Rationale:

We at NetApp added a gdb feature allowing users to specify environment variable names when invoking gdb commands 'set logging file', 'source' & 'shell'.

This has been a big help especially in gdb macros since it allows macro writers to customize macro
behavior with environment var settings.

We've implemented this feature for the above three gdb commands but it can easily be  extended to other commands such as 'set substitute-path'

Usage example below.

setenv LOG_DIR /tmp
setenv SCRIPT_DIR "."

I've created a script 'macroScript' in the current directory ("."); it is referenced in the gdb session.

Now, run the gdb with the new env var feature:

gdbWithEnvVarFeature
....
GNU gdb (GDB) 9.1
Copyright (C) 2020 Free Software Foundation, Inc.
...

(gdb) shell head ${SCRIPT_DIR}/macroScript
define macro1
       echo macro1\n
end
define macro2
       echo macro2\n
end
macro1
macro2


gdb) source ${SCRIPT_DIR}/macroScript
macro1
macro2
(gdb) set logging file ${LOG_DIR}/logFile
(gdb) show logging file
The current logfile is "${LOG_DIR}/logFile".
(gdb) set logging on
Copying output to /tmp/logFile.
Copying debug output to /tmp/logFile.

(gdb) help set
Evaluate expression EXP and assign result to variable VAR.
Usage: set VAR = EXP
This uses assignment syntax appropriate for the current language
(VAR = EXP or VAR := EXP for example)
...
set architecture -- Set architecture of target.
...

(gdb) set var $i = 1

(gdb) printf "i:%d\n", $i
i:1
(gdb) shell tail ${LOG_DIR}/logFile
set watchdog -- Set watchdog timer.
set width -- Set number of characters where GDB should wrap lines of its output.
set write -- Set writing into executable and core files.
....
Command name abbreviations are allowed if unambiguous.
...
i:1

The patch to gdb9.1 is here; the changed lines are bracket by
#if defined ALLOW_ENV_VARS
#endif

patch:

*** fsf/cli-cmds.c      2022-09-22 15:59:17.238371000 -0700
--- modifiedFsf/cli-cmds.c      2022-09-22 16:29:59.506242000 -0700
***************
*** 660,665 ****
--- 660,744 ----
    script_from_file (stream, file);
  }

+ #define ALLOW_ENV_VARS 1
+ #if defined ALLOW_ENV_VARS
+ /* return true if s uses one or more env vars, specified as ${VARNAME} */
+ extern int
+ contains_env_var(const char *s0)
+ {
+   return s0 && (strlen (s0) > 1) && (strstr(s0, "${") != NULL) &&
+         (strchr(s0, '}') != NULL);
+ }
+
+ /* substitute_env_var - given a string, return a copy of the string with
+    env vars interpolated (if any).  env vars are recognized by the
+    syntax ...${VARNAME}...
+ */
+
+
+ extern char *
+ substitute_env_var(const char *s0, const char* msg, const int from_tty)
+ {
+     int slen, i;
+
+     char *s = xstrdup(s0);
+     slen = strlen(s);
+
+     for (i=0; i<slen; i++)
+       {
+       // find the lead-in for an environment variable
+       if ((s[i] == '$') && (s[i+1] == '{'))
+         {
+           char *endbrace = index(s+i+2, '}');
+           if (endbrace == NULL)
+             {
+               break;
+             }
+           else
+             {
+               int e = endbrace - s;
+
+               //    ...${X}...
+               //       ^  ^
+               //       i  e
+
+               int varname_len = e-(i+2);
+               char *varname = xstrndup(s+i+2, varname_len);
+               char *var_value = getenv(varname);
+               if (var_value == NULL)
+                 {
+                   if (from_tty)
+                   {
+                     fprintf_unfiltered
+                       (gdb_stderr,
+                        "Env var '%s' not set but used in '%s'.\n", varname, s0);
+                   }
+                   var_value = (char*)"";
+                 }
+               xfree(varname);
+               {
+                 // resize s to hold the substitution of varname
+                 int delta = strlen(var_value) - (e-i+1);
+
+                 // new_s may be shorter than s
+                 char *new_s = (char*) xmalloc(slen+delta+1);
+                 memcpy(new_s, s, i);
+                 memcpy(new_s+i, var_value, strlen(var_value));
+                 memmove(new_s+e+1+delta, s+e+1, slen-(e+1)+1);  // include \0
+
+                 // Skip ahead to new string at pos right after replacement so
+                 // we don't try recursive replacement so avoid infinite loops.
+                 i += strlen(var_value) - 1;  // -1 to offset i++ in for()
+                 slen += delta;
+                 xfree(s);
+                 s = new_s;
+               }
+             }
+         }
+       }
+     return s;
+ }
+ #endif // ALLOW_ENV_VARS
+
  /* Worker to perform the "source" command.
     Load script FILE.
     If SEARCH_PATH is non-zero, and the file isn't found in cwd,
***************
*** 672,677 ****
--- 751,764 ----
    if (file == NULL || *file == 0)
      error (_("source command requires file name of file to source."));

+ #if defined ALLOW_ENV_VARS
+   /* support env var interpolation in source filename in user/kernel/native*/
+   if (contains_env_var(file))
+     {
+       file = substitute_env_var(file, "'source'-ing", from_tty);
+     }
+ #endif
+
    gdb::optional<open_script> opened = find_and_open_script (file, search_path);
    if (!opened)
      {
***************
*** 844,849 ****
--- 931,945 ----
        if (!arg)
        execl (user_shell, p, (char *) 0);
        else
+ #if defined ALLOW_ENV_VARS
+       /* support interpolation of env vars in shell arg */
+       if (contains_env_var(arg))
+         {
+           char* newArg = substitute_env_var(arg, "exec'ing cmd", from_tty);
+           execl (user_shell, p, "-c", newArg, (char *) 0); //do as usual
+           xfree(newArg);
+         } else
+ #endif //ALLOW_ENV_VARS
        execl (user_shell, p, "-c", arg, (char *) 0);

        fprintf_unfiltered (gdb_stderr, "Cannot execute %s: %s\n", user_shell,

*** fsf/cli-logging.c   2022-09-22 15:59:17.250302000 -0700
--- modifiedFsf/cli-logging.c   2022-09-22 16:26:12.482260000 -0700
***************
*** 139,144 ****
--- 139,151 ----
      current_uiout->redirect (gdb_stdout);
  }

+ #define ALLOW_ENV_VARS 1
+ #if defined (ALLOW_ENV_VARS)
+ extern int contains_env_var(const char *s0);
+ extern char * substitute_env_var
+ (const char *s0, const char* msg, const int from_tty);
+ #endif
+
  static void
  set_logging_on (const char *args, int from_tty)
  {
***************
*** 149,154 ****
--- 156,173 ----
        xfree (logging_filename);
        logging_filename = xstrdup (rest);
      }
+
+ #if defined (ALLOW_ENV_VARS)
+   // recognize ${<envVariable>} in log file name
+   else
+     {
+       if (contains_env_var(logging_filename))
+       {
+         logging_filename = substitute_env_var
+           (logging_filename, "Logging to", from_tty);
+       }
+     }
+ #endif // ALLOW_ENV_VARS
    handle_redirections (from_tty);
  }