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GNU cfengine

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4.8 Editing Files

A very convenient characteristic of BSD/System 5 systems is that they are configured primarily by human-readable textfiles. This makes it easy for humans to configure the system and it also simplifies the automation of the procedure. Most configuration files are line-based text files, a fact which explains the popularity of, for example, the Perl programming language. Cfengine does not attempt to compete with Perl or its peers. Its internal editing functions operate at a higher level which are designed for transparency rather than flexibility. Fortunately most editing operations involve appending a few lines to a file, commenting out certain lines or deleting lines.

For example, some administrators consider the finger service to be a threat to security and want to disable it. This could be done as follows.

 
editfiles:

      { /etc/inetd.conf

      HashCommentLinesContaining "finger"
      }

Commands containing the word `Comment' are used to `comment out' certain lines from a text-file--i.e. render a line impotent without actually deleting it. Three types of comment were supported originally: shell style (hash) `#', `%' as used in TeX and on AIX systems, and C++-style `//'.

A more flexible way of commenting is also possible, using directives which first define strings which signify the start of a comment and the end of a comment. A single command can then be used to render a comment. The default values of the comment-start string is `# ' and the default comment-end string is the empty string. For instance, to define C style comments you could write:

 
  { file

  SetCommentStart "/* "
  SetCommentEnd   " */"

  # Comment out all lines containing printf!

  CommentLinesMatching ".*printf.*"
  }

Other applications for these editing commands include monitoring and controlling root-access to hosts by editing files such as `.rhosts' and setting up standard environment variables in global shell resource files-- for example, to set the timezone. You can use the editing feature to update and distribute the message of the day file, or to configure sendmail, (see FAQS and Tips in the Reference manual).

An extremely powerful feature of cfagent is the ability to edit a similar file belonging to every user in the system. For example, as a system administrator, you sometimes need to ensure that users have a sensible login environment. Changes in the system might require all users to define a new environment variable, for instance. This is achieved with the home pseudo-wildcard. If one writes

 
  { home/.cshrc

  AppendIfNoSuchLine "# Sys admin/cfengine: put next line here"
  AppendIfNoSuchLine "setenv PRINTER newprinter"
  }

then the users' files are checked one-by-one for the given lines of text, and edited if necessary.

Files are loaded into cfagent and edited in memory. They are only saved again if modifications to the file are carried out, in which case the old file is preserved by adding a suffix to the filename. When files are edited, cfagent generates a warning for the administrator's inspection so that the reason for the change can be investigated.

The behaviour of cfagent should not be confused with that of sed or perl. Some functionality is reproduced for convenience, but the specific functions have been chosen on the basis of (i) their readability and (ii) the fact that they are `frequently-required-functions'. A typical file editing session involves the following points:

Equivalent one-line sed operations involve editing the same file perhaps many times to achieve the same results--without the safety checks in addition.


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