Format: support syntax of {$ENV_VAR} in custom format

Fix #1541

Author: CarterLi

CHANGELOG.md

@@ -16,6 +16,8 @@ Features:
 * Capitalize `{type}`'s first letter in custom format (#1543, Display)
 * Support model name detection for s390x (CPU, Linux)
 * Support more Armbian variants detection (#1547, OS, Linux)
+* Support the syntax of `{$ENV_VAR}` in custom format, which will be replaced by the value of the environment variable `ENV_VAR` (#1541)
+    * This is another way to pass 3rd-party data to fastfetch besides `Custom` module.
 
 Logo:
 * Update arch_old

src/common/format.c

@@ -259,24 +259,33 @@ void ffParseFormatString(FFstrbuf* buffer, const FFstrbuf* formatstr, uint32_t n
             continue;
         }
 
-        //test for constant, if so evaluate it
+        //test for constant or env var, if so evaluate it
         if (firstChar == '$')
         {
             char* pend = NULL;
             int32_t indexSigned = (int32_t) strtol(placeholderValue.chars + 1, &pend, 10);
-            uint32_t index = (uint32_t) indexSigned;
-            bool backward = indexSigned < 0;
-
-            if (indexSigned == 0 || *pend != '\0' || instance.config.display.constants.length < index)
+            if (pend == placeholderValue.chars + 1)
             {
-                appendInvalidPlaceholder(buffer, "{", &placeholderValue, i, formatstr->length);
-                continue;
+                // treat placeholder as an environment variable
+                char* envValue = getenv(placeholderValue.chars + 1);
+                if (envValue)
+                    ffStrbufAppendS(buffer, envValue);
+                else
+                    appendInvalidPlaceholder(buffer, "{", &placeholderValue, i, formatstr->length);
             }
+            else
+            {
+                // treat placeholder as a constant
+                uint32_t index = (uint32_t) (indexSigned < 0 ? (int32_t) instance.config.display.constants.length + indexSigned : indexSigned - 1);
 
-            FFstrbuf* item = FF_LIST_GET(FFstrbuf, instance.config.display.constants, backward
-                ? instance.config.display.constants.length - index
-                : index - 1);
-            ffStrbufAppend(buffer, item);
+                if (*pend != '\0' || instance.config.display.constants.length <= index)
+                    appendInvalidPlaceholder(buffer, "{", &placeholderValue, i, formatstr->length);
+                else
+                {
+                    FFstrbuf* item = FF_LIST_GET(FFstrbuf, instance.config.display.constants, index);
+                    ffStrbufAppend(buffer, item);
+                }
+            }
             continue;
         }
 

src/data/help_format.txt

@@ -21,6 +21,8 @@ In 2.24.0 or newer, `{~startIndex,endIndex}` can be specified to slice a string.
 If an index is omitted, 0 is used. For example, both `{~,0}` `{~0,}` and `{~,}` are same as `{~0,0}` and will always generate a empty string.
 If `,endIndex` is omitted or greater than the length of the string, the length of string is used.
 
+In 2.36.0 or newer, `{$NUM}` can be specified to reference a constant defined in `display.constants`. `{$ENV_VAR}` can be specified to reference an environment variable.
+
 If the value index is missing, meaning the placeholder is "{}", an internal counter sets the value index.
 This means that the format string "Values: {1} ({2})" is equivalent to "Values: {} ({})".
 Note that this counter only counts empty placeholders, so the format string "{2} {} {}" will contain the second value, then the first, and then the second again.

tests/format.c

@@ -113,6 +113,24 @@ int main(void)
     VERIFY("output({?1}OK{?}{/1}NOT OK{/})", "", "output(NOT OK)");
     }
 
+    {
+        ffListInit(&instance.config.display.constants, sizeof(FFstrbuf));
+        ffStrbufSetStatic(ffListAdd(&instance.config.display.constants), "CONST1");
+        ffStrbufSetStatic(ffListAdd(&instance.config.display.constants), "CONST2");
+        setenv("FF_TEST", "ENVVAR", 1);
+        VERIFY("output({$FF_TEST})", "", "output(ENVVAR)");
+        VERIFY("output({$1})", "", "output(CONST1)");
+        VERIFY("output({$FF_TEST}{$1})", "", "output(ENVVARCONST1)");
+        VERIFY("output({$1}{$FF_TEST})", "", "output(CONST1ENVVAR)");
+        VERIFY("output({$FF_TEST}{$FF_TEST})", "", "output(ENVVARENVVAR)");
+        VERIFY("output({$1}{$-1})", "", "output(CONST1CONST2)");
+
+        VERIFY("output({$FF_INVAL})", "", "output({$FF_INVAL})");
+        VERIFY("output({$9}{$0}${-9})", "", "output({$9}{$0}${-9})");
+        VERIFY("output({$1NO})", "", "output({$1NO})");
+        ffListDestroy(&instance.config.display.constants);
+    }
+
     //Success
     puts("\033[32mAll tests passed!" FASTFETCH_TEXT_MODIFIER_RESET);
 }