Bash Expansion
Contents
Brace Expansion
{a,b} is expanded to the tokens a and b. This can be used like:
cp /path/to/file/example{,.bak}
# or
touch example.{c,h}
Sequence Expansion
{1..3} is expanded to the tokens 1, 2, and 3.
An increment can be optionally specified like {1..3..1}.
If the first number is larger than the second, then the token expands to a decreasing sequence. Behind the scenes, in this case, the increment defaulted to -1.
Tilde Expansion
The tilde (~) expands to a directory. Which directory depends on what, if any, characters follow it.
Tilde Prefix |
Expansion |
~ |
$HOME |
~/foo |
$HOME/foo |
~me/foo |
subdirectory foo within the home directory of user me |
~+/foo |
$PWD/foo |
~-/foo |
if $OLDPWD is set, $OLDPWD/foo |
~N |
for an integer N, the Nth element in the directory stack (dirs +N) |
~+N |
for an integer N, the Nth element in the directory stack (dirs +N) |
~-N |
for an integer N, the -Nth element in the directory stack (dirs -N) |
Parameter Expansion
The simplest form of parameter expansion is ${parameter}.
Conditional Parameter Expansion
Conditional parameter expansions provide default values if the parameter is unset or empty.
Suppose that:
export a=a export b= unset c
Operator |
Parameter Expansion |
Value |
Side Effects |
:- |
${a:-d} |
a |
|
:- |
${b:-d} |
d |
|
:- |
${c:-d} |
d |
|
- |
${a-d} |
a |
|
- |
${b-d} |
|
|
- |
${c-d} |
d |
|
:= |
${a:=d} |
a |
|
:= |
${b:=d} |
d |
b is set to d |
:= |
${c:=d} |
d |
c is set to d |
= |
${a=d} |
a |
|
= |
${b=d} |
|
|
= |
${c=d} |
d |
c is set to d |
:? |
${a:?d} |
a |
|
:? |
${b:?d} |
d |
shell exits |
:? |
${c:?d} |
d |
shell exits |
? |
${a?d} |
a |
|
? |
${b?d} |
|
|
? |
${c?d} |
d |
shell exits |
:+ |
${a:+d} |
d |
|
:+ |
${b:+d} |
|
|
:+ |
${c:+d} |
|
|
+ |
${a+d} |
d |
|
+ |
${b+d} |
d |
|
+ |
${c+d} |
|
|
The general rules are that:
including a colon (:) checks for unset and empty values; excluding it checks for only unset values
the subtraction sign (-) uses the left value value if possible, otherwise uses the right hand value
the equals sign (=) uses the left value value if possible, otherwise uses the right hand value and set the left hand value to be equal to the right hand value
the question mark (?) uses the left value value if possible, otherwise throws an error and exits the shell
the plus sign (+) uses the right value value if the left hand value is available, otherwise is empty
Substring Parameter Expansion
Strings can be sliced in parameter expansion.
string=01234567890abcdefgh
echo ${string:7} # 7890abcdefgh
echo ${string:7:2} # 78
echo ${string:7:-2} # 7890abcdef
echo ${string: -7} # bcdefgh
echo ${string: -7:-2} # bcdefArrays use the same syntax if they are subscripted with [@] or [*].
array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
echo ${array[@]:7} # 7 8 9 0 a b c d e f g h
Indirect Parameter Expansion
If a parameter expansion begins with !, then the expansion is interpretted as input for another expansion.
export foo=bar
export bar=baz
echo ${foo} # bar
echo ${!foo} # baz
Arithmetic Expansion
$(( 1 + 1 )) expands to 2. For a description of all arithmetic operators, see here.
The tokens within an arithmetic expansion undergo all of the above and below expansions individually. Arithmetic expansion can be nested.
Variables are expanded, though an empty or unset variable expands to 0. In addition, if a variable expands to anything that is not a number, that evaluation is interpretted as input for another expansion.
A string literal will not expand, and in fact will raise an error.
Command Expansion
$(date) expands to the output of date.
The commands are executed in a subshell, so all side-effects are discarded.
Command expansion can be nested.
Process Expansion
Commands can be expanded to file-like objects.
To provide a command's STDIN as a file-like object to another command, try:
date > >(cat)
To provide a command's STDOUT as a file-like object to another command, try:
cat <(date)
Word Splitting
A command is split into tokens using $IFS. (The default is spaces, tabs, and newlines.) Leading and trailing instances of $IFS are ignored. Repeated instances of any member of $IFS are treated as a singular delimiter for splitting.
Literal null tokens ("" or '') are kept and passed to commands. On the other hand, unquoted null tokens (as resulting from expansions) are removed. The exception is if such an expansion happens within double quotes (such as "$var"), in which case a null token is kept. Note that when such an expansion is part of a larger, non-null token (such as "x$var"), the null expansion part is removed.
Filename Expansion
The unquoted characters *, ?, and [ trigger filename expansion.
* matches any 0 or more characters. When the globstar option is enabled, ** matches all files and directories and **/ matches all directories.
? matches any 1 character.
[...] matches any character contained within the brackets.
Quoting
To avoid ambiguity with variable expansion, ${ is not eligible for these types of expansion.
{ and , may be quoted within brace expansion to ensure they are interpretted as literal characters.