The Bash manual uses the term "parameter expansion" to mean "the value of a variable"
In these examples, foo is a variable, bar is a literal string.
| Syntax |
Description |
$foo |
The value of foo |
${foo} |
Same. |
${foo}bar |
The value of foo, concatenated with literal bar |
| Syntax |
Description |
${foo:-bar} |
If foo unset or null, then bar, else the value of foo. |
${foo:=bar} |
If foo unset or null, then set foo=bar, else the value of foo. |
${foo:?bar} |
If foo unset or null, then exit script and write bar to stderr, else the value of foo |
${foo:+bar} |
If foo unset or null, then nothing is substituted, else bar is substituted. |
Note: A variable can be unset (does not exist) or it can exist, but have null value. There's a difference.
if colon (:) is omitted, then only test for unset, not for null.
Some scripts may have "set -o nounset" ( No unset variables allowed).
But sometimes you don't know if a variable is going to be unset, you can test it like so:
if [ "${foo:-}" = something ]; then...
So if foo is unset, the null value is substituted, and the script won't error out.
| Syntax |
Description |
${#foo} |
length of foo |
${#foo[@]} |
number of array elements |
All "patterns" are shell "glob" type of patterns, not regular expressions.
| Syntax |
Description |
${foo#pattern} |
Remove from Beginning: Delete the shortest part that matches and return the rest. |
${foo##pattern} |
Remove from Beginning: Delete the longest part that matches and return the rest. |
${foo%pattern} |
Remove from End: Delete the shortest part that matches and return the rest. |
${foo%%pattern} |
Remove from End: Delete the longest part that matches and return the rest. |
Note: One way to remember these is to look at the keyboard keys 3,4,and 5.
# removes the beginning of string $, and % removes the end
| Syntax |
Description |
${foo:offset} |
from offset to end |
${foo:offset:length} |
from offset, length characters |
${foo[@]:offset} |
array elements from offset to last element |
${foo[@]:offset:length} |
array elements from offset, length elements |
| Syntax |
Description |
${foo/pattern/bar} |
Find pattern (greedy), within foo, replace first occurance with bar |
${foo//pattern/bar} |
Find pattern (greedy), within foo, replace all occurances with bar |
${foo/#pattern/bar} |
if foo begins with pattern, replace match with bar |
${foo/%pattern/bar} |
if foo ends with pattern, replace match with bar |
| Syntax |
Description |
${foo,pattern} |
First character in pattern to lower case |
${foo,,pattern} |
All characters in pattern to lower case |
${foo^pattern} |
First character in pattern to upper case |
${foo^^pattern} |
All characters in pattern to upper case |
Pattern is optional. If no pattern, operate on entire variable foo.
| Syntax |
Description |
${!var} |
Reference. Expands to the contents of the variable name held by var. |
${!prefix*} |
Expands to the names of variables whose names begin with prefix. |
${!prefix@} |
As above, if used within double quotes, each variable name expands to a separate word. |
${!foo[@]} |
Expands to list of array indices |
| Syntax |
Description |
${foo@U} |
all upper-case |
${foo@u} |
first character upper-case |
${foo@L} |
all lower-case |
${foo@Q} |
quoted, can be re-used as input |
${foo@E} |
expand backslash escapes like the $'…' quoting mechanism |
${foo[@]@K} |
Print key-value pairs of array |