The Hack-R Blog

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 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.

-

(A hyphen.) 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 invoking shell, not the subshell.

!

Expands to the process ID of the most recently executed background (asynchronous) command.

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 (see Shell Scripts), $0 is set to the name of that file. If Bash is started with the -c option (seeInvoking Bash), 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.

_

(An underscore.) 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.

depends on whether you want to store the command’s output (either stdout, or stdout + stderr) or its exit status (0 to 255, with 0 typically meaning “success”).

If you want to capture the output, you use command substitution:

    output=$(command)      # stdout only; stderr remains uncaptured
    output=$(command 2>&1) # both stdout and stderr will be captured

If you want the exit status, you use the special parameter $? after running the command:

    command
    status=$?

If you want both:

    output=$(command)
    status=$?

The assignment to output has no effect on command‘s exit status, which is still in $?.

If you don’t actually want to store the exit status, but simply want to take an action upon success or failure, just use if:

    if command; then
        echo "it succeeded"
    else
        echo "it failed"
    fi

Or if you want to capture stdout as well as taking action on success/failure, without explicitly storing or checking $?:

    if output=$(command); then
        echo "it succeeded"
    ...

Example: speedpad.desktop