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env: add bash-style default value operator :- #11787

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fb18c82
env: add bash-style default value operator :-
benjaminmueggenburg-serato May 10, 2026
93acbd7
tests: internal: env: Add test cases for default value operator :-
benjaminmueggenburg-serato May 10, 2026
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31 changes: 30 additions & 1 deletion src/flb_env.c
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Original file line number Diff line number Diff line change
Expand Up @@ -311,6 +311,9 @@ const char *flb_env_get(struct flb_env *env, const char *key)
/*
* Given a 'value', lookup for variables, if found, return a new composed
* sds string.
*
* Supports bash-style default substitution inside ${...}:
* ${name:-word} — if unset or empty, expand word; do not assign.
*/
flb_sds_t flb_env_var_translate(struct flb_env *env, const char *value)
{
Expand All @@ -321,6 +324,9 @@ flb_sds_t flb_env_var_translate(struct flb_env *env, const char *value)
int pre_var;
int have_var = FLB_FALSE;
const char *env_var = NULL;
char *v_sep;
const char *def_val;
int def_len;
char *v_start = NULL;
char *v_end = NULL;
char tmp[4096];
Expand Down Expand Up @@ -358,6 +364,18 @@ flb_sds_t flb_env_var_translate(struct flb_env *env, const char *value)
strncpy(tmp, v_start, v_len);
tmp[v_len] = '\0';
have_var = FLB_TRUE;

/* Bash-style default value expansion :- */
v_sep = strstr(tmp, ":");
def_val = NULL;
def_len = 0;

if (v_sep && (v_sep[1] == '-')) {
def_val = v_sep + 2;
def_len = strlen(def_val);
*v_sep = '\0';
}


/* Append pre-variable content */
pre_var = (v_start - 2) - (value + i);
Expand All @@ -374,7 +392,8 @@ flb_sds_t flb_env_var_translate(struct flb_env *env, const char *value)

/* Lookup the variable in our env-hash */
env_var = flb_env_get(env, tmp);
if (env_var) {
/* Skip env if it's empty and have a fallback defined */
if (env_var && !(def_val && strlen(env_var) == 0)) {
e_len = strlen(env_var);
s = buf_append(buf, env_var, e_len);
if (!s) {
Expand All @@ -385,6 +404,16 @@ flb_sds_t flb_env_var_translate(struct flb_env *env, const char *value)
buf = s;
}
}
else if (def_val) {
if (def_len > 0) {
s = buf_append(buf, def_val, def_len);
if (!s) {
flb_sds_destroy(buf);
return NULL;
}
buf = s;
}
}
Comment on lines +407 to +416
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@cosmo0920 cosmo0920 May 12, 2026

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This is just a nitpick comment but bash shell also supports nested style of expanding variable like:
result=${A:-${B:-default}}.

Do we need to plan for this style of nested variable for default variable?
Just leaving as-is is also better to keep simplicity of implementation.

else if (env->warn_unused == FLB_TRUE) {
flb_warn("[env] variable ${%s} is used but not set", tmp);
}
Expand Down
155 changes: 155 additions & 0 deletions tests/internal/env.c
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Original file line number Diff line number Diff line change
Expand Up @@ -413,6 +413,158 @@ void test_file_env_var_missing()
flb_env_destroy(env);
}

/* ${name:-word} when unset or empty */
void test_expand_default_hyphen()
{
struct flb_env *env;
flb_sds_t buf = NULL;
const char *v;
int ret;

env = flb_env_create();
if (!TEST_CHECK(env != NULL)) {
TEST_MSG("flb_env_create failed");
return;
}

buf = flb_env_var_translate(env, "${FLB_IT_DEFHYPH_Z:-fallback}");
if (!TEST_CHECK(buf != NULL)) {
TEST_MSG("translate failed");
flb_env_destroy(env);
return;
}
if (!TEST_CHECK(strcmp(buf, "fallback") == 0)) {
TEST_MSG("expected fallback, got=%s", buf);
}
flb_sds_destroy(buf);

v = flb_env_get(env, "FLB_IT_DEFHYPH_Z");
if (!TEST_CHECK(v == NULL)) {
TEST_MSG(":- must not assign locally");
}

