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Matt Livingston 185d33616e minor wording changes 2 years ago
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Curlfish - The CRLF shell

This is curlfish, a program just about as simple as they come. The name comes from an expansion of an abbreviation of "CR/LF shell" (CRLF shell -> crlfsh - > curlfish)

What is a "CR/LF shell"? And why did I bother doing this? Well, years ago as a sysadmin, a user came to me with a puzzling error: his shell script was doing nothing but printing this error message : File not found /bin/ksh

This was hard to understand, because /bin/ksh certainly did exist! After a little investigation, I discovered that he had transferred the file up to the unix server (running HP- UX 10.20, IIRC) via FTP from his windows desktop PC, and had done so in binary mode. Consequently, his script was full of CR/LF line endings, and when it ran, the shell found the shebang line "#!/bin/ksh^M" Naturally, when the shell could find no file of that name, it ceased executing the script, and printed this 4-part error message:

ksh: ./myscript: /bin/ksh^M: File not found
 ^        ^          ^             ^
 1        2          3             4

Of course, the ^M character was interpreted by the terminal as a carriage return, and the error message began overwriting itself from the start of the line. By a weird quirk of coincidence, part 4 of the error message was just long enough to completely overwrite parts 1 and 2 while leaving part 3 (mostly) intact, resulting in the slightly confusing message shown earlier.

So I hit upon the idea of installing a program called /bin/ksh^M that merely printed a newline and an error message explaining exactly what must have happened and then exited. The result was curlfish

In fact, curlfish is little more than "Hello World!". The only real hack is in the naming of files (filenames of symbolic links, actually). The actual binary may be installed wherever you like (e.g. /usr/local/bin) and you make multiple symlinks to it, named exactly the same as real shells and script interpreters, except followed by a carriage return character. Thus, when a script file is created under an OS which uses CRLF line terminations by default, the script has a valid interpreter to call via its #! line.

For example:

/bin/ksh^M -> /usr/local/bin/curlfish
/bin/csh^M -> /usr/local/bin/curlfish
/usr/bin/perl^M -> /usr/local/bin/curlfish
/usr/local/bin/python^M -> /usr/local/bin/curlfish

The actual creation of symlinks with those characters is (currently) left as an exercise for the reader. There are several ways of doing so, and they all require some form of root access to create the symlinks that match the system-installed shells (at least, they should. Your system is pretty well b0rked if you can do this without root access!)

I have also added a fairly distinctive return code (29), so that if scripts that call curlfish are called by methods other than an interactive shell invocation, that fact can be tested for and the error handled gracefully. Automated build systems might make use of this feature.

So here it is at last, a somewhat practical use for a "Hello World!" program!


blubrick, 05/10/16 (code actually written circa 2001)