subst (1)

subst [ -e editor ] -f substitutions victim ...

Subst makes substitutions into files, in a way that is suitable for customizing software to local conditions. Each victim file is altered according to the contents of the substitutions file.

The substitutions file contains one line per substitution. A line consists of two fields separated by one or more tabs. The first field is the name of the substitution, the second is the value. Neither should contain the character `#', and use of text-editor metacharacters like `&' and `\' is also unwise; the name in particular is best restricted to be alphanumeric. A line starting with `#' is a comment and is ignored.

In the victims, each line on which a substitution is to be made (a target line) must be preceded by a prototype line. The prototype line should be delimited in such a way that it will be taken as a comment by whatever program processes the file later. The prototype line must contain a ``prototype'' of the target line bracketed by `=()<' and `>()='; everything else on the prototype line is ignored. Subst extracts the prototype, changes all instances of substitution names bracketed by `@<' and `>@' to their values, and then replaces the target line with the result.

-e

Substitutions are done using the sed(1) editor, which must be found in either the /bin or /usr/bin directories. To specify a different executable, use the ``-e'' flag.

If the substitutions file is

FIRST	111
SECOND	222

and the victim file is

x = 2;
/* =()<y = @<FIRST>@ + @<SECOND>@;>()= */
y = 88 + 99;
z = 5;

then ``subst -f substitutions victim'' changes victim to:

x = 2;
/* =()<y = @<FIRST>@ + @<SECOND>@;>()= */
y = 111 + 222;
z = 5;

victimdir/substtmp.new new version being built

victimdir/substtmp.old old version during renaming

sed(1)

Complains and halts if it is unable to create its temporary files or if they already exist.

Written at U of Toronto by Henry Spencer.

Rich $alz added the ``-e'' flag July, 1991.

When creating a file to be substed, it's easy to forget to insert a dummy target line after a prototype line; if you forget, subst ends up deleting whichever line did in fact follow the prototype line.