  • ${PARAMETER/PATTERN/STRING}
  • ${PARAMETER//PATTERN/STRING}
  • ${PARAMETER/PATTERN}
  • ${PARAMETER//PATTERN}
• String length
  • ${#PARAMETER}
• Substring expansion
  • ${PARAMETER:OFFSET}
  • ${PARAMETER:OFFSET:LENGTH}
• Use a default value
  • ${PARAMETER:-WORD}
  • ${PARAMETER-WORD}
• Assign a default value
  • ${PARAMETER:=WORD}
  • ${PARAMETER=WORD}
• Use an alternate value
  • ${PARAMETER:+WORD}
  • ${PARAMETER+WORD}

$PARAMETER

${PARAMETER}

The easiest form is to just use a parameter's name within braces. This is identical to using $FOO like you see it everywhere, but has the advantage that it can be immediately followed by characters that would be interpreted as part of the parameter name otherwise. Compare these two expressions (WORD="car" for example), where we want to print a word with a trailing "s":

echo "The plural of $WORD is most likely $WORDs"
echo "The plural of $WORD is most likely ${WORD}s"

Why does the first one fail? It prints nothing, because a parameter (variable) named "WORDs" is undefined and thus printed as "" (nothing). Without using braces for parameter expansion, Bash will interpret the sequence of all valid characters from the introducing "$" up to the last valid character as name of the parameter. When using braces you just force Bash to only interpret the name inside your braces.

Also, please remember, that parameter names are (like nearly everything in UNIX®) case sentitive!

The second form with the curly braces is also needed to access positional parameters (arguments to a script) behind $9:

echo "Argument  1 is: $1"
echo "Argument 10 is: ${10}"

See also the article about general array syntax

For arrays you always need the braces. The arrays are expanded by individual indexes or mass arguments. An individual index behaves like a normal parameter, for the mass expansion, please read the article about arrays linked above.

• ${array[5]}
• ${array[*]}
• ${array[@]}

${!PARAMETER}

Everywhere you can name a parameter to expand, like for example

${PARAMETER}

${PARAMETER:0:3}

${!PARAMETER}

read -p "Which variable do you want to inspect? " look_var

echo "The value of \"$look_var\" is: \"${!look_var}\""

Of course the indirection also works with special variables:

# set some fake positional parameters
set one two three four

# get the LAST argument ("#" stores the number of arguments, so "!#" will reference the LAST argument)
echo ${!#}

This is also known as "variable variables" or "indirect reference". Indirect references to array names are not possible (as of Bash 4.1).

${PARAMETER^}

${PARAMETER^^}

${PARAMETER,}

${PARAMETER,,}

${PARAMETER~}

${PARAMETER~~}

These expansion operators modify the case of the letters in the expanded text.

The ^ operator modifies the first character to uppercase, the , operator to lowercase. When using the double-form (^^ and ,,), all characters are converted.

The (currently undocumented) operators ~ and ~~ reverse the case of the given text (in PARAMETER).~ reverses the case of first letter of words in the variable while ~~ reverses case for all.Thanks to Bushmills and geirha on the Freenode IRC channel for this finding.

Example: Rename all *.txt filenames to lowercase

for file in *.txt; do
  mv "$file" "${file,,}"
done

Note: The feature worked word-wise in Bash 4 RC1 (a modification of a parameter containing hello world ended up in Hello World, not Hello world). In the final Bash 4 version it works on the whole parameter, regardless of something like "words". IMHO a technically cleaner implementation. Thanks to Chet.

For array expansion, the case modification applies to every expanded element, no matter if you expand an individual index or mass-expand the whole array using @ or * subscripts. Some examples:

Assume: array=(This is some Text)

• echo "${array[@],}"
  • ⇒ this is some text
• echo "${array[@],,}"
  • ⇒ this is some text
• echo "${array[@]^}"
  • ⇒ This Is Some Text
• echo "${array[@]^^}"
  • ⇒ THIS IS SOME TEXT
• echo "${array[2]^^}"
  • ⇒ TEXT

${!PREFIX*}

${!PREFIX@}

This expands to a list of all set variable names beginning with the string PREFIX. The elements of the list are separated by the first character in the IFS-variable (<space> by default).

