prepend (Compute)

When To Use

Use prepend to concatenate a value at the beginning of a column's content, on every row of the table. Typical uses are labeling a raw value for display (value: "SerialNumber: " before the serial number column), prepending $n (the content of column n) to build composite values, or prepending the content of another source with ${source::...}.

Because a table is serialized as semicolon-separated text, prepending a value that contains ; effectively inserts new columns to the left of the target column — a deliberate and widely used trick, especially to add a constant marker column (value: MSHW;) in front of single-value SNMP results.

Syntax

sources:
  upsInputVoltage:
    type: snmpTable
    oid: 1.3.6.1.2.1.33.1.3.3.1
    selectColumns: ID
    computes:
      # Add a constant "MSHW" ID column and an "Input " label prefix
      # MSHW;Input <lineID>;
    - type: prepend
      column: 1
      value: 'MSHW;Input '

Properties

Table Transformation Example

With column: 1 and value: 'MSHW;Input ':

Semicolon trick: the first row serializes as 1; prepending MSHW;Input to column 1 yields:

MSHW;Input 1

which re-parses as 2 columns: the ; inside the value split the old column 1 into a new constant column plus the prefixed original. GenericUPS uses exactly this (value: MSHW;) to give single-value snmpGet results a stable ID column.

Recommended Pattern

• Quote values that end with a space or contain ; followed by text, so YAML preserves them verbatim: value: 'MSHW;Input '.
• Use prepend with a trailing ; (value: MSHW;) to add a constant leading key column to keyless single-value sources before a tableJoin or mapping.
• Label human-readable fields ("Model: ", "SerialNumber: ") with prepend right before mapping rather than in AWK scripts — it keeps the pipeline declarative.
• Keep a # Column1;Column2;... comment after any prepend that inserts columns, so later computes are written against the new layout.

Common Mistakes

• Forgetting that a ; in value shifts all column indexes right — every subsequent compute and the mapping section must use the new indexes.
• Forgetting that column is 1-based.
• Writing value: $2 expecting a literal $2: $n is interpreted as a column reference.
• Losing a trailing space by leaving the value unquoted: write value: "Model: ", not value: Model:.

Community Examples

• GenericUPS^[1]
• IpmiTool^[2]
• Windows^[3]

From Windows (prepending the content of another source to combine both tables for calculations), included directly from the connector source:

# Combining both sources for calculations
- type: prepend
  column: 1
  value: ${source::memoryInformation}