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| normally !. The second character is the quick substitution character, which is used as shorthand for rerunning the previous command entered, substituting one string for another in the command. The default is ^. The optional third character is the character that signifies that the remainder of the line is a comment, when found as the first character of a word, normally #. The history comment character causes history substitution to be skipped for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment. | |
| nolinks | If set, the shell does not follow symbolic links when executing commands that change the current working directory. It uses the physical directory structure instead. By default, bash follows the logical chain of directories when performing commands that change the current directory, such as cd. See also the description of the _P option to the set builtin ("Shell Built-in Commands"). |
| hostname_completion_file HOSTFILE | Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The file may be changed interactively; the next time hostname completion is attempted bash adds the contents of the new file to the already existing database. |
| noclobber | If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. This variable may be overridden when creating output files by using the redirection operator >| instead of >. (See also the _C option to the set built-in command.) |
| auto_resume | This variable controls how the shell interacts with the user and job control. If this variable is set, single word simple commands without redirections are treated as candidates for resumption of an existing stopped job. There is no ambiguity allowed; if there is more than one job beginning with the string typed, the job most recently accessed is selected. The name of a stopped job, in this context, is the command line used to start it. If set to the value exact, the string supplied must match the name of a stopped job exactly; if set to substring, the string supplied needs to match a substring of the name of a stopped job. The substring value provides functionality analogous to the %? job ID. (See "Job Control," later in this manual page.) If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the % job id. |
| no_exit_on_failed_exec | If this variable exists, a noninteractive shell will not exit if it cannot execute the file specified in the exec built-in command. An interactive shell does not exit if exec fails. |
| cdable_vars | If this is set, an argument to the cd built-in command that is not a directory is assumed to be the name of a variable whose value is the directory to change to. |
EXPANSION
Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.
The order of expansions is as follows: brace expansion, tilde expansion, parameter, variable, command, and arithmetic substitution (done in a left_to_right fashion), word splitting, and pathname expansion.
On systems that can support it, there is an additional expansion available: process substitution.
Only brace expansion, word splitting, and pathname expansion can change the number of words of the expansion; other expansions expand a single word to a single word. The single exception to this is the expansion of "$@", as explained earlier. (See "Parameters.")
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BRACE EXPANSION
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by a series of comma-separated strings between a pair of braces, followed by an optional postamble. The preamble is prepended to each string contained within the braces, and the postamble is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, a{d,c,b}e expands into ade ace abe.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.
A correctly formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma. Any incorrectly formed brace expansion is left unchanged.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the preceding example, such as
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with traditional versions of sh, the Bourne shell. sh does not treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. bash removes braces from words as a consequence of brace expansion. For example, a word entered to sh as file{1,2} appears identically in the output. The same word is output as file1 file2 after expansion by bash. If strict compatibility with sh is desired, start bash with the _nobraceexpansion flag (see "Options," earlier in this manual page) or disable brace expansion with the +o braceexpand option to the set command. (See "Shell Built-in Commands.")
TILDE EXPANSION
If a word begins with a tilde character (~), all of the characters preceding the first slash (or all characters, if there is no slash) are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the parameter HOME. If HOME is unset, the home directory of the user executing the shell is substituted instead.
If a + follows the tilde, the value of PWD replaces the tilde and + If a _ follows, the value of OLDPWD is substituted. If the value following the tilde is a valid login name, the tilde and login name are replaced with the home directory associated with that name. If the name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted instances of tildes following a : or =. In these cases, tilde substitution is also performed. Consequently, one may use pathnames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.
PARAMETER EXPANSION
The $ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
| ${parameter} | The value of parameter is substituted. The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name. |
In each of the following cases, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset.