Lesson 22 · Append redirection

Append redirection

The `>>` operator adds to a file instead of replacing it. Today's change is one flag different from yesterday, but it teaches the distinction between truncating and appending that every log file depends on.

The goal

Implement `>> file` so output is appended to the file rather than overwriting it.

Start here - the target

TO DO

Scenario: Appending to a file

Giventhe file log.txt already contains "a\n"

Whenthe line: echo b >> log.txt runs

Thenlog.txt contains "a\nb\n" (the existing content is kept)

Andby contrast, echo c > log.txt would replace it with just "c\n"

Background

>> is >’s gentler sibling: it points standard output at a file just the same, but appends to whatever is already there instead of wiping the file first. The entire difference is one flag when you open the file - truncate for >, append for >> - which is why it fits in a single lesson on top of yesterday’s work.

The distinction matters more than it looks. command > log starts the log fresh every run; command >> log accumulates history across runs. Getting the open flags right is the whole job: open with create-and-append for >>, and the operating system positions every write at the current end of the file, even if another process is appending too. Record >> as a second redirection kind alongside > in the list you built yesterday.

Make it work

// same as '>' but O_APPEND instead of O_TRUNC
int flags = O_WRONLY | O_CREAT | (append ? O_APPEND : O_TRUNC);
int fd = open(target, flags, 0644);
dup2(fd, 1);

CheckpointDONE

`>>` appends and `>` truncates. Commit and stop here.