POSIX.1-2017 Base Specifications

https://pubs.opengroup.org/onlinepubs/9699919799/
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POSIX.1-2017 is an IEEE standard that defines the portable operating system interface for Unix-like...
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### command -V example with ls

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

Uses command -V to get verbose information about ls.

```shell
command -V ls

```

--------------------------------

### command -p -V example with my_func

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

Uses command -p -V to get verbose information about my_func with a safe path.

```shell
command -p -V my_func

```

--------------------------------

### sccs -d and -p example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sccs.html

Example showing how the -d and -p options modify the pathname: 'sccs -d /x -p y get a/b' converts to 'get /x/a/y/s.b'.

```shell
sccs -d /x -p y get a/b
```

```shell
get /x/a/y/s.b
```

--------------------------------

### order_start example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap07.html

Example of order_start with forward and backward directives.

```plaintext
order_start    forward;backward

```

--------------------------------

### open() - Opening a File for Writing

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html

Example demonstrating how to open a file for writing, creating it if it does not exist, and truncating it if it does.

```APIDOC
## [open] open()

### Description
Opens a file for writing, creating it if it does not exist, or truncating it to zero length if it does.

### Method
open

### Endpoint
open(pathname, oflag, mode)

### Parameters
#### Path Parameters
- **pathname** (string) - Required - The path to the file to open.

#### Flags (oflag)
- **O_WRONLY** - Required - Open for writing only.
- **O_CREAT** - Required - Create the file if it does not exist.
- **O_TRUNC** - Required - Truncate the file to zero length if it exists.

#### Mode (mode)
- **mode** (mode_t) - Required - File permission bits. Example: S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH.

### Request Example
```c
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

#define LOCKFILE "/etc/ptmp"
...
int pfd;
char pathname[PATH_MAX+1];
...
if ((pfd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC,
    S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
    perror("Cannot open output file\n"); exit(1);
}
...
```

### Response
#### Success Response
- **pfd** (int) - File descriptor for the opened file.

#### Error Response
- Returns -1 on failure.

```

--------------------------------

### stat() - Getting Directory Information

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stat.html

Example fragment that gets status information for each entry in a directory using the stat() function.

```APIDOC
## stat() - Getting Directory Information

### Description
This example fragment gets status information for each entry in a directory using the stat() function.

### Function
stat()

### Parameters
- **path** (const char *) - The pathname of the directory entry.
- **buf** (struct stat *) - Pointer to a stat structure where the file status information will be stored.

### Example Code
```c
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>
#include <pwd.h>
#include <grp.h>
#include <time.h>
#include <locale.h>
#include <langinfo.h>
#include <stdio.h>
#include <stdint.h>

struct dirent  *dp;
struct stat     statbuf;
struct passwd  *pwd;
struct group   *grp;
struct tm      *tm;
char            datestring[256];
...
/* Loop through directory entries. */
while ((dp = readdir(dir)) != NULL) {
    /* Get entry's information. */
    if (stat(dp->d_name, &statbuf) == -1)
        continue;
    /* Print out type, permissions, and number of links. */
    printf("%10.10s", sperm (statbuf.st_mode));
    printf("%4d", statbuf.st_nlink);
    /* Print out owner's name if it is found using getpwuid(). */
    if ((pwd = getpwuid(statbuf.st_uid)) != NULL)
        printf(" %-8.8s", pwd->pw_name);
    else
        printf(" %-8d", statbuf.st_uid);
    /* Print out group name if it is found using getgrgid(). */
    if ((grp = getgrgid(statbuf.st_gid)) != NULL)
        printf(" %-8.8s", grp->gr_name);
    else
        printf(" %-8d", statbuf.st_gid);
    /* Print size of file. */
    printf(" %9jd", (intmax_t)statbuf.st_size);
    tm = localtime(&statbuf.st_mtime);
    /* Get localized date string. */
    strftime(datestring, sizeof(datestring), nl_langinfo(D_T_FMT), tm);
    printf(" %s %s\n", datestring, dp->d_name);
}
```
```

--------------------------------

### what command example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/what.html

Command to run `what` on a C source file and its compiled outputs.

```text
what f.c f.o a.out
```

--------------------------------

### Example of using popen() to start a command

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/pclose.html

Shows a call to popen() with the command "command" and mode "r" to start a child process. This is part of the rationale example where the parent and child communicate via a pipe.

```c
popen("command", "r")
```

