File: autoconf.info, Node: Limitations of Builtins, Next: Limitations of Usual Tools, Prev: Shell Functions, Up: Portable Shell 11.14 Limitations of Shell Builtins =================================== No, no, we are serious: some shells do have limitations! :) You should always keep in mind that any builtin or command may support options, and therefore differ in behavior with arguments starting with a dash. For instance, even the innocent ‘echo "$word"’ can give unexpected results when ‘word’ starts with a dash. It is often possible to avoid this problem using ‘echo "x$word"’, taking the ‘x’ into account later in the pipe. Many of these limitations can be worked around using M4sh (*note Programming in M4sh::). ‘.’ Use ‘.’ only with regular files (use ‘test -f’). Bash 2.03, for instance, chokes on ‘. /dev/null’. Remember that ‘.’ uses ‘PATH’ if its argument contains no slashes. Also, some shells, including bash 3.2, implicitly append the current directory to this ‘PATH’ search, even though Posix forbids it. So if you want to use ‘.’ on a file ‘foo’ in the current directory, you must use ‘. ./foo’. Not all shells gracefully handle syntax errors within a sourced file. On one extreme, some non-interactive shells abort the entire script. On the other, ‘zsh’ 4.3.10 has a bug where it fails to react to the syntax error. $ echo 'fi' > syntax $ bash -c '. ./syntax; echo $?' ./syntax: line 1: syntax error near unexpected token `fi' ./syntax: line 1: `fi' 2 $ ash -c '. ./syntax; echo $?' ./syntax: 1: Syntax error: "fi" unexpected $ zsh -c '. ./syntax; echo $?' ./syntax:1: parse error near `fi' 0 ‘!’ The Unix version 7 shell did not support negating the exit status of commands with ‘!’, and this feature is still absent from some shells (e.g., Solaris 10 ‘/bin/sh’). Other shells, such as FreeBSD ‘/bin/sh’ or ‘ash’, have bugs when using ‘!’: $ sh -c '! : | :'; echo $? 1 $ ash -c '! : | :'; echo $? 0 $ sh -c '! { :; }'; echo $? 1 $ ash -c '! { :; }'; echo $? {: not found Syntax error: "}" unexpected 2 Shell code like this: if ! cmp file1 file2 >/dev/null 2>&1; then echo files differ or trouble fi is therefore not portable in practice. Typically it is easy to rewrite such code, e.g.: cmp file1 file2 >/dev/null 2>&1 || echo files differ or trouble In M4sh, the ‘AS_IF’ macro provides an easy way to write these kinds of conditionals: AS_IF([cmp -s file file.new], [], [echo files differ or trouble]) This kind of rewriting is needed in code outside macro definitions that calls other macros. *Note Common Shell Constructs::. It is also useful inside macro definitions, where the “then” and “else” branches might contain macro arguments. More generally, one can always rewrite ‘! COMMAND’ as: AS_IF([COMMAND], [(exit 1)]) ‘&&’ and ‘||’ If an AND-OR list is not inside ‘AC_DEFUN’, and it contains calls to Autoconf macros, it should be rewritten using ‘AS_IF’. *Note Common Shell Constructs::. The operators ‘&&’ and ‘||’ have equal precedence and are left associative, so instead of: # This is dangerous outside AC_DEFUN. cmp a b >/dev/null 2>&1 && AS_ECHO([files are same]) >$tmpfile || AC_MSG_NOTICE([files differ, or echo failed]) you can use: # This is OK outside AC_DEFUN. AS_IF([AS_IF([cmp a b >/dev/null 2>&1], [AS_ECHO([files are same]) >$tmpfile], [false])], [AC_MSG_NOTICE([files differ, or echo failed])]) ‘{...}’ Bash 3.2 (and earlier versions) sometimes does not properly set ‘$?’ when failing to write redirected output of a compound command. This problem is most commonly observed with ‘{...}’; it does not occur with ‘(...)’. For example: $ bash -c '{ echo foo; } >/bad; echo $?' bash: line 1: /bad: Permission denied 0 $ bash -c 'while :; do echo; done >/bad; echo $?' bash: line 1: /bad: Permission denied 0 To work around the bug, prepend ‘:;’: $ bash -c ':;{ echo foo; } >/bad; echo $?' bash: line 1: /bad: Permission denied 1 Posix requires a syntax error if a brace list has no contents. However, not all shells obey this rule; and on shells where empty lists are permitted, the effect on ‘$?’ is inconsistent. To avoid problems, ensure that a brace list is never empty. $ bash -c 'false; { }; echo $?' || echo $? bash: line 1: syntax error near unexpected token `}' bash: line 1: `false; { }; echo $?' 