This will show all defined variable names (not values!) beginning with "BASH":

$ echo ${!BASH*}
BASH BASH_ARGC BASH_ARGV BASH_COMMAND BASH_LINENO BASH_SOURCE BASH_SUBSHELL BASH_VERSINFO BASH_VERSION

This list will also include array names.

${PARAMETER#PATTERN}

${PARAMETER##PATTERN}

${PARAMETER%PATTERN}

${PARAMETER%%PATTERN}

This one can expand only a part of a parameter's value, given a pattern to describe what to remove from the string. The pattern is interpreted just like a pattern to describe a filename to match (globbing). See Pattern matching for more.

Example string (just a quote from a big man):

MYSTRING="Be liberal in what you accept, and conservative in what you send"

${PARAMETER#PATTERN} and ${PARAMETER##PATTERN}

This form is to remove the described pattern trying to match it from the beginning of the string. The operator "#" will try to remove the shortest text matching the pattern, while "##" tries to do it with the longest text matching. Look at the following examples to get the idea (matched text marked striked, remember it will be removed!):

${PARAMETER%PATTERN} and ${PARAMETER%%PATTERN}

In the second form everything will be the same, except that Bash now tries to match the pattern from the end of the string:

How the heck does that help to make my life easier?

Well, maybe the most common use for it is to extract parts of a filename. Just look at the following list with examples:

• Get basename
  • ${FILENAME%.*}
  • ⇒ bash_hackers.txt
• Get extension
  • ${FILENAME##*.}
  • ⇒ bash_hackers.txt
• Get directory name
  • ${PATHNAME%/*}
  • ⇒ /home/bash/bash_hackers.txt
• Get filename
  • ${PATHNAME##*/}
  • ⇒ /home/bash/bash_hackers.txt

These are the syntaxes for filenames with a single extension. Depending on your needs, you might need to adjust shortest/longest match.

As for most parameter expansion features, working on arrays will handle each expanded element, for individual expansion and also for mass expansion.

Simple example, removing a trailing is from all array elements (on expansion):

Assume: array=(This is a text)

• echo "${array[@]%is}"
  • ⇒ Th a text
  • (it was: This is a text)

All other variants of this expansion behave the same.

${PARAMETER/PATTERN/STRING}

${PARAMETER//PATTERN/STRING}

${PARAMETER/PATTERN}

${PARAMETER//PATTERN}

This one can substitute (replace) a substring matched by a pattern, on expansion time. The matched substring will be entirely removed and the given string will be inserted. Again some example string for the tests:

MYSTRING="Be liberal in what you accept, and conservative in what you send"

The two main forms only differ in the number of slashes after the parameter name: ${PARAMETER/PATTERN/STRING} and ${PARAMETER//PATTERN/STRING}

The first one (one slash) is to only substitute the first occurrence of the given pattern, the second one (two slashes) is to substitute all occurrences of the pattern.

First, let's try to say "happy" instead of "conservative" in our example string:

${MYSTRING//conservative/happy}

Since there is only one "conservative" in that example, it really doesn't matter which of the two forms we use.

Let's play with the word "in", I don't know if it makes any sense, but let's substitute it with "by".

First form: Substitute first occurrence

${MYSTRING/in/by}

Second form: Substitute all occurrences

${MYSTRING//in/by}

Anchoring Additionally you can "anchor" an expression: A # (hashmark) will indicate that your expression is matched against the beginning portion of the string, a % (percent-sign) will do it for the end portion.

