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# ln — Make Links
## Overview
`ln` creates hard links or symbolic links between files. It supports
interactive prompting, forced overwriting, verbose output, and optional
warnings for missing symbolic link targets.
**Source**: `ln/ln.c` (single file)
**Origin**: BSD 4.4, University of California, Berkeley
**License**: BSD-3-Clause
## Synopsis
```
ln [-sfhivFLPnw] source_file [target_file]
ln [-sfhivFLPnw] source_file ... target_dir
```
## Options
| Flag | Description |
|------|-------------|
| `-s` | Create symbolic links instead of hard links |
| `-f` | Force: remove existing target files |
| `-i` | Interactive: prompt before overwriting |
| `-n` | Don't follow symlinks on the target |
| `-v` | Verbose: print each link created |
| `-w` | Warn if symbolic link target does not exist |
| `-h` | Don't follow symlink if target is a symlink to a directory |
| `-F` | Remove existing target directory before linking |
| `-L` | Follow symlinks on the source |
| `-P` | Don't follow symlinks on the source (default for hard links) |
## Source Analysis
### Data Structures
```c
struct ln_options {
bool force; /* -f: remove existing targets */
bool remove_dir; /* -F: remove existing directories */
bool no_target_follow; /* -n/-h: don't follow target symlinks */
bool interactive; /* -i: prompt before replace */
bool follow_source_symlink; /* -L: follow source symlinks */
bool symbolic; /* -s: create symlinks */
bool verbose; /* -v: print actions */
bool warn_missing; /* -w: warn on missing symlink target */
int linkch; /* Function: link or symlink */
};
```
### Functions
| Function | Purpose |
|----------|---------|
| `main()` | Parse options, determine single vs. multi-target mode |
| `linkit()` | Create one link (core logic) |
| `remove_existing_target()` | Unlink or rmdir existing target |
| `samedirent()` | Check if source and target are the same file |
| `should_append_basename()` | Determine if target is a directory |
| `stat_parent_dir()` | Stat the parent directory of a path |
| `warn_missing_symlink_source()` | Check if symlink target exists |
| `prompt_replace()` | Interactive yes/no prompt |
### Core Linking Logic
```c
static int
linkit(const char *source, const char *target,
const struct ln_options *opts)
{
/* Check if target already exists */
if (lstat(target, &sb) == 0) {
/* Same file check */
if (samedirent(source, target)) {
warnx("%s and %s are the same", source, target);
return 1;
}
/* Interactive prompt */
if (opts->interactive && !prompt_replace(target))
return 0;
/* Remove existing target */
if (opts->force || opts->interactive)
remove_existing_target(target, opts);
}
/* Create the link */
if (opts->symbolic) {
if (symlink(source, target) < 0) {
warn("symlink %s -> %s", target, source);
return 1;
}
} else {
if (link(source, target) < 0) {
warn("link %s -> %s", target, source);
return 1;
}
}
/* Warn about dangling symlinks */
if (opts->symbolic && opts->warn_missing)
warn_missing_symlink_source(source, target);
/* Verbose output */
if (opts->verbose)
printf("%s -> %s\n", target, source);
return 0;
}
```
### Sameness Detection
```c
static int
samedirent(const char *source, const char *target)
{
struct stat sb_src, sb_tgt;
if (stat(source, &sb_src) < 0)
return 0;
if (stat(target, &sb_tgt) < 0)
return 0;
return (sb_src.st_dev == sb_tgt.st_dev &&
sb_src.st_ino == sb_tgt.st_ino);
}
```
### Target Resolution
When the target is a directory, the source basename is appended:
```c
static int
should_append_basename(const char *target,
const struct ln_options *opts)
{
struct stat sb;
int (*statfn)(const char *, struct stat *);
/* -n/-h: use lstat to not follow target symlinks */
statfn = opts->no_target_follow ? lstat : stat;
if (statfn(target, &sb) == 0 && S_ISDIR(sb.st_mode))
return 1;
return 0;
}
```
## System Calls Used
| Syscall | Purpose |
|---------|---------|
| `link(2)` | Create hard link |
| `symlink(2)` | Create symbolic link |
| `lstat(2)` | Stat without following symlinks |
| `stat(2)` | Stat following symlinks |
| `unlink(2)` | Remove existing target |
| `rmdir(2)` | Remove existing target directory (`-F`) |
| `readlink(2)` | Resolve symlink for display |
## Examples
```sh
# Hard link
ln file1.txt file2.txt
# Symbolic link
ln -s /usr/local/bin/python3 /usr/bin/python
# Force overwrite
ln -sf new_target link_name
# Verbose with warning
ln -svw /opt/myapp/bin/app /usr/local/bin/app
# Multiple files into directory
ln -s file1 file2 file3 /tmp/links/
```
## Exit Codes
| Code | Meaning |
|------|---------|
| 0 | All links created successfully |
| 1 | Error creating one or more links |
|
