Linux Shell Parameter extension (Parameter Expansion), parameterexpansion
The main reference: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
Other materials: http://www.ibm.com/developerworks/cn/linux/l-bash-parameters.html
The parameter extension format is $ {expression }. Expression contains various characters until matching'}'. When the following situations occur:'}' Will not be checked for matching:
1) after the escape character \, such \{;
2) in quotation marks, such '}';
3)In arithmetic expressions, commands or variable extensions, such as $ {value}
The simplest form of parameter extension is $ {parameter}
You can modify the parameter extension in the following mode:
$ {
Parameter:-
[
Word
]}
Use Default Values. If
ParameterIs unset or null, the expansion
Word(Or an empty string if
WordIs omitted) shall be substituted; otherwise, the value
ParameterShall be substituted.
When the $ {parameter} value is null or is not set, replace it with the [word] value. Otherwise, it is the value of this expression.
[hdfs@cdh51kdc ~]$ bb=3[hdfs@cdh51kdc ~]$ echo ${aa}[hdfs@cdh51kdc ~]$ echo ${bb}3[hdfs@cdh51kdc ~]$ echo ${aa-${bb}}3[hdfs@cdh51kdc ~]$ aa=2[hdfs@cdh51kdc ~]$ echo ${aa-${bb}}2
$ {
Parameter: =
[
Word
]}
Assign Default Values. If
ParameterIs unset or null, the expansion
Word(Or an empty string if
WordIs omitted) shall be assigned
Parameter. In all cases, the final value
ParameterShall be substituted. Only variables, not positional parameters or special parameters, can be assigned in this way.
When the $ {parameter} value is null or is not set, use the [word] value to assign a value to $ {parameter} and replace the final expression. Otherwise, it is the value of this expression.
[hdfs@cdh51kdc ~]$ echo ${aa-${bb}}2[hdfs@cdh51kdc ~]$ echo ${aa:=${bb}}2[hdfs@cdh51kdc ~]$ echo ${cc}[hdfs@cdh51kdc ~]$ echo ${cc:=${bb}}3[hdfs@cdh51kdc ~]$ echo ${cc}3
$ {
Parameter:?
[
Word
]}
Indicate Error if Null or Unset. If
ParameterIs unset or null, the expansion
Word(Or a message indicating it is unset if
WordIs omitted) shall be written to standard error and the shell exits with a non-zero exit status. Otherwise, the value
ParameterShall be substituted. An interactive shell need not exit.
When the value of $ {parameter} is null or is not set, the [word] value is used as the standard error output prompt and the shell is exited and the status is not 0 is returned. Otherwise, it is the value of the expression.
[hdfs@cdh51kdc ~]$ echo ${cc:?"Value not set"}3[hdfs@cdh51kdc ~]$ echo ${dd:?"Value not set"}-bash: dd: Value not set
$ {
Parameter: +
[
Word
]}
Use Alternative Value. If
ParameterIs unset or null, null shall be substituted; otherwise, the expansion
Word(Or an empty string if
WordIs omitted) shall be substituted.
When the $ {parameter} value is null or is not set, the expression returns null. Otherwise, replace the value of the expression with [word.
[hdfs@cdh51kdc ~]$ echo ${cc:+"Value not set"}Value not set[hdfs@cdh51kdc ~]$ echo ${dd:+"Value not set"}
$ {#Parameter}
String Length. The length in characters of the value
ParameterShall be substituted. If
ParameterIs '*'Or '@', The result of the expansion is unspecified. If
ParameterIs unset and
Set
-UIs in effect, the expansion shall fail.
Returns the number of characters in the $ {parameter} value.
[hdfs@cdh51kdc ~]$ echo ${#cc}1[hdfs@cdh51kdc ~]$ echo ${#dd}0
The following four varieties of parameter expansion provide for substring processing. in each case, pattern matching notation (see Pattern Matching Notation), rather than regular expression notation, shall be used to evaluate the patterns. ifParameterIs'#','*', Or'@', The result of the expansion is unspecified. IfParameterIs unset andSet -UIs in effect, the expansion shall fail. enclosing the full parameter expansion string in double-quotes shall not cause the following four varieties of pattern characters to be quoted, whereas quoting characters within the braces shall have this effect. in each variety, ifWordIs omitted, the empty pattern shall be used.
$ {Parameter%[Word]}
Remove Smallest Suffix Pattern.
WordShall be expanded to produce a pattern. The parameter expansion shall then result in
Parameter, With the smallest portion of the suffix matched by
PatternDeleted. If present,
WordShall not begin with an unquoted '%'.
Match the suffix of $ {parameter} in the [word] mode and remove the minimum matching part.
[hdfs@cdh51kdc ~]$ echo ${cc}Value not set[hdfs@cdh51kdc ~]$ echo ${cc%"et"}Value not s
$ {
Parameter%
[
Word
]}
Remove Largest Suffix Pattern.
WordShall be expanded to produce a pattern. The parameter expansion shall then result in
Parameter, With the largest portion of the suffix matched by
PatternDeleted.
Match the suffix of $ {parameter} in the [word] mode and remove the largest matching part.
[hdfs@cdh51kdc ~]$ echo ${cc%%t*}Value no[hdfs@cdh51kdc ~]$ echo ${cc%t*}Value not se
$ {
Parameter#
[
Word
]}
Remove Smallest Prefix Pattern.
WordShall be expanded to produce a pattern. The parameter expansion shall then result in
Parameter, With the smallest portion of the prefix matched by
PatternDeleted. If present,
WordShall not begin with an unquoted '#'.
$ {
Parameter##
[
Word
]}
Remove Largest Prefix Pattern.
WordShall be expanded to produce a pattern. The parameter expansion shall then result in
Parameter, With the largest portion of the prefix matched by
PatternDeleted.
The last two are the [word] minimum matching and maximum matching for removing the prefix respectively.
[hdfs@cdh51kdc ~]$ echo ${cc#*t}set[hdfs@cdh51kdc ~]$ echo ${cc##*t}[hdfs@cdh51kdc ~]$ echo ${cc#V}alue not set
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