Fault-Tolerant Shell (ftsh) Technical Manual

27 August 2004
Version 2.0.4

Front Matter
Introduction
Basics
Structures
Miscellaneous Features

Front Matter

Introduction

Shell scripts are a vital tool for integrating software. They are indispensable for rapid prototyping and system assembly. Yet, they are extraordinarily sensitive to errors. A missing file, a broken network, or a sick file server can cause a script to barrel through its actions with surprising results. It is possible to write error-safe scripts, but only with extraordinary discipline and complexity.

The Fault Tolerant Shell (ftsh) aims to solve this problem by combining the ease of scripting with precise error semantics. Ftsh is a balance between the flexibility and power of script languages and the precision of most compiled languages. Ftsh parses complete programs in order to eliminate run-time errors. An exception-like structure allows scripts to be both succinct and safe. A focus on timed repetition simplifies the most common form of recovery in a distributed system. A carefully-vetted set of language features limits the "surprises" that haunt system programmers.

As an example, consider this innocuous script written in Bourne Shell:

        #!/bin/sh

        cd /work/foo
        rm -rf bar
        cp -r /fresh/data .

Suppose that the /work filesystem is temporarily unavailable, perhaps due to an NFS failure. The cd command will fail and print a message on the console. The shell will ignore this error result -- it is primarily designed as a user interface tool -- and proceed to execute the rm and cp in the directory it happened to be before.

Naturally, we may attempt to head off these cases with code that checks error codes, attempts to recover, and so on. However, even the disciplined programmer that leaves no value unturned must admit that this makes shell scripts incomprehensible:

        #!/bin/sh

        for attempt in 1 2 3
               cd /work/foo
               if [ ! $? ]
               then
                       echo "cd failed, trying again..."
                       sleep 5
               else
                       break
               fi
        done

        if [ ! $? ]
        then
               echo "couldn't cd, giving up..."
               return 1
        fi

And that's just the first line!

If we accept that failure, looping, timeouts, and job cancellation are fundamental concerns in distributed systems, we may both simplify and strengthen programs by making them fundamental expressions in a programming language. These concepts are embodied in the simple try command:

        #!/usr/bin/ftsh

        try for 5 minutes every 30 seconds
                cd /work/foo
                rm -rf bar
                cp -r /fresh/data .
        end

Ftsh provides simple structures that encourage explicit acknowledgement of failure while maintaining the readability of script code. You might think of this as exceptions for scripts.

Want to learn more? This document is a short introduction to the fault tolerant shell. It quickly breezes over the language features in order to get started. You can learn more about the motivation for the language in "The Ethernet Approach to Grid Computing", available from the ftsh web page For a quick introduction, read on!

Basics

Simple Commands

An ftsh program is built up from simple commands. A simple command names a program to be executed, just sh or csh. The command is separated from its arguments by whitespaces. Quotation marks may be used to escape whitespace. For example:

       ls -l /etc/hosts

or:

       cat /etc/passwd

or:

       cp "This File" "That File"

As you may know, a command (a UNIX process) returns an integer known as its "exit code." Convention dictates that an exit code of zero indicates success while any other number indicates failure. Languages tend to differ in their mapping of integers to success or failure, so from here on, we will simply use the abstract terms "success" and "failure."

A command may also fail in a variety of other ways without returning an exit code. It may be killed by a signal, or it may fail to start altogether if the program does not exist or its image cannot be loaded. These cases are also considered failures.

Groups

A "group" is simply a list of commands. Each command only runs if the previous command succeeded. Let's return our first example:

        #!/usr/bin/ftsh

        cd /work/foo
        rm -rf bar
        cp -r /fresh/data .

This group succeeds only if every command in the group succeeds. So, if cd fails, then the whole group fails and no further commands are executed.

This is called the "brittle" property of ftsh. If anything goes wrong, then processing stops instantly. When something goes wrong, you will know it, and the program will not "run away" executing more commands blindly. We will see ways to contain the brittleness of a program below.

Ftsh itself has an exit code. Ftsh returns the result of the top-level group that makes up the program. So, if any command in the top-level group fails, then ftsh itself will fail. If they all succeed, then ftsh succeeds.

Try Statements

A try statement is used to contain and retry group failure. Here is a simple try statement:

        #!/usr/bin/ftsh

        try 5 times
               cd /work/foo
               rm -rf bar
               cp -r /fresh/data .
        end

The try statement attempts to execute the enclosed group until the conditions in its header expire. Here, the group will be attempted five times. Recall that a group fails as soon as any one command fails. So, if rm fails, then the try statement will stop and attempt the group again from the beginning.

