Shell bash

-  google "shell man bash"

-  $ man bash

   $ man bash |grep "Special Parameters" -A 50

    

      The shell treats several parameters specially.  These parameters may only be referenced; assignment to them is not allowed.
       *      Expands to the positional parameters, starting from one.  When the expansion is not within double quotes, each positional parameter expands to a separate word.   In  con‐
              texts  where  it  is performed, those words are subject to further word splitting and pathname expansion.  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.  That is, "$*" is equivalent to "$1c$2c...",  where  c  is  the
              first character of the value of the IFS variable.  If IFS is unset, the parameters are separated by spaces.  If IFS is null, the parameters are joined without intervening
              separators.
       @      Expands to the positional parameters, starting from one.  When the expansion occurs within double quotes, each parameter expands to a separate word.   That  is,  "$@"  is
              equivalent  to  "$1" "$2" ...  If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original
              word, and the expansion of the last parameter is joined with the last part of the original word.  When there are no positional parameters, "$@" and $@ expand  to  nothing
              (i.e., they are removed).
       #      Expands to the number of positional parameters in decimal.
       ?      Expands to the exit status of the most recently executed foreground pipeline.
       -      Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option).
       $      Expands to the process ID of the shell.  In a () subshell, it expands to the process ID of the current shell, not the subshell.
       !      Expands  to  the  process  ID  of  the  job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin (see JOB CONTROL
              below).
       0      Expands to the name of the shell or shell script.  This is set at shell initialization.  If bash is invoked with a file of commands, $0 is set to the name of  that  file.
              If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present.  Otherwise, it is set to the filename used
              to invoke bash, as given by argument zero.
       _      At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in  the  environment  or  argument  list.   Subsequently,
              expands  to  the last argument to the previous command, after expansion.  Also set to the full pathname used to invoke each command executed and placed in the environment
              exported to that command.  When checking mail, this parameter holds the name of the mail file currently being checked.

　　

-  $  help test

$ help test
test: test [expr]
    Evaluate conditional expression.

    Exits with a status of 0 (true) or 1 (false) depending on
    the evaluation of EXPR.  Expressions may be unary or binary.  Unary
    expressions are often used to examine the status of a file.  There
    are string operators and numeric comparison operators as well.

    The behavior of test depends on the number of arguments.  Read the
    bash manual page for the complete specification.

    File operators:

      -a FILE        True if file exists.
      -b FILE        True if file is block special.
      -c FILE        True if file is character special.
      -d FILE        True if file is a directory.
      -e FILE        True if file exists.
      -f FILE        True if file exists and is a regular file.
      -g FILE        True if file is set-group-id.
      -h FILE        True if file is a symbolic link.
      -L FILE        True if file is a symbolic link.
      -k FILE        True if file has its `sticky' bit set.
      -p FILE        True if file is a named pipe.
      -r FILE        True if file is readable by you.
      -s FILE        True if file exists and is not empty.
      -S FILE        True if file is a socket.
      -t FD          True if FD is opened on a terminal.
      -u FILE        True if the file is set-user-id.
      -w FILE        True if the file is writable by you.
      -x FILE        True if the file is executable by you.
      -O FILE        True if the file is effectively owned by you.
      -G FILE        True if the file is effectively owned by your group.
      -N FILE        True if the file has been modified since it was last read.

      FILE1 -nt FILE2  True if file1 is newer than file2 (according to
                       modification date).

      FILE1 -ot FILE2  True if file1 is older than file2.

      FILE1 -ef FILE2  True if file1 is a hard link to file2.

    All file operators except -h and -L are acting on the target of a symbolic
    link, not on the symlink itself, if FILE is a symbolic link.

    String operators:

      -z STRING      True if string is empty.

      -n STRING
         STRING      True if string is not empty.

      STRING1 = STRING2
                     True if the strings are equal.
      STRING1 != STRING2
                     True if the strings are not equal.
      STRING1 < STRING2
                     True if STRING1 sorts before STRING2 lexicographically.
      STRING1 > STRING2
                     True if STRING1 sorts after STRING2 lexicographically.

    Other operators:

      -o OPTION      True if the shell option OPTION is enabled.
      -v VAR         True if the shell variable VAR is set.
      -R VAR         True if the shell variable VAR is set and is a name
                     reference.
      ! EXPR         True if expr is false.
      EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
      EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.

      arg1 OP arg2   Arithmetic tests.  OP is one of -eq, -ne,
                     -lt, -le, -gt, or -ge.

    Arithmetic binary operators return true if ARG1 is equal, not-equal,
    less-than, less-than-or-equal, greater-than, or greater-than-or-equal
    than ARG2.

    See the bash manual page bash(1) for the handling of parameters (i.e.
    missing parameters).

    Exit Status:
    Returns success if EXPR evaluates to true; fails if EXPR evaluates to
    false or an invalid argument is given.