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5. Flow Control

Contents:
if/else
for
case
select
while and until

If you are a programmer, you may have read the last chapter-with its claim at the outset that the Korn shell has an advanced set of programming capabilities - and wondered where many features from conventional languages are. Perhaps the most glaringly obvious "hole" in our coverage thus far concerns flow control constructs like if, for, while, and so on.

Flow control gives a programmer the power to specify that only certain portions of a program run, or that certain portions run repeatedly, according to conditions such as the values of variables, whether or not commands execute properly, and others. We call this the ability to control the flow of a program's execution.

Almost every shell script or function shown thus far has had no flow control-they have just been lists of commands to be run! Yet the Korn shell, like the C and Bourne shells, has all of the flow control abilities you would expect and more; we will examine them in this chapter. We'll use them to enhance the solutions to some of the programming tasks we saw in the last chapter and to solve tasks that we will introduce here.

Although we have attempted to explain flow control so that non-programmers can understand it, we also sympathize with programmers who dread having to slog through yet another tabula rasa explanation. For this reason, some of our discussions relate the Korn shell's flow-control mechanisms to those that programmers should know already. Therefore you will be in a better position to understand this chapter if you already have a basic knowledge of flow control concepts.

The Korn shell supports the following flow control constructs:

if/else

Execute a list of statements if a certain condition is/is not true

for

Execute a list of statements a fixed number of times

while

Execute a list of statements repeatedly while a certain condition holds true

until

Execute a list of statements repeatedly until a certain condition holds true

case

Execute one of several lists of statements depending on the value of a variable

In addition, the Korn shell provides a new type of flow-control construct:

select

Allow the user to select one of a list of possibilities from a menu

We will cover each of these, but be warned: the syntax is not pretty.

5.1 if/else

The simplest type of flow control construct is the conditional, embodied in the Korn shell's if statement. You use a conditional when you want to choose whether or not to do something, or to choose among a small number of things to do, according to the truth or falsehood of conditions. Conditions test values of shell variables, characteristics of files, whether or not commands run successfully, and other factors. The shell has a large set of built-in tests that are relevant to the task of shell programming.

The if construct has the following syntax:

if condition
then
    statements
[elif condition
    then statements...]
[else
    statements]
fi

The simplest form (without the elif and else parts, a.k.a. clauses) executes the statements only if the condition is true. If you add an else clause, you get the ability to execute one set of statements if a condition is true or another set of statements if the condition is false. You can use as many elif (a contraction of "else if") clauses as you wish; they introduce more conditions, and thus more choices for which set of statements to execute. If you use one or more elifs, you can think of the else clause as the "if all else fails" part.

5.1.1 Exit Status and Return

Perhaps the only aspect of this syntax that differs from that of conventional languages like C and Pascal is that the "condition" is really a list of statements rather than the more usual Boolean (true or false) expression. How is the truth or falsehood of the condition determined? It has to do with a general UNIX concept that we haven't covered yet: the exit status of commands.

Every UNIX command, whether it comes from source code in C, some other language, or a shell script/function, returns an integer code to its calling process-the shell in this case-when it finishes. This is called the exit status. 0 is usually the "OK" exit status, while anything else (1 to 255) usually denotes an error. [1]

[1] Because this is a "convention" and not a "law," there are exceptions. For example, diff (1) (find differences between two files) returns 0 for "no differences," 1 for "differences found," or 2 for an error such as an invalid filename argument.

if checks the exit status of the last statement in the list following the if keyword. [2] (The list is usually just a single statement.) If the status is 0, the condition evaluates to true; if it is anything else, the condition is considered false. The same is true for each condition attached to an elif statement (if any).

[2] LISP programmers will find this idea familiar.

This enables us to write code of the form:

if command ran successfully
then
    normal processing
else
    error processing
fi

More specifically, we can now improve on the pushd function that we saw in the last chapter:

function pushd {		# push current directory onto stack
    dirname=$1
    cd ${dirname:?"missing directory name."}
    DIRSTACK="$dirname ${DIRSTACK:-$PWD}"
    print $DIRSTACK
}

This function requires a valid directory as its argument. Let's look at how it handles error conditions: if no argument is given, the second line of code prints an error message and exits. This is fine.

However, the function reacts deceptively when an argument is given that isn't a valid directory. In case you didn't figure it out when reading the last chapter, here is what happens: the cd fails, leaving you in the same directory you were in. This is also appropriate. But then the third line of code pushes the bad directory onto the stack anyway, and the last line prints a message that leads you to believe that the push was successful.

