Table 3-3. Special bash variables
| Character | Definition |
|---|---|
| $* | Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. |
| $@ | Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. |
| $# | Expands to the number of positional parameters in decimal. |
| $? | Expands to the exit status of the most recently executed foreground pipeline. |
| $- | A hyphen expands to the current option flags as specified upon invocation, by the set built-in command, or those set by the shell itself (such as the -i). |
| $$ | Expands to the process ID of the shell. |
| $! | Expands to the process ID of the most recently executed background (asynchronous) command. |
| $0 | Expands to the name of the shell or shell script. |
| $_ | The underscore variable is set at shell startup and contains the absolute file name of the shell or script being executed as passed in the argument list. Subsequently, it expands to the last argument to the previous command, after expansion. It is also set to the full pathname of each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file. |
|
$* vs. $@ |
|---|---|
|
The implementation of "$*" has always been a problem and realistically should have been replaced with the behavior of "$@". In almost every case where coders use "$*", they mean "$@". "$*" Can cause bugs and even security holes in your software. |
3.2.2. Creating variables
Variables are case sensitive and capitalized by default. Giving local variables a lowercase name is a convention which is sometimes applied. However, you are free to use the names you want or to mix cases. Variables can also contain digits, but a name starting with a digit is not allowed:
prompt> export 1number=1 bash: export: `1number=1': not a valid identifier |
To set a variable in the shell, use
VARNAME="value"
Putting spaces around the equal sign will cause errors. It is a good habit to quote content strings when assigning values to variables: this will reduce the chance that you make errors.
Some examples using upper and lower cases, numbers and spaces:
franky ~> MYVAR1="2" franky ~> echo $MYVAR1 2 franky ~> first_name="Franky" franky ~> echo $first_name Franky franky ~> full_name="Franky M. Singh" franky ~> echo $full_name Franky M. Singh franky ~> MYVAR-2="2" bash: MYVAR-2=2: command not found franky ~> MYVAR1 ="2" bash: MYVAR1: command not found franky ~> MYVAR1= "2" bash: 2: command not found franky ~> unset MYVAR1 first_name full_name franky ~> echo $MYVAR1 $first_name $full_name <--no output--> franky ~> |
3.2.3. Exporting variables
A variable created like the ones in the example above is only available to the current shell. It is a local variable: child processes of the current shell will not be aware of this variable. In order to pass variables to a subshell, we need to export them using the export built-in command. Variables that are exported are referred to as environment variables. Setting and exporting is usually done in one step:
export VARNAME="value"
A subshell can change variables it inherited from the parent, but the changes made by the child don't affect the parent. This is demonstrated in the example:
franky ~> full_name="Franky M. Singh" franky ~> bash franky ~> echo $full_name franky ~> exit franky ~> export full_name franky ~> bash franky ~> echo $full_name Franky M. Singh franky ~> export full_name="Charles the Great" franky ~> echo $full_name Charles the Great franky ~> exit franky ~> echo $full_name Franky M. Singh franky ~> |
When first trying to read the value of full_name in a subshell, it is not there (echo shows a null string). The subshell quits, and full_name is exported in the parent - a variable can be exported after it has been assigned a value. Then a new subshell is started, in which the variable exported from the parent is visible. The variable is changed to hold another name, but the value for this variable in the parent stays the same.