Running scripts across shells in *nix

A friend asked me about running csh scripts on Linux. Apparently, they were porting from Solaris to Linux (how did a great Operating system like Solaris end up here? Another of those grave mistakes Sun Microsoft did!!). The linux box had korn shell as the default shell(1). When they ran it, it was giving a lot of errors. They were thinking of rewriting the script in ksh syntax. This definitely looked like a momentous task. I knew you can run one shell from another. (Any doubt? Simply type csh when you are in K-Shell. Now, you are in C-Shell – as long as it’s installed). Then why was the script failing with a lot of errors? We sat down to troubleshoot. In the end it turned out to be a simple PATH issue! If you are facing similar issues with your old shell scripts, then read on!

If your machine doesn’t have the target shell, you need to install it first (2). There is a linux port of the C-Shell called tcsh. Download and install it. See here for instructions on how to. It’s pretty straight forward. Though the program is tcsh, the installation creates some symbolic links with the name csh in /usr/bin and /bin, so you can run it as just csh.

If you try to simply run your csh script in another shell (Bash or ksh), it will fail. There are many differences between the 2 shells. For e.g. to define variables in csh, you will use setenv. To do this in ksh (and in bash) you will have to do export. So a csh script will not run inside ksh shell and vice versa. You will have to either rewrite or force it to run inside the corresponding shell.

To force a script to run in a specific shell, you typically add shebang as the first line inside the script. This helps the script to run with the right script interpreter automatically. (instead of associating specific types with programs, as we do in Windows). But, this doesn’t work when you are running the script inside a different shell, it tries to interpret it using current shell. To run a script inside a target shell, you need to get into the target shell, then type the script to run. You can do this in one step, by “sourcing” the script into the target shell, by using the fast (-f) option as,

csh -f <script>

Now back the problem my friend had. She was indeed using the above syntax to run her C-Shell script. Still it failed with several “command not found” errors, particularly on date command. Hmm! This script used to work in C-Shell on Solaris and date is a common unix utility, it must exist everywhere! I went into C-Shell(just type csh on command prompt, it will switch to C-Shell) and tried date command. It wasn’t there! So, now we have a more specific issue. Find out where the date program is.

to do this, I typed “which date” on K-Shell and the mystery resolved. This command used to be in /usr/bin directory on Solaris and on the linux box it was in /bin. The PATH variable used in the script included /usr/bin, but not /bin. This was the reason why date command wasn’t working. Once we fixed the PATH, everything was fine again.

The lesson is, you don’t always have to rewrite your script when you are changing machines/shells. You *can* invoke any type of shell scripts, from any other type of shells. Chances are there is a port for your favorite shell on your new *nix machine.

Also, when you are getting “command not found” type of errors, try to use the which command to find out where a program is and try to include its path in the environment variable, $PATH. You can also look at the man pages for the command you cannot find. If the command is installed, man will list the path information also.

And finally, never assume anything in the *nix world!

(1) Suse Linux typically comes with Bash as the default shell. Since our standard is ksh, I think they must have installed and changed default to ksh. See here for how to switch shell associated with a login.

(2) C-shell is an older shell which was very popular in the early days of Unix. Several places have phased out of this, because of its limitations. Ksh or Bash is more modern and Bash is typically default on Linux boxes. So, unless there is a real need for it, or you are a C-Shell junkie, you don’t really want to switch to C-Shell. See here and here for some notes on why you shouldn’t be using C-shell.


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