0e81e488f
Denys Vlasenko
2010-05-17
Subject
shell/README: describe special builtins
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shell/README+82 -108
| @@ -1,108 +1,82 @@ | |
| - | Various bits of what is known about busybox shells, in no particular order. |
| - | |
| - | 2008-02-14 |
| - | ash: does not restore tty pgrp if killed by HUP. Symptom: Midnight Commander |
| - | is backgrounded if you started ash under it, and then killed it with HUP. |
| - | |
| - | 2007-11-23 |
| - | hush: fixed bogus glob handling; fixed exec <"$1"; added test and echo builtins |
| - | |
| - | 2007-06-13 |
| - | hush: exec <"$1" doesn't do parameter subst |
| - | |
| - | 2007-05-24 |
| - | hush: environment-related memory leak plugged, with net code size |
| - | decrease. |
| - | |
| - | 2007-05-24 |
| - | hush: '( echo ${name )' will show syntax error message, but prompt |
| - | doesn't return (need to press <enter>). Pressing Ctrl-C, <enter>, |
| - | '( echo ${name )' again, Ctrl-C segfaults. |
| - | |
| - | 2007-05-21 |
| - | hush: environment cannot be handled by libc routines as they are leaky |
| - | (by API design and thus unfixable): hush will leak memory in this script, |
| - | bash does not: |
| - | pid=$$ |
| - | while true; do |
| - | unset t; |
| - | t=111111111111111111111111111111111111111111111111111111111111111111111111 |
| - | export t |
| - | ps -o vsz,pid,comm | grep " $pid " |
| - | done |
| - | The fix is to not use setenv/putenv/unsetenv but manipulate env ourself. TODO. |
| - | hush: meanwhile, first three command subst bugs mentioned below are fixed. :) |
| - | |
| - | 2007-05-06 |
| - | hush: more bugs spotted. Comparison with bash: |
| - | bash-3.2# echo "TEST`date;echo;echo`BEST" |
| - | TESTSun May 6 09:21:05 CEST 2007BEST [we dont strip eols] |
| - | bash-3.2# echo "TEST`echo '$(echo ZZ)'`BEST" |
| - | TEST$(echo ZZ)BEST [we execute inner echo] |
| - | bash-3.2# echo "TEST`echo "'"`BEST" |
| - | TEST'BEST [we totally mess up this one] |
| - | bash-3.2# echo `sleep 5` |
| - | [Ctrl-C should work, Ctrl-Z should do nothing][we totally mess up this one] |
| - | bash-3.2# if true; then |
| - | > [Ctrl-C] |
| - | bash-3.2# [we re-issue "> "] |
| - | bash-3.2# if echo `sleep 5`; then |
| - | > true; fi [we execute sleep before "> "] |
| - | |
| - | 2007-05-04 |
| - | hush: made ctrl-Z/C work correctly for "while true; do true; done" |
| - | (namely, it backgrounds/interrupts entire "while") |
| - | |
| - | 2007-05-03 |
| - | hush: new bug spotted: Ctrl-C on "while true; do true; done" doesn't |
| - | work right: |
| - | # while true; do true; done |
| - | [1] 0 true <-- pressing Ctrl-C several times... |
| - | [2] 0 true |
| - | [3] 0 true |
| - | Segmentation fault |
| - | |
| - | 2007-05-03 |
| - | hush: update on "sleep 1 | exit 3; echo $?" bug. |
| - | parse_stream_outer() repeatedly calls parse_stream(). |
| - | parse_stream() is now fixed to stop on ';' in this example, |
| - | fixing it (parse_stream_outer() will call parse_stream() 1st time, |
| - | execute the parse tree, call parse_stream() 2nd time and execute the tree). |
| - | But it's not the end of story. |
| - | In more complex situations we _must_ parse way farther before executing. |
| - | Example #2: "{ sleep 1 | exit 3; echo $?; ...few_lines... } >file". |
| - | Because of redirection, we cannot execute 1st pipe before we parse it all. |
| - | We probably need to learn to store $var expressions in parse tree. |
| - | Debug printing of parse tree would be nice too. |
| - | |
| - | 2007-04-28 |
| - | hush: Ctrl-C and Ctrl-Z for single NOFORK commands are working. |
| - | Memory and other resource leaks (opendir) are not addressed |
| - | (testcase is "rm -i" interrupted by ctrl-c). |
| - | |
| - | 2007-04-21 |
| - | hush: "sleep 5 | sleep 6" + Ctrl-Z + fg seems to work. |
