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# hostname — Get or Set the System Hostname
## Overview
`hostname` reads or sets the system hostname. On Linux it uses `uname(2)` to
read and `sethostname(2)` to write. The `-f` (FQDN) option is explicitly
unsupported because resolving a fully qualified domain name requires
NSS/DNS, which is outside the scope of a core utility.
**Source**: `hostname/hostname.c` (single file)
**Origin**: BSD 4.4, University of California, Berkeley
**License**: BSD-3-Clause
## Synopsis
```
hostname [-s | -d] [name-of-host]
```
## Options
| Flag | Description |
|------|-------------|
| `-s` | Print short hostname (truncate at first `.`) |
| `-d` | Print domain part only (after first `.`) |
| `-f` | **Not supported on Linux** — exits with error |
## Source Analysis
### Data Structures
```c
struct options {
bool short_name; /* -s: truncate at first dot */
bool domain_only; /* -d: print after first dot */
bool set_mode; /* hostname was provided as argument */
const char *new_hostname;
};
```
### Functions
| Function | Purpose |
|----------|---------|
| `main()` | Dispatch between get/set modes |
| `parse_args()` | `getopt(3)` option parsing |
| `dup_hostname()` | Fetch hostname from `uname(2)` and duplicate it |
| `print_hostname()` | Print full, short, or domain part |
| `set_hostname()` | Set hostname via `sethostname(2)` |
| `linux_hostname_max()` | Query max hostname length from UTS namespace |
### Reading the Hostname
```c
static char *
dup_hostname(void)
{
struct utsname uts;
if (uname(&uts) < 0)
err(1, "uname");
return strdup(uts.nodename);
}
```
### Setting the Hostname
```c
static void
set_hostname(const char *name)
{
size_t max_len = linux_hostname_max();
size_t len = strlen(name);
if (len > max_len)
errx(1, "hostname too long: %zu > %zu", len, max_len);
if (sethostname(name, len) < 0)
err(1, "sethostname");
}
```
### Short/Domain Modes
```c
static void
print_hostname(const char *hostname, const struct options *opts)
{
if (opts->short_name) {
/* Truncate at first '.' */
const char *dot = strchr(hostname, '.');
if (dot)
printf("%.*s\n", (int)(dot - hostname), hostname);
else
puts(hostname);
} else if (opts->domain_only) {
/* Print after first '.' or empty */
const char *dot = strchr(hostname, '.');
puts(dot ? dot + 1 : "");
} else {
puts(hostname);
}
}
```
### Max Hostname Length
```c
static size_t
linux_hostname_max(void)
{
long val = sysconf(_SC_HOST_NAME_MAX);
return (val > 0) ? (size_t)val : 64;
}
```
## System Calls Used
| Syscall | Purpose |
|---------|---------|
| `uname(2)` | Read current hostname |
| `sethostname(2)` | Set new hostname (requires `CAP_SYS_ADMIN`) |
| `sysconf(3)` | Query `_SC_HOST_NAME_MAX` |
## Examples
```sh
# Print hostname
hostname
# Print short hostname
hostname -s
# Print domain part
hostname -d
# Set hostname (requires root)
hostname myserver.example.com
```
## Exit Codes
| Code | Meaning |
|------|---------|
| 0 | Success |
| 1 | Error (sethostname failed, invalid option) |
## Differences from GNU hostname
- No `-f` / `--fqdn` — Linux requires NSS for FQDN resolution
- No `--ip-address` / `-i`
- No `--alias` / `-a`
- No `--all-fqdns` / `--all-ip-addresses`
- Simpler: read or set only, no DNS lookups
|
