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copied!<p><sup><em>Previously, the answer was presented with what's now the first section as the last section.</em></sup></p> <h3>POSIX Shell includes a <code>!</code> operator</h3> <p>Poking around the shell specification for other issues, I recently (September 2015) noticed that the POSIX shell supports a <code>!</code> operator. For example, it is listed as a <a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_04" rel="noreferrer">reserved word</a> and can appear at the start of a <a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_02" rel="noreferrer">pipeline</a> — where a simple command is a special case of 'pipeline'. It can, therefore, be used in <code>if</code> statements and <code>while</code> or <code>until</code> loops too — in POSIX-compliant shells. Consequently, despite my reservations, it is probably more widely available than I realized back in 2008. A quick check of POSIX 2004 and SUS/POSIX 1997 shows that <code>!</code> was present in both those versions.</p> <p>Note that the <code>!</code> operator must appear at the <em>beginning</em> of the pipeline and negates the status code of the entire pipeline (i.e. the <em>last</em> command). Here are some examples.</p> <pre><code># Simple commands, pipes, and redirects work fine. $ ! some-command succeed; echo $? 1 $ ! some-command fail | some-other-command fail; echo $? 0 $ ! some-command < succeed.txt; echo $? 1 # Environment variables also work, but must come after the !. $ ! RESULT=fail some-command; echo $? 0 # A more complex example. $ if ! some-command < input.txt | grep Success > /dev/null; then echo 'Failure!'; recover-command; mv input.txt input-failed.txt; fi Failure! $ ls *.txt input-failed.txt </code></pre> <h3>Portable answer — works with antique shells</h3> <p>In a Bourne (Korn, POSIX, Bash) script, I use:</p> <pre><code>if ...command and arguments... then : it succeeded else : it failed fi </code></pre> <p>This is as portable as it gets. The 'command and arguments' can be a pipeline or other compound sequence of commands.</p> <h3>A <code>not</code> command</h3> <p>The '!' operator, whether built-in to your shell or provided by the o/s, is not universally available. It isn't dreadfully hard to write, though - the code below dates back to at least 1991 (though I think I wrote a previous version even longer ago). I don't tend to use this in my scripts, though, because it is not reliably available.</p> <pre><code>/* @(#)File: $RCSfile: not.c,v $ @(#)Version: $Revision: 4.2 $ @(#)Last changed: $Date: 2005/06/22 19:44:07 $ @(#)Purpose: Invert success/failure status of command @(#)Author: J Leffler @(#)Copyright: (C) JLSS 1991,1997,2005 */ #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <sys/wait.h> #include "stderr.h" #ifndef lint static const char sccs[] = "@(#)$Id: not.c,v 4.2 2005/06/22 19:44:07 jleffler Exp $"; #endif int main(int argc, char **argv) { int pid; int corpse; int status; err_setarg0(argv[0]); if (argc <= 1) { /* Nothing to execute. Nothing executed successfully. */ /* Inverted exit condition is non-zero */ exit(1); } if ((pid = fork()) < 0) err_syserr("failed to fork\n"); if (pid == 0) { /* Child: execute command using PATH etc. */ execvp(argv[1], &argv[1]); err_syserr("failed to execute command %s\n", argv[1]); /* NOTREACHED */ } /* Parent */ while ((corpse = wait(&status)) > 0) { if (corpse == pid) { /* Status contains exit status of child. */ /* If exit status of child is zero, it succeeded, and we should exit with a non-zero status */ /* If exit status of child is non-zero, if failed and we should exit with zero status */ exit(status == 0); /* NOTREACHED */ } } /* Failed to receive notification of child's death -- assume it failed */ return (0); } </code></pre> <p>This returns 'success', the opposite of failure, when it fails to execute the command. We can debate whether the 'do nothing successfully' option was correct; maybe it should report an error when it isn't asked to do anything. The code in '<code>"stderr.h"</code>' provides simple error reporting facilities - I use it everywhere. Source code on request - see my profile page to contact me.</p>

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