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PostgreSQL的学习心得和知识总结（六十六）|关于PostgreSQL数据库 psql获取环境变量的元命令\getenv 的实现方案

Gascognya 2022-03-21 阅读 62
postgresql\getenv

目录结构

注：提前言明 本文借鉴了以下博主、书籍或网站的内容，其列表如下：

PostgreSQL数据库 psql获取环境变量的元命令\getenv

文章快速说明索引

学习目标：

做数据库内核开发久了就会有一种 少年得志，年少轻狂 的错觉，然鹅细细一品觉得自己其实不算特别优秀 远远没有达到自己想要的。也许光鲜的表面掩盖了空洞的内在，每每想到于此，皆有夜半临渊如履薄冰之感。为了睡上几个踏实觉，即日起 暂缓其他基于PostgreSQL数据库的兼容功能开发，近段时间 将着重于学习分享Postgres的基础知识和实践内幕。

备注1：


遇一本好书，很难得
读一本好书，很重要

我给大家推荐一本书：《PostgreSQL指南：内幕探索》，点击前往

学习内容：（详见目录）

1、PostgreSQL数据库 psql获取环境变量的元命令\getenv 的实现

学习时间：

2022年03月20日 16:46:28

学习产出：

1、PostgreSQL数据库基础知识回顾 1个
2、CSDN 技术博客 1篇
3、PostgreSQL数据库内核深入学习

注：下面我们所有的学习环境是Centos7+PostgreSQL14.2+Oracle11g+MySQL8.0

postgres=# select version();
                                   version                                   
-----------------------------------------------------------------------------
 PostgreSQL 14.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.1.0, 64-bit
(1 row)

postgres=#

#-----------------------------------------------------------------------------#

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE	11.2.0.1.0	Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

SQL>

#-----------------------------------------------------------------------------#

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.27    |
+-----------+
1 row in set (0.06 sec)

mysql>

问题描述背景说明

在开始之前，先来看一下现在的PostgreSQL数据库里面 psql 工具所支持的元命令：

postgres=# select version();
                                                version                                                 
--------------------------------------------------------------------------------------------------------
 PostgreSQL 14.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-4), 64-bit
(1 row)

postgres=# \?
General
  \copyright             show PostgreSQL usage and distribution terms
  \crosstabview [COLUMNS] execute query and display results in crosstab
  \errverbose            show most recent error message at maximum verbosity
  \g [(OPTIONS)] [FILE]  execute query (and send results to file or |pipe);
                         \g with no arguments is equivalent to a semicolon
  \gdesc                 describe result of query, without executing it
  \gexec                 execute query, then execute each value in its result
  \gset [PREFIX]         execute query and store results in psql variables
  \gx [(OPTIONS)] [FILE] as \g, but forces expanded output mode
  \q                     quit psql
  \watch [SEC]           execute query every SEC seconds

Help
  \? [commands]          show help on backslash commands
  \? options             show help on psql command-line options
  \? variables           show help on special variables
  \h [NAME]              help on syntax of SQL commands, * for all commands

Query Buffer
  \e [FILE] [LINE]       edit the query buffer (or file) with external editor
  \ef [FUNCNAME [LINE]]  edit function definition with external editor
  \ev [VIEWNAME [LINE]]  edit view definition with external editor
  \p                     show the contents of the query buffer
  \r                     reset (clear) the query buffer
  \s [FILE]              display history or save it to file
  \w FILE                write query buffer to file

Input/Output
  \copy ...              perform SQL COPY with data stream to the client host
  \echo [-n] [STRING]    write string to standard output (-n for no newline)
  \i FILE                execute commands from file
  \ir FILE               as \i, but relative to location of current script
  \o [FILE]              send all query results to file or |pipe
  \qecho [-n] [STRING]   write string to \o output stream (-n for no newline)
  \warn [-n] [STRING]    write string to standard error (-n for no newline)

Conditional
  \if EXPR               begin conditional block
  \elif EXPR             alternative within current conditional block
  \else                  final alternative within current conditional block
  \endif                 end conditional block

