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