--------------------------------

### Getting the Value of an Environment Variable

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getenv.html

Example from the EXAMPLES section that gets the value of the HOME environment variable using getenv(). It includes the necessary #include <stdlib.h> and declares a const char *name and char *value before calling getenv(name).

```c
#include <stdlib.h>
...
const char *name = "HOME";
char *value;

value = getenv(name);
```

--------------------------------

### Creating a directory with mkdir()

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/mkdir.html

Example from the POSIX documentation showing how to create a directory named /home/cnd/mod1 with permissions S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH. Includes required headers and a status variable.

```c
#include <sys/types.h>
#include <sys/stat.h>

int status;
...
status = mkdir("/home/cnd/mod1", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);

```

--------------------------------

### Getting an IPC key with ftok()

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/ftok.html

Example from the EXAMPLES section that gets a key based on the pathname /tmp and the ID value 'a', assigning it to the semkey variable for later use with semget(), msgget(), or shmget(). It checks for failure and prints an error message if ftok() returns (key_t)-1.

```c
#include <sys/ipc.h>
...
key_t semkey;

if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
    perror("IPC error: ftok"); exit(1);
}
```

--------------------------------

### Input files for combinatorial example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/join.html

Two input files 'fa' and 'fb' with multiple matching keys, used to demonstrate combinatorial output.

```text
fa:
    a x
    a y
    a z
fb:
    a p

```

--------------------------------

### Installation time script to set POSIX shell pathname

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sh.html

Recommended approach for installing executable scripts: use getconf PATH to determine the shell pathname and update the #! script appropriately during installation, for example with sed.

```shell
#
# Installation time script to install correct POSIX shell pathname
#

```

--------------------------------

### Selecting Options from the Command Line

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/optind.html

Example code fragment showing how to select the type of database routines based on the Options argument using getopt() and optind.

```APIDOC
## Selecting Options from the Command Line

### Description
This example demonstrates how to use the getopt() function and the optind variable to select an option from a predefined string of options. It sets the dbtype variable based on the position of the selected option in the Options string.

### Usage
This is a code example, not a callable API. It illustrates the use of the optind variable in a C program.

### Code Example
c
#include <unistd.h>
#include <string.h>
...
const char *Options = "hdbtl";
...
int dbtype, c;
char *st;
...
dbtype = 0;
while ((c = getopt(argc, argv, Options)) != -1) {
    if ((st = strchr(Options, c)) != NULL) {
        dbtype = st - Options;
        break;
    }
}


### Notes
- The optind variable is used implicitly by getopt() to track the current argument index.
- The example selects the database type based on the option character found in the Options string.
```

--------------------------------

### C: Equivalent command-line invocations for getopt() example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/opterr.html

These are the equivalent command-line forms accepted by the preceding getopt() example. They show various ways to combine options, including using a single dash with combined options, separate options, and the '--' terminator.

```text
cmd -ao arg path path
cmd -a -o arg path path
cmd -o arg -a path path
cmd -a -o arg -- path path
cmd -a -oarg path path
cmd -aoarg path path

```

--------------------------------

### Example commands for built-in utilities

Source: https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap01.html

Counter-examples showing utilities like cd, kill, and true used in pipelines, demonstrating that even unusual cases have value.

```shell
`find . -type d -exec cd {} \; -exec foo {} \;
   ` (which invokes "foo" on accessible directories)`  

ps ... | sed ... | xargs kill
  

find . -exec true \; -a ...
   ` (where "true" is used for temporary debugging)

```

--------------------------------

### command -V example with command

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

Uses command -V to get verbose information about command.

```shell
command -V command

```

--------------------------------

### what output example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/what.html

Example output from the `what` command showing the identification strings for each file.

```text
f.c:
    identification information
    ...
f.o:
    identification information
    ...
a.out:
    identification information
    ...
```

--------------------------------

### command -V example with my_func

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

Uses command -V to get verbose information about my_func.

```shell
command -V my_func

```

--------------------------------

### set -e script example

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

Example script showing the use of set -e within a function. The script starts a server and echoes a success message, but the set -e behavior can cause unexpected failures when the function is called with a fallback.

```shell
set -e
start() {
    some_server
    echo some_server started successfully
}
start || echo >&2 some_server failed

```

--------------------------------

### head -- *

Source: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/head.html

Example from the EXAMPLES section: writes the first ten lines of all files (except those with a leading period) in the current directory.

```shell
head -- *
```
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