ret = flb_env_set(env, "FLB_IT_DEFHYPH_Z", "");
if (!TEST_CHECK(ret >= 0)) {
TEST_MSG("flb_env_set empty failed");
flb_env_destroy(env);
return;
}
buf = flb_env_var_translate(env, "${FLB_IT_DEFHYPH_Z:-empty_fallback}");
if (!TEST_CHECK(buf != NULL)) {
TEST_MSG("translate empty failed");
flb_env_destroy(env);
return;
}
if (!TEST_CHECK(strcmp(buf, "empty_fallback") == 0)) {
TEST_MSG("empty local should use default, got=%s", buf);
}
flb_sds_destroy(buf);
flb_env_destroy(env);
}


/* ${name:-} should return an empty string if name is unset or empty */
void test_expand_empty_default()
{
struct flb_env *env;
flb_sds_t buf = NULL;
int ret;

env = flb_env_create();
if (!TEST_CHECK(env != NULL)) {
TEST_MSG("flb_env_create failed");
return;
}

/* Case 1: Variable is unset */
buf = flb_env_var_translate(env, "val=${VAR_NOT_SET:-}");
if (!TEST_CHECK(buf != NULL)) {
TEST_MSG("translate failed");
flb_env_destroy(env);
return;
}
if (!TEST_CHECK(strcmp(buf, "val=") == 0)) {
TEST_MSG("expected 'val=', got='%s'", buf);
}
flb_sds_destroy(buf);

/* Case 2: Variable is explicitly set to empty string */
ret = flb_env_set(env, "VAR_EMPTY", "");
if (!TEST_CHECK(ret >= 0)) {
TEST_MSG("flb_env_set failed");
flb_env_destroy(env);
return;
}

buf = flb_env_var_translate(env, "${VAR_EMPTY:-}");
if (!TEST_CHECK(buf != NULL)) {
TEST_MSG("translate failed");
flb_env_destroy(env);
return;
}

/* Result should be length 0 */
if (!TEST_CHECK(strlen(buf) == 0)) {
TEST_MSG("expected empty string, got='%s'", buf);
}

flb_sds_destroy(buf);
flb_env_destroy(env);
}


void test_expand_default_hyphen_from_os()
{
struct flb_env *env;
flb_sds_t buf = NULL;
char *var_name = "FLB_IT_DEFHYPH_OS";
char *var_val = "from_process";
int ret;

/* Set OS environment inline */
#ifdef FLB_SYSTEM_WINDOWS
ret = _putenv_s(var_name, var_val);
#else
ret = setenv(var_name, var_val, 1);
#endif

if (!TEST_CHECK(ret == 0)) {
TEST_MSG("putenv failed");
return;
}

env = flb_env_create();
if (!TEST_CHECK(env != NULL)) {
TEST_MSG("flb_env_create failed");
#ifdef FLB_SYSTEM_WINDOWS
_putenv_s(var_name, "");
#else
unsetenv(var_name);
#endif
return;
}

buf = flb_env_var_translate(env, "${FLB_IT_DEFHYPH_OS:-fallback}");
if (!TEST_CHECK(buf != NULL)) {
TEST_MSG("translate failed");
}
else {
if (!TEST_CHECK(strcmp(buf, var_val) == 0)) {
TEST_MSG("expected from_process, got=%s", buf);
}
flb_sds_destroy(buf);
}

flb_env_destroy(env);

/* Unset OS environment inline */
#ifdef FLB_SYSTEM_WINDOWS
_putenv_s(var_name, "");
#else
unsetenv(var_name);
#endif
}


TEST_LIST = {
{ "translate_long_env" , test_translate_long_env},
Expand All @@ -421,5 +573,8 @@ TEST_LIST = {
{ "file_env_var_uri" , test_file_env_var_uri},
{ "mixed_env_vars" , test_mixed_env_vars},
{ "file_env_var_missing" , test_file_env_var_missing},
{ "expand_default_hyphen" , test_expand_default_hyphen},
{ "expand_empty_default", test_expand_empty_default},
{ "expand_default_hyphen_from_os", test_expand_default_hyphen_from_os},
{ NULL, NULL }
};
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