Stage 3 · Build

Building CLI Tools

Output Formats

Table, JSON, YAML, and templating with Go templates for flexible CLI output.

6 min readGo for SREBuild

Format Selection Pattern

Most SRE tools support multiple output formats. The standard pattern is a --output flag that selects table, JSON, YAML, or a custom template. This lets users pipe output to jq, yq, or other tools.

Gooutput-format-handling.go

22 linesLn 1, Col 1Go

A single switch handles all output formats. The caller decides the format based on the --output flag. This keeps format logic centralized and consistent across commands.

Table Output

Tables are the default output format for human-readable CLI output. Use text/tabwriter or a table library for aligned columns.

Gotable-output-with-tabwriter.go

19 linesLn 1, Col 1Go

tabwriter aligns columns based on tab stops. The minwidth, padwidth, and padding parameters control alignment. Flush writes the buffered output — always call it or nothing appears.

JSON Output

JSON output is essential for piping to jq and integrating with other tools. Use encoding/json with proper indentation for readability.

Gojson-output.go

23 linesLn 1, Col 1Go

json.Encoder writes directly to an io.Writer, which is more efficient than Marshal + Write. SetIndent makes the output human-readable. The json struct tags control field names in the output.

Use json.Encoder for streaming

json.Encoder writes directly to a Writer without allocating a string. For large outputs, this is significantly more memory-efficient than json.Marshal followed by fmt.Println.

YAML Output

YAML output is useful for Kubernetes-related tools where users paste results into YAML files. Use gopkg.in/yaml.v3 for consistent formatting.

Goyaml-output.go

15 linesLn 1, Col 1Go

yaml.NewEncoder writes YAML documents to the Writer. SetIndent controls indentation. Close flushes any remaining data. For multiple documents, Encode writes each as a separate YAML document.

Go Templates

Go templates let users define custom output formats. The --format flag accepts a template string, and the tool renders it against the data.

Gotemplate-output.go

14 linesLn 1, Col 1Go

Go templates use {{}} syntax for interpolation. The data interface can be a struct, slice, or map. Users can create complex output formats without changing code.

Colorizing Output

Color makes output scannable — green for healthy, red for error, yellow for warning. Use lipgloss or color for terminal colors with automatic detection.

Gocolorized-status-output.go

19 linesLn 1, Col 1Go

lipgloss detects whether the terminal supports color. If not, it renders plain text. This means your colorized output is safe for pipes and non-terminal environments.

Format consistency across commands

Use the same --output flag name and the same format options across all commands. Users should not have to guess whether a command supports JSON output. Document the supported formats in the command help.

Mark this lesson complete to store local progress and unlock a cleaner resume path the next time you visit.

Stage 3 · Build

Building CLI Tools

Output Formats

Table, JSON, YAML, and templating with Go templates for flexible CLI output.

6 min readGo for SREBuild

Format Selection Pattern

Most SRE tools support multiple output formats. The standard pattern is a --output flag that selects table, JSON, YAML, or a custom template. This lets users pipe output to jq, yq, or other tools.

Gooutput-format-handling.go

22 linesLn 1, Col 1Go

A single switch handles all output formats. The caller decides the format based on the --output flag. This keeps format logic centralized and consistent across commands.

Table Output

Tables are the default output format for human-readable CLI output. Use text/tabwriter or a table library for aligned columns.

Gotable-output-with-tabwriter.go

19 linesLn 1, Col 1Go

tabwriter aligns columns based on tab stops. The minwidth, padwidth, and padding parameters control alignment. Flush writes the buffered output — always call it or nothing appears.

JSON Output

JSON output is essential for piping to jq and integrating with other tools. Use encoding/json with proper indentation for readability.

Gojson-output.go

23 linesLn 1, Col 1Go

json.Encoder writes directly to an io.Writer, which is more efficient than Marshal + Write. SetIndent makes the output human-readable. The json struct tags control field names in the output.

Use json.Encoder for streaming

json.Encoder writes directly to a Writer without allocating a string. For large outputs, this is significantly more memory-efficient than json.Marshal followed by fmt.Println.

YAML Output

YAML output is useful for Kubernetes-related tools where users paste results into YAML files. Use gopkg.in/yaml.v3 for consistent formatting.

Goyaml-output.go

15 linesLn 1, Col 1Go

yaml.NewEncoder writes YAML documents to the Writer. SetIndent controls indentation. Close flushes any remaining data. For multiple documents, Encode writes each as a separate YAML document.

Go Templates

Go templates let users define custom output formats. The --format flag accepts a template string, and the tool renders it against the data.

Gotemplate-output.go

14 linesLn 1, Col 1Go

Go templates use {{}} syntax for interpolation. The data interface can be a struct, slice, or map. Users can create complex output formats without changing code.

Colorizing Output

Color makes output scannable — green for healthy, red for error, yellow for warning. Use lipgloss or color for terminal colors with automatic detection.

Gocolorized-status-output.go

19 linesLn 1, Col 1Go

lipgloss detects whether the terminal supports color. If not, it renders plain text. This means your colorized output is safe for pipes and non-terminal environments.

Format consistency across commands

Mark this lesson complete to store local progress and unlock a cleaner resume path the next time you visit.