Diff for "BashGuide/CommandsAndArguments"

Commands And Arguments

BASH reads commands from its input (which is either a terminal or a file). These commands can be aliases, functions, builtins, keywords, or executables.

• Aliases: Aliases are a way of shortening commands. They are only used in interactive shells, not in scripts. An alias is a name that is mapped to a certain string. Whenever that name is used as a command name, it is replaced by the string before executing the command.

  • So, instead of executing:

         $ nmap -P0 -A --osscan_limit 192.168.0.1

    You could use an alias like this:

         $ alias nmapp='nmap -P0 -A --osscan_limit'
         $ nmapp 192.168.0.1

• Functions: Functions in BASH are somewhat like aliases, but more powerful. Unlike aliases they can be used in scripts. A function contains shell commands, very much like a small script. When a function is called, the commands in it are executed.

• Builtins: BASH has some basic commands built into it, such as cd (change directory), if (conditional command execution), and so on. You can think of them as functions that are provided already.

• Keywords: Keywords are quite like builtins, but the main difference is that special parsing rules apply to them. For example, [ is a bash builtin, while [[ is bash keyword. They are both used for testing stuff, but since [[ is a keyword rather than a builtin, it benefits from a few special parsing rules which make it a lot better:

  •      $ [ a < b ]
        -bash: b: No such file or directory
         $ [[ a < b ]]

    The first example returns an error because bash tries to redirect the file b to the command [ a ] (See File Redirection). The second example actually does what you expect it to. The special character < no longer has its special meaning of File Redirection operator.

• Executables: The last option of commands that can be executed in bash is executables. The command name of an executable is always the pathname of to the executable to execute. If the executable is in the current directory; use ./myprogram. If it's in the /usr/local/bin directory, use /usr/local/bin/myprogram.

  • To make life a little easier for you, though, BASH uses a variable that tells it where to find applications in case you just use the name of the application but not its full pathname. This variable is called PATH, and it is a set of directory names separated by colons -- for example, /bin:/usr/bin. When a command is specified in BASH without a pathname (e.g. myprgram, or ls), and it isn't an alias, function, builtin or keyword, BASH searches through the directories in PATH, in order from left to right, to see whether they contain an executable by the name of the command name you typed.

Each command can be followed by arguments. Arguments are words you specify after the command name. Arguments are separated from the command name and from each other by white space. This is important to remember. For example, the following is wrong:

    $ [-f file]

You want the [ command name to be separated from the arguments -f, file and ]. If you do not separate [ and -f from each other with whitespace, bash will think you are trying to execute the command name [-f and look in PATH for a program named [-f. Additionally, the arguments file and ] also need to be separated by spaces. The [ command expects the last argument to be ]. The correct command separates all arguments with spaces:

    $ [ -f file ]

NOTE:
It is very important that you understand how this works exactly. If you don't grasp these concepts well, the quality of your code will degrade significantly and you will introduce very dangerous bugs. Read Argument Splitting very carefully.

    $ ls
    a  b  c

ls is a command that lists files in the current directory. It's intended to be used only for producing human-readable results. Please don't try to parse, pipe, grep, capture, read, or loop over the output of ls in a script. It's dangerous and there's always a better way. While an invaluable tool on the interactive shell, ls should therefore never be used in scripts. You will understand why as you go through this guide.

    $ mkdir d
    $ cd d
    $ ls

mkdir is a command that creates a new directory. We specified the argument d to that command. This way, the application mkdir is instructed to create a directory called d. After that, we use the builtin command cd to change the shell's current directory to d. ls shows us that the current directory (which is now d) is empty, since it doesn't display any filenames.

In BASH scripts, arguments that were passed to the script are saved in 'Positional Parameters'. You can read these by using $1, $2, and so on for the respective argument. You can also use $@ and $* but more about this later on.

• Tip:
  You can use the type command to figure out the type of a command.
  For example:

    $ type rm
    rm is hashed (/bin/rm)
    $ type cd
    cd is a shell builtin

• In The Manual: Simple Commands

• Alias: A name that is mapped to a string. Whenever that name is used as a command, it is replaced by the string it has mapped.
  Function: A name that is mapped to a script. Whenever that name is used as a command, the script is called with the arguments provided to the function's name on the command line.
  Builtin: Certain features have been built into BASH. These are handled internally whenever they are executed on the command line (and often do not create a new process).
  Application: A binary that can be executed by referring to it (/bin/ls) or if its location is in your PATH variable, you can execute it simply by using its name (ls).