If the five times are exceeded, then the try statement itself fails, and (if it is the top-level try-statement) the whole shell program itself will fail. If you prefer, you may catch and react to a try statement in a manner similar to an exception. The failure keyword may be used to cause a new exception, just like throw in other languages.

        try 5 times
               cd /work/foo
               rm -rf bar
               cp -r /fresh/data .
        catch
               echo "Oops, it failed.  Oh well!"
               failure
        end

Try statements come in several forms. They may limit the number of times the group is executed. For example:

        try for 10 times

A try statement may allow an unlimited number of loops, terminated by a maximum amount of time, given in seconds, minutes, hours, or days:

        try for 45 seconds

Both may be combined, yielding a try statement that stops when either the loop limit or the time limit expires:

        try for 3 days or 100 times

Note that such an statement does not limit the length of any single attempt to execute the contained group. If a single command is delayed for three days, the try statement will wait that long and then kill the command. To force individual attempts to be shorter, try statements may be nested. For example:

        try for 3 days or 100 times
                try for 1 time or 1 minute
                        /bin/big-simulation
                end
        end

Here, big-simulation will be executed for no more than a minute at a time. Such one-minute attempts will be tried for up to three days or one hundred attempts before the outer try statement fails.

By default, ftsh uses an exponential backoff. If a group fails once, ftsh will wait one second, and then try again. If it fails again, it will wait 2 seconds, then 4 seconds, and so on, doubling the waiting time after each failure, up to a maximum of one hour. This prevents failures from consuming excessive resources in fruitless retries.

If you prefer to have the retries occur at regular intervals (though we don't recommend it) use the every keyword to control how frequently errors are retried. For example:

        try for 3 days every 1 hour

        try for 10 times every 30 seconds
 
        try for 1 minute or 3 times every 15 seconds

If a time limit expires in the middle of a try statement, then the currently running command is forcibly cancelled. If an every clause is used, it merely ensures that each attempt is at least that long. However, group will not be cancelled merely to satisfy an every clause. To ensure that a single loop attempt will be time limited, you may combine two try statements as above:

        try for 3 days or 100 times every 1 minute
                try for 1 time or 1 minute
                        /bin/big-simulation
                end
        end

Try statements themselves return either success or failure in the same way as a simple command. If the enclosed group finally succeeds, then the try expression itself succeeds. If the try expression exhausts its attempts, then the try statement itself fails. We will make use of this success or failure value in the next section.

Cancelling a process is somewhat more complicated than one might think. For all the details on how this actually works, see the section on cancelling processes below.

In (almost) all cases, a try statement absolutely controls what comes inside of it. There are two ways for a subprogram to break out of the control of a try. The first is to invoke exit, which causes the entire ftsh process to exit immediately with the given exit code. The second is to call exec, which causes the given process to be run in place of the current shell process, thus voiding any surrounding controls.

Redirection

Ftsh uses Bourne shell style I/O redirection. For example:

        echo "hello" > outfile

...sends the output hello into the file outfile, likewise, ftsh supports many of the more arcane redirections of the Bourne shell, such as the redirection of explicit file descriptors:

        grep needle 0<infile 1>outfile 2>errfile

...appending to output files:

        grep needle >>outfile 2>>errfile

...redirection to an open file descriptor:

        grep needle >outfile 2>&1

...and redirection of both input and output at once:

        grep needle >& out-and-err-file

Variables

Ftsh provides variables similar to that of the Bourne shell. For example,

        name=Douglas
        echo "Hello, ${name}!"
        echo "Hello, $(name)!"
        echo "Hello, $name!"

Ftsh also allows variables to be the source and target of redirections. That is, a variable may act as a file! The benefit of this approach is that ftsh manages the storage and name space of variables for you. You don't have to worry about cleaning up or clashing with other programs.

Variable redirection looks just like file redirection, except a dash is put in front of the redirector. For example, For example, suppose that we want to capture the output of grep and then run it through sort:

        grep needle /tmp/haystack -> needles
        sort -< needles

This sort of operation takes the place of a pipeline, which ftsh does not have (yet). However, by using variables instead of pipelines, different retry conditions may be placed on each stage of the work:

        try for 5 times
                grep needle /tmp/haystack -> needles
        end
        try for 1 hour
                sort -< needles
        end

All of the variations on file redirection are available for variable redirection, including -> and 2-> and ->> and 2->> and ->& and ->>&.

Like the Bourne shell, several variable names are reserved. Simple integers are used to refer to the command line arguments given to ftsh itself. $$ names the current process. $# gives the number of arguments passed to the program. $* gives all of the unquoted current arguments, while "$@" gives all of the arguments individually quoted. The shift command can be used to pop off the first positional argument.

Variables are implemented by creating temporary files and immediately unlinking them after creation. Thus, no matter how ftsh exits -- even if it crashes -- the kernel deletes buffer space after you. This prevents both the namespace and garbage collection problem left by scripts that manually read and write to files.

