Bash: Parameter Expansion (Substitution)

Default Usage

${PARAMETER}

Expand parameter value. It can be used for:

• Separating characters from variable names
  • cd /home/${user}
• Positional parameter higher than 9
  • echo "Argument 10 is: ${10}"

Indirection

${!PARAMETER}

Expands to the value of the variable named by the value of parameter.

var=var1
var1=temp

$ echo ${!var}
temp

Case Modification

${PARAMETER^}, ${PARAMETER^^} ${PARAMETER,}, ${PARAMETER,,} ${PARAMETER~}, ${PARAMETER~~}

Operator	Action
^	Changes first character to upper case
^^	Changes all characters to upper case
,	Changes first character to lower case
,,	Changes all characters to lower case
~	Inverts case of first character
~~	Inverts case of all characters

Variable Name (Prefix) Expansion

${!PREFIX*} ${!PREFIX@}

Expands to a list of all set variable names beginning with the string in PREFIX.

$ echo ${!XDG*}
XDG_CURRENT_DESKTOP XDG_MENU_PREFIX XDG_RUNTIME_DIR XDG_SEAT XDG_SESSION_DESKTOP XDG_SESSION_ID XDG_SESSION_TYPE XDG_VTNR

Substring Removal

${PARAMETER#PATTERN} ${PARAMETER##PATTERN} ${PARAMETER%PATTERN} ${PARAMETER%%PATTERN}

With #, it removes the mathing pattern from the beggining of the variable, where # removes the shortest match, and ## removes the longest.

Note the empty spaces in the example below.

substr="the quick brown fox jumps over the lazy dog"

$ echo ${substr#* }
quick brown fox jumps over the lazy dog

$ echo ${substr##* }
dog

Removing path from a file:

mlog=/var/log/clamav/freshclam.log

$ echo ${mlog##*/}
freshclam.log

The operator % does the same, but at the end of the file.

$ echo ${substr% *}
the quick brown fox jumps over the lazy

$ echo ${substr%% *}
the

Changing the extension of a file

file=123.txt

$ echo ${file%.*}
123

$ echo ${file%.*}.log
123.log

Search and Replace

${PARAMETER/PATTERN/STRING} ${PARAMETER//PATTERN/STRING} ${PARAMETER/PATTERN} ${PARAMETER//PATTERN}

The main diffence is that a single / substitutes the first occurrence, while double // substitute all occurences:

$ echo $substr 
the quick brown fox jumps over the lazy dog

$ echo ${substr/the/da}
da quick brown fox jumps over the lazy dog

$ echo ${substr//the/da}
da quick brown fox jumps over da lazy dog

$ echo ${substr/the}
quick brown fox jumps over the lazy dog

$ echo ${substr//the}
quick brown fox jumps over lazy dog

Anchoring

You can use # and % to anchor to the beginning and end respectively

var1=00000000

$ echo ${var1/#0/1}
10000000

$ echo ${var1/%0/1}
00000001

Offset and Lenght

${PARAMETER:OFFSET} ${PARAMETER:OFFSET:LENGTH}

• Offset removes the amount of characters as specified
• Lenght prints the specified character lenght after offset

Note: You can also use a negative value for offset and lenght, which will be calculated from the end of the file

$ echo ${substr}
the quick brown fox jumps over the lazy dog
123456789...

Removes first 9 chars

$ echo ${substr:9}
brown fox jumps over the lazy dog

Removes first 3 chars and print the next 5

$ echo ${substr:4:5}
quick

Use Default

${PARAMETER:-WORD} ${PARAMETER-WORD}

If parameter unset (or null if using :), expand to word.

$ my_var=gru
$ echo ${my_var:-yyz}
gru

$ unset my_var
$ echo ${my_var:-yyz}
yyz

Use Alternate Value

${PARAMETER:+WORD} ${PARAMETER+WORD}

If parameter is set (or null if using :), expand to word.

$ my_var=gru
$ echo ${my_var:+yyz}
yyz

$ unset my_var
$ echo ${my_var:+yyz}

Use Default And Assign

${PARAMETER:=WORD} ${PARAMETER=WORD}

If parameter unset (or null if using :), expand to word and assign parameter to the value of word.

my_var=gru

$ echo ${my_var:=yyz}
gru

$ unset my_var
$ echo ${my_var:=yyz}
yyz

$ echo $my_var
yyz

Display Error

${PARAMETER:?WORD} ${PARAMETER?WORD}

If parameter unset (or null if using :), display error with word as appendix, otherwise expand parameter.

my_var=gru

$ echo ${my_var:?Not set}
gru

$ unset my_var
$ echo ${my_var:?Not set}
bash: my_var: Not set

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Reference:

• wiki.bash-hackers.org
• tldp.org
• gnu.org