Have you tried to compile something that wasn’t in the repositories, or otherwise run a shell script, and gotten this error?

mark@atlantis:~/code/mpeg4ip-1.5.0.1$ ./bootstrap
dir: .
SDL appears to be installed
./bootstrap: 76: Syntax error: Bad fd number

This “Bad fd number” error occurs because Ubuntu Edgy uses dash as /bin/sh. In Ubuntu 6.06 “Dapper Drake” and earlier versions used bash instead, which ran scripts in a special backwards-compatibility mode that wasn’t truly backwards compatible (i.e. some bash-specific code was still accepted, even though it wasn’t part of the POSIX 1003.2 and 1003.2a specifications for shells). Ubuntu Edgy is the first version of Ubuntu that symlinks /bin/sh to /bin/dash instead of /bin/bash. The packages in the repositories have been checked for compatibility, but this may still cause problems with improperly-written third-party shell scripts.

Confused yet? There’s a simple solution: run the script through bash yourself.

mark@atlantis:~/code/mpeg4ip-1.5.0.1$ bash bootstrap

If you’re developing your own shell script and your users are reporting this error to you, you should change the first line of your script to #!/bin/bash, since you’ve been relying on bash-specific extensions without realizing it. Or better yet, take the time to learn what those extensions are, and fix them so your script works in POSIX-compliant shells. You can manually install dash and test compatibility by changing the first line of your script to #!/bin/dash, then change it back to #!/bin/sh once everything works again. I did this with my podencoder script and found a few surprises.

This tip has been brought to you by someone who knows just enough shell scripting to be dangerous.

Further reading:

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Seven comments here (latest comments)

  1. You’re running edgy already? Blimey.

    — Stuart Langridge #

  2. From the Dash as /bin/sh Ubuntu Wiki Page: The default user shell (that set by adduser) would remain as /bin/bash, as would the shell of existing accounts including root.
    Did you create a new account when you installed Edgy, or does that mean it doesn’t work as described?

    — Nicolas Chachereau #

  3. Most of the scripts use #!/bin/sh and on Edgy that is a link to /bin/dash so it does not interfer with the shell the user has only for scrpting.

    — 2tone #

  4. I typically manually run bash for shell scripts, except for a few of my own private scripts that require csh. However, when installing a package, there could be scripts that explicitly launch other scripts by calling sh.

    — W^L+ #

  5. What shell construct raises “Bad fd number” ? And why?

    — Nilton Volpato #

  6. The particular script where I noticed the problem was attempting to redirect FAAC’s help output (printed to stderr, I believe) to a file.

    faac --help >&faac_help

    This is the line that caused the “Bad fd number” error, but I don’t know enough about shell scripting to know why.

    — Mark #

  7. ‘>& file’ is a csh way to redirect both stdout and stderrto a file, and bash allows the same construct.

    sh uses >& in a more restrictive manner. When sh sees >&, it accepts only a file descriptor number after the >& and freaks out when it sees a non-number.

    The sh redirection of both stdout and stderr to a file is ‘> file 2>&1′, meaning redirect stdout to file and then redirect sdterr to stdout.

    — Matthew Ernest #

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