2 $ zsh -c 'false; { }; echo $?' || echo $? 1 $ pdksh -c 'false; { }; echo $?' || echo $? 0 ‘break’ The use of ‘break 2’ etc. is safe. ‘case’ If a ‘case’ command is not inside ‘AC_DEFUN’, and it contains calls to Autoconf macros, it should be rewritten using ‘AS_CASE’. *Note Common Shell Constructs::. Instead of: # This is dangerous outside AC_DEFUN. case $filename in *.[ch]) AC_MSG_NOTICE([C source file]);; esac use: # This is OK outside AC_DEFUN. AS_CASE([$filename], [[*.[ch]]], [AC_MSG_NOTICE([C source file])]) You don't need to quote the argument; no splitting is performed. You don't need the final ‘;;’, but you should use it. Posix requires support for ‘case’ patterns with opening parentheses like this: case $file_name in (*.c) echo "C source code";; esac but the ‘(’ in this example is not portable to a few obsolescent Bourne shell implementations, which is a pity for those of us using tools that rely on balanced parentheses. For instance, with Solaris 10 ‘/bin/sh’: $ case foo in (foo) echo foo;; esac error→syntax error: `(' unexpected The leading ‘(’ can be omitted safely. Unfortunately, there are contexts where unbalanced parentheses cause other problems, such as when using a syntax-highlighting editor that searches for the balancing counterpart, or more importantly, when using a case statement as an underquoted argument to an Autoconf macro. *Note Balancing Parentheses::, for trade-offs involved in various styles of dealing with unbalanced ‘)’. Zsh handles pattern fragments derived from parameter expansions or command substitutions as though quoted: $ pat=\?; case aa in ?$pat) echo match;; esac $ pat=\?; case a? in ?$pat) echo match;; esac match Because of a bug in its ‘fnmatch’, Bash fails to properly handle backslashes in character classes: bash-2.02$ case /tmp in [/\\]*) echo OK;; esac bash-2.02$ This is extremely unfortunate, since you are likely to use this code to handle Posix or MS-DOS absolute file names. To work around this bug, always put the backslash first: bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac OK bash-2.02$ case /tmp in [\\/]*) echo OK;; esac OK Many Bourne shells cannot handle closing brackets in character classes correctly. Some shells also have problems with backslash escaping in case you do not want to match the backslash: both a backslash and the escaped character match this pattern. To work around this, specify the character class in a variable, so that quote removal does not apply afterwards, and the special characters don't have to be backslash-escaped: $ case '\' in [\<]) echo OK;; esac OK $ scanset='[<]'; case '\' in $scanset) echo OK;; esac $ Even with this, Solaris ‘ksh’ matches a backslash if the set contains any of the characters ‘|’, ‘&’, ‘(’, or ‘)’. Conversely, Tru64 ‘ksh’ (circa 2003) erroneously always matches a closing parenthesis if not specified in a character class: $ case foo in *\)*) echo fail ;; esac fail $ case foo in *')'*) echo fail ;; esac fail Some shells, such as Ash 0.3.8, are confused by an empty ‘case’/‘esac’: ash-0.3.8 $ case foo in esac; error→Syntax error: ";" unexpected (expecting ")") Posix requires ‘case’ to give an exit status of 0 if no cases match. However, ‘/bin/sh’ in Solaris 10 does not obey this rule. Meanwhile, it is unclear whether a case that matches, but contains no statements, must also change the exit status to 0. The M4sh macro ‘AS_CASE’ works around these inconsistencies. $ bash -c 'case `false` in ?) ;; esac; echo $?' 0 $ /bin/sh -c 'case `false` in ?) ;; esac; echo $?' 255 ‘cd’ Posix 1003.1-2001 requires that ‘cd’ must support the ‘-L’ ("logical") and ‘-P’ ("physical") options, with ‘-L’ being the default. However, traditional shells do not support these options, and their ‘cd’ command has the ‘-P’ behavior. Portable scripts should assume neither option is supported, and should assume neither behavior is the default. This can be a bit tricky, since the Posix default behavior means that, for example, ‘ls ..’ and ‘cd ..’ may refer to different directories if the current logical directory is a symbolic link. It is safe to use ‘cd DIR’ if DIR contains no ‘..’ components. Also, Autoconf-generated scripts check for this problem when computing variables like ‘ac_top_srcdir’ (*note Configuration Actions::), so it is safe to ‘cd’ to these variables. Posix states that behavior is undefined if ‘cd’ is given an explicit empty argument. Some shells do nothing, some change to the first entry in ‘CDPATH’, some change to ‘HOME’, and some exit the shell rather than returning an error. Unfortunately, this means that if ‘$var’ is empty, then ‘cd "$var"’ is less predictable than ‘cd $var’ (at least the latter is well-behaved in all shells at changing to ‘HOME’, although this is probably not what you wanted in a script). You should check that a directory name was supplied before trying to change locations. *Note Special Shell Variables::, for portability problems involving ‘cd’ and the ‘CDPATH’ environment variable. Also please see the discussion of the ‘pwd’ command. ‘echo’ The simple ‘echo’ is probably the most surprising source of portability troubles. It is not possible to use ‘echo’ portably unless both options and escape sequences are omitted. Don't expect any option. Do not use backslashes in the arguments, as there is no consensus on their handling. For ‘echo '\n' | wc -l’, the ‘sh’ of Solaris 10 outputs 2, but Bash and Zsh (in ‘sh’ emulation mode) output 1. The problem is truly ‘echo’: all the shells understand ‘'\n'’ as the string composed of a backslash and an ‘n’. Within a command substitution, ‘echo 'string\c'’ will mess up the internal state of ksh88 on AIX 6.1 so that it will print the first character ‘s’ only, followed by a newline, and then entirely drop the output of the next echo in a command substitution. Because of these problems, do not pass a string containing arbitrary characters to ‘echo’. For example, ‘echo "$foo"’ is safe only if you know that FOO's value cannot contain backslashes and cannot start with ‘-’. Normally, ‘printf’ is safer and easier to use than ‘echo’ and ‘echo -n’. Thus, you should use ‘printf "%s\n"’ instead of ‘echo’, and similarly use ‘printf %s’ instead of ‘echo -n’. Older scripts, written before ‘printf’ was portable, sometimes used a here-document as a safer alternative to ‘echo’, like this: cat <1 | head -n1 sh: syntax error at line 1: `;' unexpected $ make bad list='a b' a b $ make good $ make good list='a b' a b In Solaris 10 ‘/bin/sh’, when the list of arguments of a ‘for’ loop starts with _unquoted_ tokens looking like variable assignments, the loop is not executed on those tokens: $ /bin/sh -c 'for v in a=b c=d x e=f; do echo $v; done' x e=f Thankfully, quoting the assignment-like tokens, or starting the list with other tokens (including unquoted variable expansion that results in an assignment-like result), avoids the problem, so it is easy to work around: $ /bin/sh -c 'for v in "a=b"; do echo $v; done' a=b $ /bin/sh -c 'x=a=b; for v in $x c=d; do echo $v; done' a=b c=d ‘if’ If an ‘if’ command is not inside ‘AC_DEFUN’, and it contains calls to Autoconf macros, it should be rewritten using ‘AS_IF’. *Note Common Shell Constructs::. Using ‘if ! ...’ is not portable. *Note ‘!’ notes: !. Some very old shells did not reset the exit status from an ‘if’ with no ‘else’: $ if (exit 42); then true; fi; echo $? 42 whereas a proper shell should have printed ‘0’. Although this is no longer a portability problem, as any shell that supports functions gets it correct, it explains why some makefiles have lengthy constructs: if test -f "$file"; then install "$file" "$dest" else : fi ‘printf’ A format string starting with a ‘-’ can cause problems. Bash interprets it as an option and gives an error. And ‘--’ to mark the end of options is not good in the NetBSD Almquist shell (e.g., 0.4.6) which takes that literally as the format string. Putting the ‘-’ in a ‘%c’ or ‘%s’ is probably easiest: printf %s -foo AIX 7.2 ‘sh’ mishandles octal escapes in multi-byte locales by treating them as characters instead of bytes. For example, in a locale using the UTF-8 encoding, ‘printf '\351'’ outputs the two bytes C3, A9 (the UTF-8 encoding for U+00E9) instead of the desired single byte E9. To work around the bug, use the C locale. Bash 2.03 mishandles an escape sequence that happens to evaluate to ‘%’: $ printf '\045' bash: printf: `%': missing format character Large outputs may cause trouble. On Solaris 10, for example, ‘/usr/bin/printf’ is buggy, so when using ‘/bin/sh’ the command ‘printf %010000x 123’ normally dumps core. Since ‘printf’ is not always a shell builtin, there is a potential speed penalty for using ‘printf '%s\n'’ as a replacement for an ‘echo’ that does not interpret ‘\’ or leading ‘-’. With Solaris ‘ksh’, it is possible to use ‘print -r --’ for this role instead. *Note Limitations of Shell Builtins: echo, for a discussion of portable alternatives to both ‘printf’ and ‘echo’. ‘pwd’ With modern shells, plain ‘pwd’ outputs a "logical" directory name, some of whose components may be symbolic links. These directory names are in contrast to "physical" directory names, whose components are all directories. Posix 1003.1-2001 requires that ‘pwd’ must support the ‘-L’ ("logical") and ‘-P’ ("physical") options, with ‘-L’ being the default. However, traditional shells do not support these options, and their ‘pwd’ command has the ‘-P’ behavior. Portable scripts should assume neither option is supported, and should assume neither behavior is the default. Also, on many hosts ‘/bin/pwd’ is equivalent to ‘pwd -P’, but Posix does not require this behavior and portable scripts should not rely on it. Typically it's best to use plain ‘pwd’. On modern hosts this outputs logical directory names, which have the following advantages: • Logical names are what the user specified. • Physical names may not be portable from one installation host to another due to network file system gymnastics. • On modern hosts ‘pwd -P’ may fail due to lack of permissions to some parent directory, but plain ‘pwd’ cannot fail for this reason. Also please see the discussion of the ‘cd’ command. ‘read’ No options are portable, not even support ‘-r’ (Solaris 10 ‘/bin/sh’ for example). Tru64/OSF 5.1 ‘sh’ treats ‘read’ as a special built-in, so it may exit if input is redirected from a non-existent or unreadable file. ‘set’ With the FreeBSD 6.0 shell, the ‘set’ command (without any options) does not sort its output. The ‘set’ builtin faces the usual problem with arguments starting with a dash. Modern shells such as Bash or Zsh understand ‘--’ to specify the end of the options (any argument after ‘--’ is a parameter, even ‘-x’ for instance), but many traditional shells (e.g., Solaris 10 ‘/bin/sh’) simply stop option processing as soon as a non-option argument is found. Therefore, use ‘dummy’ or simply ‘x’ to end the option processing, and use ‘shift’ to pop it out: set x $my_list; shift Avoid ‘set -’, e.g., ‘set - $my_list’. Posix no longer requires support for this command, and in traditional shells ‘set - $my_list’ resets the ‘-v’ and ‘-x’ options, which makes scripts harder to debug. Some nonstandard shells do not recognize more than one option (e.g., ‘set -e -x’ assigns ‘-x’ to the command line). It is better to combine them: set -ex The ‘-e’ option has historically been under-specified, with enough ambiguities to cause numerous differences across various shell implementations; see for example this overview (https://www.in-ulm.de/~mascheck/various/set-e/), or this link (https://www.austingroupbugs.net/view.php?id=52), documenting a change to Posix 2008 to match ‘ksh88’ behavior. Note that mixing ‘set -e’ and shell functions is asking for surprises: set -e doit() { rm file echo one } doit || echo two According to the recommendation, ‘one’ should always be output regardless of whether the ‘rm’ failed, because it occurs within the body of the shell function ‘doit’ invoked on the left side of ‘||’, where the effects of ‘set -e’ are not enforced. Likewise, ‘two’ should never be printed, since the failure of ‘rm’ does not abort the function, such that the status of ‘doit’ is 0. The BSD shell has had several problems with the ‘-e’ option. Older versions of the BSD shell (circa 1990) mishandled ‘&&’, ‘||’, ‘if’, and ‘case’ when ‘-e’ was in effect, causing the shell to exit unexpectedly in some cases. This was particularly a problem with makefiles, and led to circumlocutions like ‘sh -c 'test -f file || touch file'’, where the seemingly-unnecessary ‘sh -c '...'’ wrapper works around the bug (*note Failure in Make Rules::). Even relatively-recent versions of the BSD shell (e.g., OpenBSD 3.4) wrongly exit with ‘-e’ if the last command within a compound statement fails and is guarded by an ‘&&’ only. For example: #! /bin/sh set -e foo='' test -n "$foo" && exit 1 echo one if :; then test -n "$foo" && exit 1 echo two test -n "$foo" && exit 1 fi echo three does not print ‘three’. One workaround is to change the last instance of ‘test -n "$foo" && exit 1’ to be ‘if test -n "$foo"; then exit 1; fi’ instead. Another possibility is to warn BSD users not to use ‘sh -e’. When ‘set -e’ is in effect, a failed command substitution in Solaris 10 ‘/bin/sh’ cannot be ignored, even with ‘||’. $ /bin/sh -c 'set -e; foo=`false` || echo foo; echo bar' $ bash -c 'set -e; foo=`false` || echo foo; echo bar' foo bar Moreover, a command substitution, successful or not, causes this shell to exit from a failing outer command even in presence of an ‘&&’ list: $ bash -c 'set -e; false `true` && echo notreached; echo ok' ok $ sh -c 'set -e; false `true` && echo notreached; echo ok' $ Portable scripts should not use ‘set -e’ if ‘trap’ is used to install an exit handler. This is because Tru64/OSF 5.1 ‘sh’ sometimes enters the trap handler with the exit status of the command prior to the one that triggered the errexit handler: $ sh -ec 'trap '\''echo $?'\'' 0; false' 0 $ sh -c 'set -e; trap '\''echo $?'\'' 0; false' 1 Thus, when writing a script in M4sh, rather than trying to rely on ‘set -e’, it is better to use ‘AS_EXIT’ where it is desirable to abort on failure. Job control is not provided by all shells, so the use of ‘set -m’ or ‘set -b’ must be done with care. When using ‘zsh’ in native mode, asynchronous notification (‘set -b’) is enabled by default, and using ‘emulate sh’ to switch to Posix mode does not clear this setting (although asynchronous notification has no impact unless job monitoring is also enabled). Also, ‘zsh’ 4.3.10 and earlier have a bug where job control can be manipulated in interactive shells, but not in subshells or scripts. Furthermore, some shells, like ‘pdksh’, fail to treat subshells as interactive, even though the parent shell was. $ echo $ZSH_VERSION 4.3.10 $ set -m; echo $? 0 $ zsh -c 'set -m; echo $?' set: can't change option: -m $ (set -m); echo $? set: can't change option: -m 1 $ pdksh -ci 'echo $-; (echo $-)' cim c Use of ‘set -n’ (typically via ‘sh -n script’) to validate a script is not foolproof. Modern ‘ksh93’ tries to be helpful by informing you about better syntax, but switching the script to use the suggested syntax in order to silence the warnings would render the script no longer portable to older shells: $ ksh -nc '``' ksh: warning: line 1: `...` obsolete, use $(...) 0 Autoconf itself uses ‘sh -n’ within its testsuite to check that correct scripts were generated, but only after first probing for other shell features (such as ‘test ${BASH_VERSION+y}’) that indicate a reasonably fast and working implementation. ‘shift’ Not only is ‘shift’ing a bad idea when there is nothing left to shift, but in addition it is not portable: the shell of MIPS RISC/OS 4.52 refuses to do it. Don't use ‘shift 2’ etc.; while it in the SVR1 shell (1983), it is also absent in many pre-Posix shells. ‘source’ This command is not portable, as Posix does not require it; use ‘.’ instead. ‘test’ The ‘test’ program is the way to perform many file and string tests. It is often invoked by the alternate name ‘[’, but using that name in Autoconf code is asking for trouble since it is an M4 quote character. The ‘-a’, ‘-o’, ‘(’, and ‘)’ operands are not present in all implementations, and have been marked obsolete by Posix 2008. This is because there are inherent ambiguities in using them. For example, ‘test "$1" -a "$2"’ looks like a binary operator to check whether two strings are both non-empty, but if ‘$1’ is the literal ‘!’, then some implementations of ‘test’ treat it as a negation of the unary operator ‘-a’. Thus, portable uses of ‘test’ should never have more than four arguments, and scripts should use shell constructs like ‘&&’ and ‘||’ instead. If you combine ‘&&’ and ‘||’ in the same statement, keep in mind that they have equal precedence, so it is often better to parenthesize even when this is redundant. For example: # Not portable: test "X$a" = "X$b" -a \ '(' "X$c" != "X$d" -o "X$e" = "X$f" ')' # Portable: test "X$a" = "X$b" && { test "X$c" != "X$d" || test "X$e" = "X$f"; } ‘test’ does not process options like most other commands do; for example, it does not recognize the ‘--’ argument as marking the end of options. It is safe to use ‘!’ as a ‘test’ operator. For example, ‘if test ! -d foo; ...’ is portable even though ‘if ! test -d foo; ...’ is not. ‘test’ (files) To enable ‘configure’ scripts to support cross-compilation, they shouldn't do anything that tests features of the build system instead of the host system. But occasionally you may find it necessary to check whether some arbitrary file exists. To do so, use ‘test -f’, ‘test -r’, or ‘test -x’. Do not use ‘test -e’, because Solaris 10 ‘/bin/sh’ lacks it. To test for symbolic links on systems that have them, use ‘test -h’ rather than ‘test -L’; either form conforms to Posix 1003.1-2001, but ‘-h’ has been around longer. For historical reasons, Posix reluctantly allows implementations of ‘test -x’ that will succeed for the root user, even if no execute permissions are present. Furthermore, shells do not all agree on whether Access Control Lists should affect ‘test -r’, ‘test -w’, and ‘test -x’; some shells base test results strictly on the current user id compared to file owner and mode, as if by ‘stat(2)’; while other shells base test results on whether the current user has the given right, even if that right is only granted by an ACL, as if by ‘faccessat(2)’. Furthermore, there is a classic time of check to time of use race between any use of ‘test’ followed by operating on the just-checked file. Therefore, it is a good idea to write scripts that actually attempt an operation, and are prepared for the resulting failure if permission is denied, rather than trying to avoid an operation based solely on whether ‘test’ guessed that it might not be permitted. ‘test’ (strings) Posix says that ‘test "STRING"’ succeeds if STRING is not null, but this usage is not portable to traditional platforms like Solaris 10 ‘/bin/sh’, which mishandle strings like ‘!’ and ‘-n’. However, it _is_ portable to test if a variable is set to a non-empty value, by using ‘test ${var+y}’, since all known implementations properly distinguish between no arguments and a known-safe string of ‘y’. Posix also says that ‘test ! "STRING"’, ‘test -n "STRING"’ and ‘test -z "STRING"’ work with any string, but many shells (such as Solaris 10, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4, etc.) get confused if STRING looks like an operator: $ test -n = test: argument expected $ test ! -n test: argument expected $ test -z ")"; echo $? 0 Similarly, Posix says that both ‘test "STRING1" = "STRING2"’ and ‘test "STRING1" != "STRING2"’ work for any pairs of strings, but in practice this is not true for troublesome strings that look like operators or parentheses, or that begin with ‘-’. It is best to protect such strings with a leading ‘X’, e.g., ‘test "XSTRING" != X’ rather than ‘test -n "STRING"’ or ‘test ! "STRING"’. It is common to find variations of the following idiom: test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" && ACTION to take an action when a token matches a given pattern. Such constructs should be avoided by using: AS_CASE([$ac_feature], [[*[!-a-zA-Z0-9_]*]], [ACTION]) If the pattern is a complicated regular expression that cannot be expressed as a shell pattern, use something like this instead: expr "X$ac_feature" : 'X.*[^-a-zA-Z0-9_]' >/dev/null && ACTION ‘expr "XFOO" : "XBAR"’ is more robust than ‘echo "XFOO" | grep "^XBAR"’, because it avoids problems when ‘FOO’ contains backslashes. ‘trap’ It is safe to trap at least the signals 1, 2, 13, and 15. You can also trap 0, i.e., have the ‘trap’ run when the script ends (either via an explicit ‘exit’, or the end of the script). The trap for 0 should be installed outside of a shell function, or AIX 5.3 ‘/bin/sh’ will invoke the trap at the end of this function. Posix says that ‘trap - 1 2 13 15’ resets the traps for the specified signals to their default values, but many common shells (e.g., Solaris 10 ‘/bin/sh’) misinterpret this and attempt to execute a "command" named ‘-’ when the specified conditions arise. Posix 2008 also added a requirement to support ‘trap 1 2 13 15’ to reset traps, as this is supported by a larger set of shells, but there are still shells like ‘dash’ that mistakenly try to execute ‘1’ instead of resetting the traps. Therefore, there is no portable workaround, except for ‘trap - 0’, for which ‘trap '' 0’ is a portable substitute. Although Posix is not absolutely clear on this point, it is widely admitted that when entering the trap ‘$?’ should be set to the exit status of the last command run before the trap. The ambiguity can be summarized as: "when the trap is launched by an ‘exit’, what is the _last_ command run: that before ‘exit’, or ‘exit’ itself?" Bash considers ‘exit’ to be the last command, while Zsh and Solaris 10 ‘/bin/sh’ consider that when the trap is run it is _still_ in the ‘exit’, hence it is the previous exit status that the trap receives: $ cat trap.sh trap 'echo $?' 0 (exit 42); exit 0 $ zsh trap.sh 42 $ bash trap.sh 0 The portable solution is then simple: when you want to ‘exit 42’, run ‘(exit 42); exit 42’, the first ‘exit’ being used to set the exit status to 42 for Zsh, and the second to trigger the trap and pass 42 as exit status for Bash. In M4sh, this is covered by using ‘AS_EXIT’. The shell in FreeBSD 4.0 has the following bug: ‘$?’ is reset to 0 by empty lines if the code is inside ‘trap’. $ trap 'false echo $?' 0 $ exit 0 Fortunately, this bug only affects ‘trap’. Several shells fail to execute an exit trap that is defined inside a subshell, when the last command of that subshell is not a builtin. A workaround is to use ‘exit $?’ as the shell builtin. $ bash -c '(trap "echo hi" 0; /bin/true)' hi $ /bin/sh -c '(trap "echo hi" 0; /bin/true)' $ /bin/sh -c '(trap "echo hi" 0; /bin/true; exit $?)' hi Likewise, older implementations of ‘bash’ failed to preserve ‘$?’ across an exit trap consisting of a single cleanup command. $ bash -c 'trap "/bin/true" 0; exit 2'; echo $? 2 $ bash-2.05b -c 'trap "/bin/true" 0; exit 2'; echo $? 0 $ bash-2.05b -c 'trap ":; /bin/true" 0; exit 2'; echo $? 2 Be aware that a trap can be called from any number of places in your script, and therefore the trap handler should not make assumptions about shell state. For some examples, if your script temporarily modifies ‘IFS’, then the trap should include an initialization back to its typical value of space-tab-newline (autoconf does this for generated ‘configure’ files). Likewise, if your script changes the current working directory at some point after the trap is installed, then your trap cannot assume which directory it is in, and should begin by changing directories to an absolute path if that is important to the cleanup efforts (autotest does this for generated ‘testsuite’ files). ‘true’ Don't worry: as far as we know ‘true’ is portable. Nevertheless, it's not always a builtin (e.g., Bash 1.x), and the portable shell community tends to prefer using ‘:’. This has a funny side effect: when asked whether ‘false’ is more portable than ‘true’ Alexandre Oliva answered: In a sense, yes, because if it doesn't exist, the shell will produce an exit status of failure, which is correct for ‘false’, but not for ‘true’. Remember that even though ‘:’ ignores its arguments, it still takes time to compute those arguments. It is a good idea to use double quotes around any arguments to ‘:’ to avoid time spent in field splitting and file name expansion. ‘unset’ In some nonconforming shells (e.g., Solaris 10 ‘/bin/ksh’ and ‘/usr/xpg4/bin/sh’, NetBSD 5.99.43 sh, or Bash 2.05a), ‘unset FOO’ fails when ‘FOO’ is not set. This can interfere with ‘set -e’ operation. You can use FOO=; unset FOO if you are not sure that ‘FOO’ is set. A few ancient shells lack ‘unset’ entirely. For some variables such as ‘PS1’, you can use a neutralizing value instead: PS1='$ ' Usually, shells that do not support ‘unset’ need less effort to make the environment sane, so for example is not a problem if you cannot unset ‘CDPATH’ on those shells. However, Bash 2.01 mishandles ‘unset MAIL’ and ‘unset MAILPATH’ in some cases and dumps core. So, you should do something like ( (unset MAIL) || exit 1) >/dev/null 2>&1 && unset MAIL || : *Note Special Shell Variables::, for some neutralizing values. Also, see *note Limitations of Builtins: export, for the case of environment variables. ‘wait’ The exit status of ‘wait’ is not always reliable.
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