Special variables included in Shell

Special variables that come with Shell-General Linux technology-Linux programming and kernel information. The following is a detailed description. $ PATH: the search PATH of the executable file, usually/usr/bin/,/usr/X11R6/bin/,/usr/local/bin, and so on.

$ PWD: working directory (your current directory)

$ UID: User ID

$ SHLVL: Shell level, that is, the depth of Bash nesting. if it is in the command line, $ SHLVL is 1. if it is in the script, $ SHLVL is 2.

$ SECONDS: The time when the script has been run (in SECONDS ).

$ IFS: Internal Domain Separator. this variable is used to determine how Bash recognizes the domain or word boundary when interpreting strings.

$ IFS is blank (space, tab, and line break) by default, but this can be modified. for example, you can set it to a comma when analyzing data files separated by commas. note that $ * uses the first character saved in $ IFS.

$0, $1, $2,... $9: location parameter. it is passed from the command line to the script, to the function, or to the variable.

$ #: Number of command line or location parameters

$ *: All location parameters are regarded as a word.

$? : Exit status code of the command, function, or script itself (success is 0)

$: Script process ID

$! : PID (process ID) of the last job running in the background)