Informational
  (options: S = show system objects, + = additional detail)
  \d[S+]                 list tables, views, and sequences
  \d[S+]  NAME           describe table, view, sequence, or index
  \da[S]  [PATTERN]      list aggregates
  \dA[+]  [PATTERN]      list access methods
  \dAc[+] [AMPTRN [TYPEPTRN]]  list operator classes
  \dAf[+] [AMPTRN [TYPEPTRN]]  list operator families
  \dAo[+] [AMPTRN [OPFPTRN]]   list operators of operator families
  \dAp[+] [AMPTRN [OPFPTRN]]   list support functions of operator families
  \db[+]  [PATTERN]      list tablespaces
  \dc[S+] [PATTERN]      list conversions
  \dC[+]  [PATTERN]      list casts
  \dd[S]  [PATTERN]      show object descriptions not displayed elsewhere
  \dD[S+] [PATTERN]      list domains
  \ddp    [PATTERN]      list default privileges
  \dE[S+] [PATTERN]      list foreign tables
  \des[+] [PATTERN]      list foreign servers
  \det[+] [PATTERN]      list foreign tables
  \deu[+] [PATTERN]      list user mappings
  \dew[+] [PATTERN]      list foreign-data wrappers
  \df[anptw][S+] [FUNCPTRN [TYPEPTRN ...]]
                         list [only agg/normal/procedure/trigger/window] functions
  \dF[+]  [PATTERN]      list text search configurations
  \dFd[+] [PATTERN]      list text search dictionaries
  \dFp[+] [PATTERN]      list text search parsers
  \dFt[+] [PATTERN]      list text search templates
  \dg[S+] [PATTERN]      list roles
  \di[S+] [PATTERN]      list indexes
  \dl                    list large objects, same as \lo_list
  \dL[S+] [PATTERN]      list procedural languages
  \dm[S+] [PATTERN]      list materialized views
  \dn[S+] [PATTERN]      list schemas
  \do[S+] [OPPTRN [TYPEPTRN [TYPEPTRN]]]
                         list operators
  \dO[S+] [PATTERN]      list collations
  \dp     [PATTERN]      list table, view, and sequence access privileges
  \dP[itn+] [PATTERN]    list [only index/table] partitioned relations [n=nested]
  \drds [ROLEPTRN [DBPTRN]] list per-database role settings
  \dRp[+] [PATTERN]      list replication publications
  \dRs[+] [PATTERN]      list replication subscriptions
  \ds[S+] [PATTERN]      list sequences
  \dt[S+] [PATTERN]      list tables
  \dT[S+] [PATTERN]      list data types
  \du[S+] [PATTERN]      list roles
  \dv[S+] [PATTERN]      list views
  \dx[+]  [PATTERN]      list extensions
  \dX     [PATTERN]      list extended statistics
  \dy[+]  [PATTERN]      list event triggers
  \l[+]   [PATTERN]      list databases
  \sf[+]  FUNCNAME       show a function's definition
  \sv[+]  VIEWNAME       show a view's definition
  \z      [PATTERN]      same as \dp

Formatting
  \a                     toggle between unaligned and aligned output mode
  \C [STRING]            set table title, or unset if none
  \f [STRING]            show or set field separator for unaligned query output
  \H                     toggle HTML output mode (currently off)
  \pset [NAME [VALUE]]   set table output option
                         (border|columns|csv_fieldsep|expanded|fieldsep|
                         fieldsep_zero|footer|format|linestyle|null|
                         numericlocale|pager|pager_min_lines|recordsep|
                         recordsep_zero|tableattr|title|tuples_only|
                         unicode_border_linestyle|unicode_column_linestyle|
                         unicode_header_linestyle)
  \t [on|off]            show only rows (currently off)
  \T [STRING]            set HTML <table> tag attributes, or unset if none
  \x [on|off|auto]       toggle expanded output (currently off)

Connection
  \c[onnect] {[DBNAME|- USER|- HOST|- PORT|-] | conninfo}
                         connect to new database (currently "postgres")
  \conninfo              display information about current connection
  \encoding [ENCODING]   show or set client encoding
  \password [USERNAME]   securely change the password for a user

Operating System
  \cd [DIR]              change the current working directory
  \setenv NAME [VALUE]   set or unset environment variable
  \timing [on|off]       toggle timing of commands (currently off)
  \! [COMMAND]           execute command in shell or start interactive shell

Variables
  \prompt [TEXT] NAME    prompt user to set internal variable
  \set [NAME [VALUE]]    set internal variable, or list all if no parameters
  \unset NAME            unset (delete) internal variable

Large Objects
  \lo_export LOBOID FILE
  \lo_import FILE [COMMENT]
  \lo_list
  \lo_unlink LOBOID      large object operations
postgres=#