| - | "rm -i" + Ctrl-C, "sleep 5" + Ctrl-Z still doesn't work |
| - | for SH_STANDALONE case :( |
| - | |
| - | 2007-04-21 |
| - | hush: fixed non-backgrounding of "sleep 1 &" and totally broken |
| - | "sleep 1 | sleep 2 &". Noticed a bug where successive jobs |
| - | get numbers 1,2,3 even when job #1 has exited before job# 2 is started. |
| - | (bash reuses #1 in this case) |
| - | |
| - | 2007-04-21 |
| - | hush: "sleep 1 | exit 3; echo $?" prints 0 because $? is substituted |
| - | _before_ pipe gets executed!! run_list_real() already has "pipe;echo" |
| - | parsed and handed to it for execution, so it sees "pipe"; "echo 0". |
| - | |
| - | 2007-04-21 |
| - | hush: removed setsid() and made job control sort-of-sometimes-work. |
| - | Ctrl-C in "rm -i" works now except for SH_STANDALONE case. |
| - | "sleep 1 | exit 3" + "echo $?" works, "sleep 1 | exit 3; echo $?" |
| - | shows exitcode 0 (should be 3). "sleep 1 | sleep 2 &" fails horribly. |
| - | |
| - | 2007-04-14 |
| - | lash, hush: both do setsid() and as a result don't have ctty! |
| - | Ctrl-C doesn't work for any child (try rm -i), etc... |
| - | lash: bare ">file" doesn't create a file (hush works) |
| + | http://www.opengroup.org/onlinepubs/9699919799/ |
| + | Open Group Base Specifications Issue 7 |
| + | |
| + | |
| + | http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html |
| + | Shell & Utilities |
| + | |
| + | It says that any of the standard utilities may be implemented |
| + | as a regular shell built-in. It gives a list of utilities which |
| + | are usually implemented that way (and some of them can only |
| + | be implemented as built-ins, like "alias"): |
| + | |
| + | alias |
| + | bg |
| + | cd |
| + | command |
| + | false |
| + | fc |
| + | fg |
| + | getopts |
| + | jobs |
| + | kill |
| + | newgrp |
| + | pwd |
| + | read |
| + | true |
| + | umask |
| + | unalias |
| + | wait |
| + | |
| + | |
| + | http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html |
| + | Shell Command Language |
| + | |
| + | It says that shell must implement special built-ins. Special built-ins |
| + | differ from regular ones by the fact that variable assignments |
| + | done on special builtin is *PRESERVED*. That is, |
| + | |
| + | VAR=VAL special_builtin; echo $VAR |
| + | |
| + | should print VAL. |
| + | |
| + | (Another distinction is that an error in special built-in should |
| + | abort the shell, but this is not such a critical difference, |
| + | and moreover, at least bash's "set" does not follow this rule, |
| + | which is even codified in autoconf now...). |
| + | |
| + | List of special builtins: |
| + | |
| + | . file |
| + | : [argument...] |
| + | break [n] |
| + | continue [n] |
| + | eval [argument...] |
| + | exec [command [argument...]] |
| + | exit [n] |
| + | export name[=word]... |
| + | export -p |
| + | readonly name[=word]... |
| + | readonly -p |
| + | return [n] |
| + | set [-abCefhmnuvx] [-o option] [argument...] |
| + | set [+abCefhmnuvx] [+o option] [argument...] |
| + | set -- [argument...] |
| + | set -o |
| + | set +o |
| + | shift [n] |
| + | times |
| + | trap n [condition...] |
| + | trap [action condition...] |
| + | unset [-fv] name... |
| + | |
| + | In practice, no one uses this obscure feature - none of these builtins |
| + | gives any special reasons to play such dirty tricks. |
| + | |
| + | However. This section says that *function invocation* should act |
| + | similar to special built-in. That is, variable assignments |
| + | done on function invocation should be preserved after function invocation. |
| + | |
| + | This is significant: it is not unthinkable to want to run a function |
| + | with some variables set to special values. But because of the above, |
| + | it does not work: variable will "leak" out of the function. |