substring (Compute)

When To Use

Use substring to keep only part of a column's content, defined by a 1-based starting character position and a number of characters. It replaces the column content in place, on every row. Reach for it when a fixed-width fragment of a value is meaningful on its own — for example, keeping the first 5 characters of an AIX disk device path to obtain the parent controller identifier.

For separator-based splitting (take field n of a |-delimited value), use the extract compute instead; for pattern-based extraction, use awk.

Syntax

sources:
  diskControllers:
    type: commandLine
    commandLine: /usr/sbin/lsdev -c disk -F 'name physloc'
    computes:
      # Keep only the first 5 chars of the disk path to obtain the controller ID
      # DeviceID;controllerID;
    - type: substring
      column: 2
      start: 1
      length: 5

Properties

Table Transformation Example

With column: 2, start: 1, and length: 5:

Recommended Pattern

• Use duplicateColumn first when you need both the full value and its fragment (duplicate, then substring one of the copies).
• Comment the resulting column layout (# DeviceID;controllerID;) so the intent of the truncation stays visible.
• Prefer extract over substring when the interesting part is delimited by a separator rather than located at a fixed position.

Common Mistakes

• Using 0-based indexes: both column and start are 1-based; start: 0 does not mean “beginning of the string”.
• Confusing length with an end position: length is the number of characters kept, not the index of the last character.
• Applying substring to variable-format values where the fragment is not at a fixed position — use extract or awk instead.

Community Examples