MYSTRING=xxxxxxxxxx
echo ${MYSTRING/#x/y}  # RESULT: yxxxxxxxxx
echo ${MYSTRING/%x/y}  # RESULT: xxxxxxxxxy

If the replacement part is completely omitted, like, the matches are replaced by the nullstring, i.e. they are removed. This is equivalent to specifying an empty replacement:

echo ${MYSTRING//conservative/}
# is equivalent to
echo ${MYSTRING//conservative}

This parameter expansion type applied to arrays applies to all expanded elements, no matter if an individual element is expanded, or all elements using the mass expansion syntaxes.

A simple example, changing the (lowercase) letter t to d:

Assume: array=(This is a text)

• echo "${array[@]/t/d}"
  • ⇒ This is a dext
• echo "${array[@]//t/d}"
  • ⇒ This is a dexd

${#PARAMETER}

When you use this form, the length of the parameter's value is expanded. Again, a quote from a big man, to have a test text:

MYSTRING="Be liberal in what you accept, and conservative in what you send"

Using echo ${#MYSTRING}...

⇒ 64

There's not much to say about it, mh?

For arrays, this expansion type has two meanings:

• For individual elements, it reports the string length of the element (as for every "normal" parameter)
• For the mass subscripts @ and * it reports the number of set elements in the array

Example:

Assume: array=(This is a text)

• echo ${#array[1]}
  • ⇒ 2 (the word "is" has a length of 2)
• echo ${#array[@]}
  • ⇒ 4 (the array contains 4 elements)

Attention: The number of used elements does not need to conform to the highest index. Sparse arrays are possible in Bash, that means you can have 4 elements, but with indexes 1, 7, 20, 31. You can't loop through such an array with a counter loop based on the number of elements!

${PARAMETER:OFFSET}

${PARAMETER:OFFSET:LENGTH}

This one can expand only a part of a parameter's value, given a position to start and maybe a length. If LENGTH is omitted, the parameter will be expanded up to the end of the string.

OFFSET and LENGTH can be any arithmetic expression. Take care: The OFFSET starts at 0, not at 1!

Example string (a quote from a big man):

MYSTRING="Be liberal in what you accept, and conservative in what you send"

In the first form, the expansion is used without a length value, note that the offset 0 is the first character:

echo ${MYSTRING:34}

In the second form we also give a length value:

echo ${MYSTRING:34:13}

If the given offset is negative, it's counted from the end of the string, i.e. an offset of -1 is the last character. In that case, the length still counts forward, of course. One special thing is to do when using a negative offset: You need to separate the (negative) number from the colon:

${MYSTRING: -10:5}
${MYSTRING:(-10):5}

For arrays, this expansion type has again 2 meanings:

• For individual elements, it expands to the specified substring (as for every “normal” parameter)
• For the mass subscripts @ and * it mass-expands individual array elements denoted by the 2 numbers given (starting element, number of elements)

Example:

Assume: array=(This is a text)

• echo ${array[0]:2:2}
  • ⇒ is (the "is" in "This", array element 0)
• echo ${array[@]:1:2}
  • ⇒ is a (from element 1 inclusive, 2 elements are expanded, i.e. element 1 and 2)

${PARAMETER:-WORD}

${PARAMETER-WORD}

If the parameter PARAMETER is unset (never was defined) or null (empty), this one expands to WORD, otherwise it expands to the value of PARAMETER, as if it just was ${PARAMETER}. If you omit the : (colon), like shown in the second form, the default value is only used when the parameter was unset, not when it was empty.

echo "Your home directory is: ${HOME:-/home/$USER}."
echo "${HOME:-/home/$USER} will be used to store your personal data."

If HOME is unset or empty, everytime you want to print something useful, you need to put that parameter syntax in.

#!/bin/bash

read -p "Enter your gender (just press ENTER to not tell us): " GENDER
echo "Your gender is ${GENDER:-a secret}."

It will print "Your gender is a secret." when you don't enter the gender. Note that the default value is used on expansion time, it is not assigned to the parameter.

For arrays, the behaviour is very similar. Again, you have to make a difference between expanding an individual element by a given index and mass-expanding the array using the @ and * subscripts.

