This is part seventeen of a series on how to approach bash programming in a way that’s safer and more structured than your basic script.

See part 1 if you want to catch the series from the start.

Last time, we recapped what has been discussed and what’s next. This time, let’s go over bash command processing.

First Things First

So, after typing a command and hitting enter, what’s the first thing that happens? And the next, and the next?

It can actually be a bit tough to get the full picture, even from fairly good docs on the web. The following is compiled from a few different sources. Here’s one that is particularly enlightening, from the developer himself.

Here’s what happens, in order:

  • history expansion - substitute a reference to an older command - e.g. if the last command was echo hello, expand sudo !! to:
sudo echo hello
  • alias substitution - substitute a string for the first word of a simple command - e.g. for the alias alias ls=’ls -l’, expand ls to:

    ls -l
    

    Aliases can resolve into other aliases, but evaluation stops if it resolves into itself, hence no recursion for the alias given here.

  • lexing/parsing - identify important syntactical tokens such as semicolons, parentheses, braces, pipe and redirection characters and determine the grammatical structure of the resulting statement

  • brace expansion - permute a string with a comma-separated (or double-dot-separated) list of strings in braces - e.g. expand echo /mypath/{one,two} to:

echo /mypath/one /mypath/two
  • tilde expansion - substitute the path to a user’s home directory for a word with a tilde as the leading character, using the text between the tilde and the first colon, slash or whitespace character as a username, or the current user if there is no such text - e.g. expand echo ~me/path to:
echo /home/me/path
  • simultaneously in left-to-right order:

    • parameter and variable expansion - substitute variable or parameter ($1, $2, $@, etc.) values for a $name construct (or a ${special_syntax} construct for special expansions) - e.g. for the assignment myvar=”hello there”, expand echo “$myvar” to:

      echo "hello there"
      
    • arithmetic expansion - calculate an arithmetic expression enclosed in a $((expression)) construct and substitute its result - e.g. expand echo $((1 + 1)) to:

      echo 2
      
    • command substitution - substitute the stdout of a command for a $(command) construct - e.g. expand echo “$(cat myfile)” to:

      echo "contents of myfile"
      
    • process substitution - substitute a system-generated filename for a <(command) construct, which when opened, presents the output of the command as the file contents - e.g. vim <(echo hello) starting up vim with the file content:

      hello
      
  • word splitting - split the results of expansions from the last step on any letters in the special IFS variable - e.g. for the assignment myvar=”hello there” and IFS with a space in it (one of the defaults along with tab and newline), split echo $myvar to:

    echo hello there
    

    Note that hello and there above are separate words in the result because the expansion was not double-quoted.

  • pathname expansion - expand a path pattern into a list of matching directory and filenames, also known as globbing - e.g. expand echo ./* to:

echo ./file1.txt ./file2.txt
  • quote removal - remove any of the types of quotation marks, as well as backslashes (which are not the result of an expansion)

  • command resolution - for each resulting statement, determine whether the command to be run exists, in order, as a:

    • function

    • built-in command

    • command in the path

Whew! That’s a lot. All just to get from cd ~ to a prompt in your home directory.

Guess what, I lied, there’s more! Here are even more details on command execution once the command is determined. Hooboy!

Fortunately we don’t have to pay attention to all of these. History expansion, for example, is an interactive feature that isn’t of much use in a script, and so is disabled by default.

For the most part we’ll discuss the ones which require you to program carefully, namely the word-splitting and the expansions.

Continue with part 18 - word splitting