We need to prevent the bad directory from being pushed and to print an error message. Here is how we can do this:

function pushd {                # push current directory onto stack
    dirname=$1
    if cd ${dirname:?"missing directory name."}   # if cd was successful
    then
        DIRSTACK="$dirname ${DIRSTACK:-$PWD}"
        print $DIRSTACK
    else
        print still in $PWD.
    fi
}

The call to cd is now inside an if construct. If cd is successful, it will return 0; the next two lines of code are run, finishing the pushd operation. But if the cd fails, it returns with exit status 1, and pushd will print a message saying that you haven't gone anywhere.

You can usually rely on built-in commands and standard UNIX utilities to return appropriate exit statuses, but what about your own shell scripts and functions? For example, what if you wrote a cd function that overrides the built-in command?

Let's say you have the following code in your .profile or environment file:

function _cd {
    "cd" $*
    print $OLDPWD -> $PWD
}
alias cd=_cd

The function _cd simply changes directories and prints a message saying where you were and where you are now. Because functions have lower priority than built-in commands in the shell's order of command lookup, we need to define cd itself as an alias so that it overrides the built-in cd.

The function calls the built-in cd command, but notice that it's surrounded in double quotes: that prevents the shell from looking it up as an alias. (This may seem like a kludge in the aliasing mechanism, but it's really just a ramification of the shell's command-line processing rules, which we list in Chapter 7, Input/Output and Command-line Processing.) [3] If it did find cd as an alias, the shell would go into an "infinite recursion" in which the alias is expanded to _cd, which runs the function, which calls cd, which the shell expands to the alias again, etc.

[3] A related result of command-line processing is that if you surround a command with single quotes, the shell won't look it up as an alias or as a function.

Anyway, we want this function to return the same exit status that the built-in cd returns. The problem is that the exit status is reset by every command, so it "disappears" if you don't save it immediately. In this function, the built-in cd's exit status disappears when the print statement runs (and sets its own exit status).

Therefore, we need to save the status that cd sets and use it as the entire function's exit status. Two shell features we haven't seen yet provide the way. First is the special shell variable ?, whose value ($?) is the exit status of the last command that ran. For example:

cd baddir
print $?

causes the shell to print 1, while:

cd gooddir
print $?

causes the shell to print 0.

5.1.1.1 Return

The second feature we need is the statement return N, which causes the surrounding script or function to exit with exit status N. N is actually optional; it defaults to 0. Scripts that finish without a return statement (i.e., every one we have seen so far) return whatever the last statement returns. If you use return within a function, it will just exit the function. (In contrast, the statement exit N exits the entire script, no matter how deeply you are nested in functions.)

Getting back to our example: if the call to "real" cd were last in our _cd function, it would behave properly. Unfortunately, we really need the assignment statement where it is, so that we can avoid lots of ugly error processing. Therefore we need to save cd's exit status and return it as the function's exit status. Here is how to do it:

function _cd {
    "cd" $*
    es=$?
    print $OLDPWD -> $PWD
    return $es
}

The second line saves the exit status of cd in the variable es; the fourth returns it as the function's exit status. We'll see a more substantial "wrapper" for cd in Chapter 7.

Exit statuses aren't very useful for anything other than their intended purpose. In particular, you may be tempted to use them as "return values" of functions, as you would with functions in C or Pascal. That won't work; you should use variables or command substitution instead to simulate this effect.

5.1.2 Combinations of Exit Statuses

One of the more obscure parts of Korn shell syntax allows you to combine exit statuses logically, so that you can test more than one thing at a time.

The syntax statement1 && statement2 means, "execute statement1, and if its exit status is 0, execute statement2." The syntax statement1 || statement2 is the converse: it means, "execute statement1, and if its exit status is not 0, execute statement2."

At first, these look like "if/then" and "if not/then" constructs, respectively. But they are really intended for use within conditions of if constructs-as C programmers will readily understand.

It's much more useful to think of these constructs as "and" and "or," respectively. Consider this:

if statement1 && statement2
then
    ...
fi

In this case, statement1 is executed. If it returns a 0 status, then presumably it ran without error. Then statement2 runs. The then clause is executed if statement2 returns a 0 status. Conversely, if statement1 fails (returns a non-0 exit status), then statement2 doesn't even run; the "last statement" in the condition was statement1, which failed-so the then clause doesn't run. Taken all together, it's fair to conclude that the then clause runs if statement1 and statement2 both succeeded.

Similarly, consider this:

if statement1 || statement2
then
    ...
fi

If statement1 succeeds, then statement2 does not run. This makes statement1 the last statement, which means that the then clause runs. On the other hand, if statement1 fails, then statement2 runs, and whether the then clause runs or not depends on the success of statement2. The upshot is that the then clause runs if statement1 or statement2 succeeds.