Structures

Complex programs are built up by combining basic elements with programming structures. Ftsh has most of the decision elements of other programming languages, such as conditionals and loops. Each of these elements behaves in a very precise way with respect to successes and failures.

For-Statements

A for-statement executes a command group once for each word in a list. For example:

        for food in bread wine meatballs
                echo "I like ${food}"
        end

Of course, the list of items may also come from a variable:

        packages="bread.tar.gz wine.tar.gz meatballs.tar.gz"

        for p in ${packages
                echo "Unpacking package ${p}..."
                tar xvzf ${p}
        end

The more interesting variations are forany and forall. A forany attempts to make a group succeed once for any of the options given in the header, chosen randomly. After the for-statement has run, the branch that succeeds in made available through the control variable:

        hosts="mirror1.wisc.edu mirror2.wisc.edu mirror3.wisc.edu"
        forany h in ${hosts}
                echo "Attempting host ${host}"
                wget http://${h}/some-file       
        end
        echo "Got file from ${h}"

A forall attempts to make a group succeed for all of the options given in the header, simultaneously:

        forall h in ${hosts}
                ssh ${h} reboot
        end

Both for and forall are brittle with respect to failures. If any instance fails, then the entire for-statement fails. A try-statement may be added in one of two ways. If you wish to make each iteration resilient, place the try-statement inside the for-statement:

        for p in ${packages}
                try for 1 hour every 5 minutes
                        echo "Unpacking package ${p}..."
                        tar xvzf ${p}
                end
        end

Or, if you wish to make the entire for-statement restart after a failure, place it outside:

        try for 1 hour every 5 minutes
                for p in ${packages}
                        echo "Unpacking package ${p}..."
                        tar xvzf ${p}
                end
        end

Loops, Conditionals, and Expressions

Ftsh has loops and conditionals similar to other languages. For example:

        n=0
        while $n .lt. 10
                echo "n is now ${n}"
                n=$n .add. 1
        end

And:

        if $n .lt. 1000
                echo "n is less than 1000"
        else if $n .eq. 1000
                echo "n is equal to 1000"
        else
                echo "n is greater than 1000"
        end

You'll notice right away that arithmetic expressions look a little different than other languages. Here's how it works:

The arithmetic operators .add. .sub. .mul. .div. .mod. .pow. represent addition, subtraction, multiplication, division, modulus, and exponentiation, including parenthesis and the usual order of operations. For example:

        a=$x .mul. ( $y .add. $z )

The comparison operators .eq. .ne. .le. .lt. .ge. .gt represent equal, not-equal, less-than-or-equal, less-than, greater-than-or-equal, and greater-than. These return the literal strings "true" and "false".

        uname -s -> n
        if $n .ne. Linux
                ...
        end

For integer comparison, use the operators .eql. and .neql..

	if $x .eql. 5
		...
	end

The Boolean operators .and. .or .not. have the usual meaning. An exception is thrown if they are given arguments that are not the boolean strings "true" or "false".

        while ( $x .lt. 10 ) .and. ( $y .gt. 20 )
                ...
        end

The unary file operators .exists. .isr. .isw. .isx. test whether a filename exists, is readable, writeable, or executable, respectively. The similar operators .isfile. .isdir. .issock. .isfile. .isblock. .ischar. test for the type of a named file. All these operators throw exceptions if the named file is unavailable for examination.

        f=/etc/passwd
        if ( .isfile. $f ) .and. ( .isr. $f )
                ...
        end

Finally, the .to. and .step. operators are conveniences for generating numeric lists to be used with for-loops:

        forall x in 1 .to. 100
                ssh c$x reboot
        end

        for x in 1 .to. 100 .step. 5
                y=$x .mul. $x
                echo "$x times $x is $y"
        end

Notice that, unlike other shells, there is a distinction between expressions, which compute a value or throw an exception, and external commands, which return no value. Therefore, you cannot do this:

        # !!! This is wrong !!!
        if rm $f
                echo "Removed $f"
        else
                echo "Couldn't remove $f"
        end

Instead, you want this:

        try
                rm $f
                echo "Removed $f"
        catch
                echo "Couldn't remove $f"
        end

Functions

Simple functions are named groups of commands that may be called in the same manner as an external program. The arguments passed to the function are available in the same way as arguments to the shell:

function compress_and_move
        echo "Working on ${1}..."
        gzip ${1}
        mv ${1}.gz ${2}
end

compress_and_move /etc/hosts /tmp/hosts.gz
compress_and_move /etc/passwd /tmp/passwd.gz
compress_and_move /usr/dict/words /tmp/dict.gz

A function may also be used to compute and return a value:

function fib
	if $1 .le. 1
		return 1
	else
		return fib($1 .sub. 1) .add. fib($1 .sub. 2)
	end
end

value=fib(100)
echo $value

Functions, like groups, are brittle with respect to failures. A failure inside a function causes the entire function to stop and fail immediately. As in most languages, functions may be both nested and recursive. However, ftsh aborts recursive function calls deeper than 1000 steps. If a function is used in an expression but does not return a value, then the expression evaluation fails.

