---
name: "subst"
section: "1"
platform: "linux"
package: "inn"
version: "1:1.7.2q-52"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/subst"
---

# subst(1)

substitute definitions into file(s)

## NAME

subst - substitute definitions into file(s)

## SYNOPSIS

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

## DESCRIPTION

*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 *victim*s, 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.

## OPTIONS

- **-e**

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

## EXAMPLE

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;
```

## FILES

*victimdir*/substtmp.new	new version being built\
 *victimdir*/substtmp.old	old version during renaming

## SEE ALSO

[sed(1)](/sed/1.md)

## DIAGNOSTICS

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

## HISTORY

Written at U of Toronto by Henry Spencer.

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

## BUGS

When creating a file to be *subst*ed, 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.