第43页 | Learning the Bash Shell | 阅读 ‧ 电子书库

Help

A feature in bash that no other shell has is an online help system. The help command gives information on commands in bash. If you type help by itself, you'll get a list of the built-in shell commands along with their options.

If you provide help with a shell command name it will give you a detailed description of the command:

$ help cd
cd: cd [-L | -P] [dir]
 Change the current directory to DIR. The variable $HOME is the
 default DIR. The variable $CDPATH defines the search path for
 the directory containing DIR. Alternative directory names in
 CDPATH are separated by a colon (:). A null directory name is
 the same as the current directory, i.e. `.'. If DIR begins with
 a slash (/), then $CDPATH is not used. If the directory is not
 found, and the shell option `cdable_vars' is set, then try the
 word as a variable name. If that variable has a value, then cd
 to the value of that variable. The -P option says to use the
 physical directory structure instead of following symbolic links;
 the -L option forces symbolic links to be followed.

You can also provide help with a partial name, in which case it will return details on all commands matching the partial name. For example, help re will provide details on read, readonly, and return. The partial name can also include wildcards. You'll need to quote the name to ensure that the wildcard is not expanded to a filename. So the last example is equivalent to help `re*', and help `re??' will only return details on read.

Sometimes help will show more than a screenful of information and it will scroll the screen. You can use the more command to show one screenful at a time by typing help command | more.