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9.1 Viewing And Editing

To view a list of files that meet certain criteria, simply run your file viewing program with the file names as arguments. Shells substitute a command enclosed in backquotes with its output, so the whole command looks like this:

less `find /usr/include -name '*.h' | xargs grep -l mode_t`

You can edit those files by giving an editor name instead of a file viewing program:

emacs `find /usr/include -name '*.h' | xargs grep -l mode_t`

Because there is a limit to the length of any individual command line, there is a limit to the number of files that can be handled in this way. We can get around this difficulty by using xargs like this:

find /usr/include -name '*.h' | xargs grep -l mode_t > todo
xargs --arg-file=todo emacs

Here, xargs will run emacs as many times as necessary to visit all of the files listed in the file todo. Generating a temporary file is not always convenient, though. This command does much the same thing without needing one:

find /usr/include -name '*.h' | xargs grep -l mode_t |
xargs sh -c 'emacs "$@" < /dev/tty' Emacs

The example above illustrates a useful trick; Using sh -c you can invoke a shell command from xargs. The $@ in the command line is expanded by the shell to a list of arguments as provided by xargs. The single quotes in the command line protect the $@ against expansion by your interactive shell (which will normally have no arguments and thus expand $@ to nothing). The capitalised ‘Emacs’ on the command line is used as $0 by the shell that xargs launches.

Please note that the implementations in GNU xargs and at least BSD support the ‘-o’ option as extension to achieve the same, while the above is the portable way to redirect stdin to /dev/tty.

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