OK，我们这里着重看一下这一行：

  \setenv NAME [VALUE]   set or unset environment variable

使用如下：

postgres=# 
postgres=# \setenv ONE 1
postgres=#

那怎么看一下 当前操作系统的环境变量 和 \setenv 设置的环境变量呢？

暂时并没有什么好的办法可以直接去查看，其实 这里也很奇怪，既然有 \setenv ，怎么没有元命令： \getenv ？

设计实现思路说明

我们此次的开发需求，大致如下：

  1. 实现 \getenv 元命令
  2. 可以获取 当前操作系统的环境变量 和 \setenv 设置的环境变量
  3. 直接打印在 bash 端即可
  4. 完善 psql 工具的相关汉化
  5. …

稍微分析一下，元命令调用如下：

// src/bin/psql/command.c

HandleSlashCmds
	|
	exec_command
		|
		exec_command_setenv

/*
 * \setenv -- set environment variable
 */
static backslashResult
exec_command_setenv(PsqlScanState scan_state, bool active_branch,
					const char *cmd)
{
	bool		success = true;

	if (active_branch)
	{
		char	   *envvar = psql_scan_slash_option(scan_state,
													OT_NORMAL, NULL, false);
		char	   *envval = psql_scan_slash_option(scan_state,
													OT_NORMAL, NULL, false);

		if (!envvar)
		{
			pg_log_error("\\%s: missing required argument", cmd);
			success = false;
		}
		else if (strchr(envvar, '=') != NULL)
		{
			pg_log_error("\\%s: environment variable name must not contain \"=\"",
						 cmd);
			success = false;
		}
		else if (!envval)
		{
			/* No argument - unset the environment variable */
			unsetenv(envvar);
			success = true;
		}
		else
		{
			/* Set variable to the value of the next argument */
			setenv(envvar, envval, 1);
			success = true;
		}
		free(envvar);
		free(envval);
	}
	else
		ignore_slash_options(scan_state);

	return success ? PSQL_CMD_SKIP_LINE : PSQL_CMD_ERROR;
}

内部再行调用的是下面这两个函数：
在这里插入图片描述
而最后实际工作的函数为系统库函数：

// /usr/include/stdlib.h

#if defined __USE_SVID || defined __USE_XOPEN
/* The SVID says this is in <stdio.h>, but this seems a better place.	*/
/* Put STRING, which is of the form "NAME=VALUE", in the environment.
   If there is no `=', remove NAME from the environment.  */
extern int putenv (char *__string) __THROW __nonnull ((1));
#endif

因为同时存在库函数getenv，如下：

__BEGIN_NAMESPACE_STD
/* Return the value of envariable NAME, or NULL if it doesn't exist.  */
extern char *getenv (const char *__name) __THROW __nonnull ((1)) __wur;
__END_NAMESPACE_STD

因此我们就可以直接利用该函数进行开发元命令！

注：这个做起来实在是太过简单，具体实现思路就不再说了！有兴趣的小伙伴们可以自行前去开发！

功能实现结果测试

在这里插入图片描述

postgres=# select version();
                                   version                                   
-----------------------------------------------------------------------------
 PostgreSQL 14.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.1.0, 64-bit
(1 row)

postgres=# \getenv LANG
en_US.UTF-8
postgres=# 
postgres=# \getenv LC_ALL
postgres=# 
postgres=# \setenv ONE 1
postgres=# 
postgres=# \getenv ONE
1
postgres=# 
postgres=# \setenv ONE
postgres=# 
postgres=# \getenv ONE
postgres=# 
postgres=#

注：这一块内容可以参见我的好哥们的博客：PostgreSQL数据库之国际化语言支持学习总结，点击前往 和 我之前的博客：PostgreSQL的学习心得和知识总结（十三）|数据库配置参数说明，点击前往

第一步，重新进行configure，如下：

[postgres@localhost postgres]$ ./configure --prefix=/home/postgres/test --enable-debug --enable-nls="zh_CN"

第二步，翻译并重新编译

第三步，配置中文环境 并重新初始化，如下：

[postgres@localhost bin]$ echo $LANG
zh_CN.UTF-8
[postgres@localhost bin]$ echo $LC_ALL
zh_CN.UTF-8
[postgres@localhost bin]$

在这里插入图片描述

注：后面 我会上传资源patch，有兴趣的小伙伴们可以自行前去开发！

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