Miscellaneous Features

Environment

Variables may be exported into the environment, just like the Bourne shell:

        PATH="/usr/bin:/usr/local/bin"
        export PATH

Nested Shells

Ftsh is perfectly safe to nest. That is, an ftsh script may safely call other scripts written in ftsh. One ftsh passes all of its options to sub-shells using environment variables, so logs, error settings, and timeouts are uniform from top to bottom. If a sub-shell provides its own arguments, these override the environment settings of the parent.

Error Logging

Ftsh may optionally keep a log that describes all the details of a script's execution. The -f option specifies a log file. Logs are open for appending, so parallel and sub-shells may share the same log. The time, process number, script, and line number are all recorded with every event.

Note: Logs shared between processes must not be recorded in NFS or AFS filesystems. NFS is not designed to support shared appending: your logs will be corrupted sooner or later. AFS is not designed to support simultaneous write sharing of a file: you will end up with the log of one process or another, but not both. These are deliberate design limitations of these filesystems and are not bugs in UNIX or ftsh.

The amount of detail kept in a log is controled with the -l option. These logging levels are currently defined:

0 - Nothing is logged.
10 - Display failed commands and structures.
20 - Display executed commands and their exit codes.
30 - Display structural elements such as TRY and IF-THEN.
40 - Display process activities such as signals and completions.

Command-Line Arguments

Ftsh accepts the following command-line arguments:

-f <file> The name of a log file for tracing the execution of a script. This log file is opened in append mode. Equivalent to the environment variable FTSH_LOG_FILE.
-l <level> The logging level, on a scale of 0 to 100. Higher numbers log more data about a script. Equivalent to the environment variable FTSH_LOG_LEVEL.
-k <mode> - Controls whether ftsh trusts the operating system to actually kill a process. If set to 'weak', ftsh will assume that processes die without checking. If set to 'strong', ftsh will issue SIGKILL repeatedly until a process actually dies. Equivalent to the environment variable FTSH_KILL_MODE.
-t <secs> - The amount of time ftsh will wait between requesting a process to exit (SIGTERM) and killing it forcibly (SIGKILL). Equivalent to the environment variable FTSH_KILL_TIMEOUT.
-p Parse, but do not execute the script. This option may be used to test the validity of an Ftsh script.
-v Show the version of ftsh.
-h Show the help screen.

Cancelling Processes

Cancelling a running process in UNIX is rather quite complex. Although starting and stopping one single process is fairly easy, there are several complications to manging a tree of processes, as well as dealing with the various failures that can occur in the transmission of a signal.

Ftsh can clean up any set of processes that it starts, given the following restrictions:

Your programs must not create a new UNIX "process session" with the setsid() system call. If you don't know what this is, then don't worry about it.

The operating system must actually kill a process when ftsh asks it to. Some variants of Linux won't kill processes using distributed file systems. Consider using the "weak" mode of ftsh.

Rogue system administrators must not forcibly kill an ftsh with a SIGKILL. However, you may safely send a SIGTERM, SIGHUP, SIGINT, or SIGQUIT to an ftsh, and it will clean up its children and exit.

Ftsh starts every command as a separate UNIX process in its own process session (i.e. setsid). This simplifies the administration of large process tress. To cancel a command, ftsh sends a SIGTERM to every process in the group. Ftsh then waits up to thirty seconds for the child to exit willingly. At the end of that time, it forcibly terminates the entire process group with a SIGKILL.

Surprisingly, SIGKILL is not always effective. Some operating systems have bugs in which signals are occasionally lost or the process may be in such a state that it cannot be killed at all. By default, ftsh tries very hard to kill processes by issuing SIGKILL repeatedly until the process actually dies. This is called the "strong" kill mode. If you do not wish to have this behavior -- perhaps you have a bug resulting in unkillable processes -- then you may run ftsh in the "weak" kill mode, using the "-k weak" option.

Ftsh may be safely nested. That is, an ftsh may invoke another program written using ftsh. However, this child needs to clean up faster than its parents. If the parent shell issues forcible kills after waiting for 30 seconds, then the child must issue forcible kills before that. This problem is handled transparently for you. Each ftsh informs its children of the current kill timeout by setting the FTSH_KILL_TIMEOUT variable to five seconds less than the current timeout. Thus, subshells are progressively less tolerant of programs that refuse to exit cleanly.