53b513331
Denys Vlasenko
2010-05-20
Subject
hush: explain various parameter expansion ops in comments
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shell/hush.c+36 -8
| @@ -2614,8 +2614,16 @@ static NOINLINE int expand_vars_to_list(o_string *output, int n, char *arg, char | |
| debug_printf_expand("%s\n", val); | |
| } else if (exp_op) { | |
| if (exp_op == '%' || exp_op == '#') { | |
| + | /* Standard-mandated substring removal ops: |
| + | * ${parameter%word} - remove smallest suffix pattern |
| + | * ${parameter%%word} - remove largest suffix pattern |
| + | * ${parameter#word} - remove smallest prefix pattern |
| + | * ${parameter##word} - remove largest prefix pattern |
| + | * |
| + | * Word is expanded to produce a glob pattern. |
| + | * Then var's value is matched to it and matching part removed. |
| + | */ |
| if (val) { | |
| - | /* we need to do a pattern match */ |
| bool match_at_left; | |
| char *loc; | |
| scan_t scan = pick_scan(exp_op, *exp_word, &match_at_left); | |
| @@ -2629,18 +2637,38 @@ static NOINLINE int expand_vars_to_list(o_string *output, int n, char *arg, char | |
| *loc = '\0'; | |
| } | |
| } else { /* one of :-=+? */ | |
| + | //TODO: check validity of exp_op. Currently it can be anything. |
| //TODO: handle ${VAR:N[:M]} here. N, M can be expressions similar to $((EXPR)): 2+2, 2+var etc | |
| - | /* we need to do an expansion */ |
| - | int exp_test = (!val || ((exp_save == ':') && !val[0])); |
| + | /* Standard-mandated substitution ops: |
| + | * ${var?word} - indicate error if unset |
| + | * If var is unset, word (or a message indicating it is unset |
| + | * if word is null) is written to standard error |
| + | * and the shell exits with a non-zero exit status. |
| + | * Otherwise, the value of var is substituted. |
| + | * ${var-word} - use default value |
| + | * If var is unset, word is substituted. |
| + | * ${var=word} - assign and use default value |
| + | * If var is unset, word is assigned to var. |
| + | * In all cases, final value of var is substituted. |
| + | * ${var+word} - use alternative value |
| + | * If var is unset, null is substituted. |
| + | * Otherwise, word is substituted. |
| + | * |
| + | * Word is subjected to tilde expansion, parameter expansion, |
| + | * command substitution, and arithmetic expansion. |
| + | * If word is not needed, it is not expanded. |
| + | * |
| + | * Colon forms (${var:-word}, ${var:=word} etc) do the same, |
| + | * but also treat null var as if it is unset. |
| + | */ |
| + | int use_word = (!val || ((exp_save == ':') && !val[0])); |
| if (exp_op == '+') | |
| - | exp_test = !exp_test; |
| + | use_word = !use_word; |
| debug_printf_expand("expand: op:%c (null:%s) test:%i\n", exp_op, | |
| - | (exp_save == ':') ? "true" : "false", exp_test); |
| - | if (exp_test) { |
| + | (exp_save == ':') ? "true" : "false", use_word); |
| + | if (use_word) { |
| if (exp_op == '?') { | |
| //TODO: how interactive bash aborts expansion mid-command? | |
| - | /* ${var?[error_msg_if_unset]} */ |
| - | /* ${var:?[error_msg_if_unset_or_null]} */ |
| /* mimic bash message */ | |
| die_if_script("%s: %s", | |
| var, | |