LESSON 366 OF 404 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: cat v4 — Concatenate Multiple Files in Go

Practice

The real cat command can take many files as arguments and prints them one after another. Build that behavior. Given a list of "files" where each file is itself a list of lines, flatten all lines in order and join them with newlines.

A file can be empty (it adds no lines), and the list can hold no files at all; the result is then an empty string.

Function signature:

func concatCat(allLines [][]string) string

Example:

Input: allLines = [["hello", "world"], ["foo", "bar", "baz"]]

Output: "hello\nworld\nfoo\nbar\nbaz"

When printed:
hello
world
foo
bar
baz

Collect every line into a flat []string with a nested loop, preserving order. Then join the pieces with "\n".

The strings package provides strings.Join, so once the lines are flattened you can stitch them together directly:

flat := []string{}
for _, fileLines := range allLines {
    for _, line := range fileLines {
        flat = append(flat, line)
    }
}
return strings.Join(flat, "\n")

The full solution:

func concatCat(allLines [][]string) string {
    flat := []string{}
    for _, fileLines := range allLines {
        for _, line := range fileLines {
            flat = append(flat, line)
        }
    }
    return strings.Join(flat, "\n")
}

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CLI Tool: cat v4 — Concatenate Multiple Files in Go

Cat V4 in Go