4.12.1.2. Special variables
There are several special parameters , or special variables , that bash sets automatically; Table 4-17 contains a list of the most important ones.
Table 4-17. Important special variables
| Name | Description | Notes |
|---|---|---|
| $$ | Process ID of the shell | Since process IDs are unique (at any one point in time), this can be used to ensure unique filenames (e.g., /tmp/$$.txt will never conflict with the same filename used by another copy of the same script). |
| $0 | Name of the script | Useful to generate error messages, and when one script is invoked through more than one name. |
| $1, $2, $3, ... | Arguments given on the script's command line | The shift command will eliminate $1 and then shift all of the parameters accordingly ($2 becomes $1, $3 becomes $2, and so forth). |
| $# | Number of arguments from the script's command line | If $# is 0, then no options were given on the command line. |
| $* $@ | All of the arguments from the script's command line | When quoted, "$*" becomes a single block of text containing all of the arguments, while "$@" becomes separate words. If the script is called with the arguments "green" and "yellow", then "$*" would evaluate to "green yellow", while "$@" would evaluate to "green" "yellow". |
| $? | Exit status of the last command | Manpages document the possible exit-status values for most commands. |