Shell command arguments, globbing and quoting

In general, a shell command specifies an executable (such as grep) followed by arguments to the command which are separated from each other by whitespace. Arguments are typically filenames but may be other command options (usually starting with -).

Globbing filenames

• * stands for any sequence of characters in a filename, so *.c is replaced by a list of all files in the current directory whose names end in .c; *alone matches all files in a directory whose names do not begin with '.'.
• ? stands for any single character, so *.? would expand to a list of filenames that end in . followed by a single character.
• [chars] matches any single character listed between the brackets; [^chars] matches any single character not listed, so *.[ch] matches filenames that end in .c or .h.

When globbing is used, the glob expression is expanded to a list of filenames which is passed to the command (so the command itself does not see or interpret the glob); if the glob expression does not match any filenames, then it is passed unchanged to the command.

Since many characters have special meaning to the shell (such as *, ?, <, >), and whitespace is considered to separate arguments, there are a few ways to prevent their special interpretation so they can be included in command arguments:

• \c always treats the next character c literally
• 'text' also treats the enclosed characters literally
• "text" treats the enclosed characters literally, but allows substitution of shell variables ($VAR) and a few other constructs

Another special quoting construct is `command`, which runs the given command and parses its output into command arguments, i.e. kill -HUP `cat /var/run/inetd.pid`.

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