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hush: explain various parameter expansion ops in comments

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2010-05-20 21:46:45 +02:00
parent ee0775dd13
commit 53b513331a
1 changed files with 36 additions and 8 deletions
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44
shell/hush.c
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@ -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,