• For individual elements, it's the very same: If the expanded element is NULL or unset (watch the :- and - variants), the default text is expanded
• For mass-expansion syntax, the default text is expanded if the array
  • contains no element or is unset (the :- and - variants mean the same here)
  • contains only elements that are the nullstring (the :- variant)

In other words: The basic meaning of this expansion type is applied as consistent as possible to arrays.

Example code (please try the example cases yourself):

####
# Example cases for unset/empty arrays and nullstring elements
####


### CASE 1: Unset array (no array)

# make sure we have no array at all
unset array

echo ${array[@]:-This array is NULL or unset}
echo ${array[@]-This array is NULL or unset}

### CASE 2: Set but empty array (no elements)

# declare an empty array
array=()

echo ${array[@]:-This array is NULL or unset}
echo ${array[@]-This array is NULL or unset}


### CASE 3: An array with only one element, a nullstring
array=("")

echo ${array[@]:-This array is NULL or unset}
echo ${array[@]-This array is NULL or unset}


### CASE 4: An array with only two elements, a nullstring and a normal word
array=("" word)

echo ${array[@]:-This array is NULL or unset}
echo ${array[@]-This array is NULL or unset}

${PARAMETER:=WORD}

${PARAMETER=WORD}

This one works like the using default values, but the default text you give is not only expanded, but also assigned to the parameter, if it was unset or null. Equivalent to using a default value, when you omit the : (colon), as shown in the second form, the default value will only be assigned when the parameter was unset.

echo "Your home directory is: ${HOME:=/home/$USER}."
echo "$HOME will be used to store your personal data."

After the first expansion here (${HOME:=/home/$USER}), HOME is set and usable.

Let's change our code example from above:

#!/bin/bash

read -p "Enter your gender (just press ENTER to not tell us): " GENDER
echo "Your gender is ${GENDER:=a secret}."
echo "Ah, in case you forgot, your gender is really: $GENDER"

For arrays this expansion type is limited. For an individual index, it behaves like for a "normal" parameter, the default value is assigned to this one element. The mass-expansion subscripts @ and * can not be used here because it's not possible to assign to them!

${PARAMETER:+WORD}

${PARAMETER+WORD}

This form expands to nothing if the parameter is unset or empty. If it is set, it does not expand to the parameter's value, but to some text you can specify:

echo "The Java application was installed and can be started.${JAVAPATH:+ NOTE: JAVAPATH seems to be set}"

Some more unrealistic example... Ask for some flags (for whatever reason), and then, if they were set, print a warning and also print the flags:

#!/bin/bash

read -p "If you want to use special flags, enter them now: " SPECIAL_FLAGS
echo "The installation of the application is finished${SPECIAL_FLAGS:+ (NOTE: there are special flags set: $SPECIAL_FLAGS)}."

If you omit the colon, as shown in the second form (${PARAMETER+WORD}), the alternate value will not be used when the parameter is empty, only if it is unset! You can use it, for example, to complain if variables you need (and that can be empty) are undefined:

# test that with the three stages:

# unset foo
# foo=""
# foo="something"

if [[ ${foo+isset} = isset ]]; then
  echo "foo is set..."
else
  echo "foo is not set..."
fi

Similar to the cases for arrays to expand to a default value, this expansion behaves like for a "normal" parameter when using individual array elements by index, but reacts differently when using the mass-expansion subscripts @ and *:

• For individual elements, it's the very same: If the expanded element is not NULL or unset (watch the :+ and + variants), the alternate text is expanded
• For mass-expansion syntax, the alternate text is expanded if the array
  • contains elements where min. one element is not a nullstring (the :+ and + variants mean the same here)
  • contains only elements that are not the nullstring (the :+ variant)

For some cases to play with, please see the code examples in the description for using a default value.

Removing the first 6 characters from a text string:

STRING="Hello world"

# only print it
echo "${STRING#?????}"

# store it
STRING=${STRING#?????}

• Internal: Introduction to expansion and substitution
• Internal: Arrays
• Dictionary, internal: Parameter