Variable expansion

$NAME

Expands to the value of the variable NAME, or to an empty string, if it is unset.

${NAME}

Same as above.

The following forms cause xenv to test for a variable that is unset or null. Omitting the colon results in a test only for a variable that is unset.

${variable:-word}

Use Default Values. If variable is unset or null, the expansion of word is substituted. Otherwise, the value of variable is substituted.

${variable:=word}

Assign Default Values. If variable is unset or null, the expansion of word is assigned to variable. The value of variable is then substituted.

${variable:?word}

Display Error if Null or Unset. If variable is null or unset, the expansion of word (or a message to that effect if word is not present) is output to standard error. Otherwise, the value of variable is substituted.

${variable:+word}

Use Alternate Value. If variable is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

${variable:|word1|word2}

Unless variable is null or unset, substitutes the expansion of word1, otherwise the expansion of word2.

The above notation is consistent with the POSIX shell, except for the ${variable:|word1|word2} form, which is an extension of xenv.

Comments

Preprocessor statements.

Changing sigil

The sigil character can be changed using the $$sigil statement. Its argument is the desired sigil, e.g.:

my user name is $USER
$$sigil %
my home dir is %HOME
%%sigil #
using shell #SHELL

    

Not all characters can be used as a sigil. See the description of the -S option (OPTIONS below), for a list of eligible characters. If invalid sigil value is requested, the $$sigil statement will be reproduced on output as any regular text.

Verbatim block

The following construct introduces verbatim text block:

$$verbatim
TEXT
$$end

    

It expands to TEXT unchanged. To insert verbatim text in a line, use the $( ... ) construct (see Quoting and escaping, below).

The following conditional statements expand to a given fragment of text depending on whether an environment variable is defined.

Expand if defined

The construct

$$ifdef NAME
TEXT1
$$else
TEXT2
$$endif

    

is replaced with the expansion of TEXT1 if the environment variable NAME is defined (including if it has a null value) and to TEXT2 otherwise.

If the construct begins with $$ifndef, the sense is inverted.

Expand if set

The construct

$$ifset NAME
TEXT1
$$else
TEXT2
$$endif

    

is replaced with the expansion of TEXT1 if the environment variable NAME is set and has a non-null value and to TEXT2 otherwise.

If the construct begins with $$ifnset, the sense is inverted.

In the conditional constructs above, the $$else part is optional.

Optional whitespace is allowed between the beginning of the line and the $$ marker, as well as between it and the keyword. This allows for indenting the nested constructs in a more natural way, e.g.:

$$ifdef LOG_HOST
$$ ifdef LOG_PORT
  logger $LOG_HOST:$LOG_PORT;
$$ else
  logger $LOG_HOST
$$ endif
$$endif

Setting and unsetting variables

$$set NAME

Sets the variable NAME to empty string.

$$set NAME "STRING"

Sets the variable NAME STRING. Within STRING, each occurrence of a backslash followed by another character is replaced by that character alone.

$$unset NAME

Unsets the variable NAME.

Quoting and escaping

\${NAME}

Backslash followed by another backslash is replaced by single backslash. Backslash followed by any other character is reproduced verbatim.

To reproduce a portion of text verbatim, enclose it in $( and ). This has the same effect as the $$verbatim block, except that it allows the verbatim portion to appear within a line. Newlines and balanced parentheses are allowed within the $( ... ) construct.

Changing the sigil

@variable
@{variable}
@{variable:-word}
@{variable:=word}
@{variable:?word}
@{variable:+word}
@{variable:|word1|word2}

Additionally, comments will be denoted by @{* ... *}, and @( ... ) will introduce inline verbatim text.

The same holds true for preprocessor statements, i.e. $$set becomes @@set, and so on.

Another way to change sigil is by using the $$sigil statement (see Preprocessor statements above).