As a simple example, assume that we need to write a script that checks a file for the presence of two words and just prints a message saying whether either word is in the file or not. We can use grep for this: it returns exit status 0 if it found the given string in its input, non-0 if not:

filename=$1
word1=$2
word2=$3
if grep $word1 $filename || grep $word2 $filename
then
    print "$word1 or $word2 is in $filename."
fi

The then clause of this code runs if either grep statement succeeds. Now assume that we want the script to say whether or not the input file contains both words. Here's how to do it:

filename=$1
word1=$2
word2=$3
if grep $word1 $filename && grep $word2 $filename
then
    print "$word1 and $word2 are both in $filename."
fi

We'll see more examples of these logical operators later in this chapter and in the code for the kshdb debugger in Chapter 9, Debugging Shell Programs.

5.1.3 Condition Tests

Exit statuses are the only things an if construct can test. But that doesn't mean you can check only whether or not commands ran properly. The shell provides a way of testing a variety of conditions with the [[ ]] construct. [4]

[4] The Korn shell also accepts the external [] and test commands. The [[ ]] construct has many more options and is better integrated into the Korn shell language: specifically, word splitting and wildcard expansion aren't done within [[ and ]], making quoting less necessary.

You can use the construct to check many different attributes of a file (whether it exists, what type of file it is, what its permissions and ownership are, etc.), compare two files to see which is newer, do comparisons and pattern matching on strings, and more.

[[ condition ]] is actually a statement just like any other, except that the only thing it does is return an exit status that tells whether condition is true or not. Thus it fits within the if construct's syntax of if statements.

5.1.3.1 String comparisons

The double square brackets ([[]]) surround expressions that include various types of operators. We will start with the string comparison operators, which are listed in Table 5.1. (Notice that there are no operators for "greater than or equal" or "less than or equal.") In the table, str refers to an expression with a string value, and pat refers to a pattern that can contain wildcards (just like the patterns in the string-handling operators we saw in the last chapter).

Table 5.1: String Comparison Operators
Operator True if...
str = pat[5]str matches pat.
str != patstr does not match pat.
str1 < str2str1 is less than str2.
str1 > str2str1 is greater than str2.
-n strstr is not null (has length greater than 0).
-z strstr is null (has length 0).

[5] Note that there is only one equal sign (=). This is a common source of errors.

We can use one of these operators to improve our popd function, which reacts badly if you try to pop and the stack is empty. Recall that the code for popd is:

function popd {			# pop directory off the stack, cd there
    DIRSTACK=${DIRSTACK#* }
    cd ${DIRSTACK%% *}
    print "$PWD"
}

If the stack is empty, then $DIRSTACK is the null string, as is the expression ${DIRSTACK%% *}. This means that you will change to your home directory; instead, we want popd to print an error message and do nothing.

To accomplish this, we need to test for an empty stack, i.e., whether $DIRSTACK is null or not. Here is one way to do it:

function popd {                 # pop directory off the stack, cd there
    if [[ -n $DIRSTACK ]]; then
        DIRSTACK=${DIRSTACK#* }
        cd ${DIRSTACK%% *}
        print "$PWD"
    else
        print "stack empty, still in $PWD."
    fi
}

Notice that instead of putting then on a separate line, we put it on the same line as the if after a semicolon, which is the shell's standard statement separator character.

We could have used operators other than -n. For example, we could have used -z and switched the code in the then and else clauses. We also could have used: [6]

[6] Note that this code does not work under the older [ ] or test syntax, which will complain about a missing argument if the variable is null. This means that it is no longer necessary to surround both sides with double quotes (or to use hacks like [ x$DIRSTACK = x ]) as you had to with the Bourne shell; the Korn shell's [[/]] syntax handles null values correctly.

if [[ $DIRSTACK = "" ]]; then
        ...

While we're cleaning up code we wrote in the last chapter, let's fix up the error handling in the highest script (Task 4-1). The code for that script is:

filename=${1:?"filename missing."}
howmany=${2:-10}
sort -nr $filename | head -$howmany

Recall that if you omit the first argument (the filename), the shell prints the message highest: 1: filename missing. We can make this better by substituting a more standard "usage" message:

if [[ -z $1 ]]; then
    print 'usage: howmany filename [-N]'
else
    filename=$1
    howmany=${2:-10}
    sort -nr $filename | head -$howmany
fi

It is considered better programming style to enclose all of the code in the if-then-else, but such code can get confusing if you are writing a long script in which you need to check for errors and bail out at several points along the way. Therefore, a more usual style for shell programming is this:

if [[ -z $1 ]]; then
    print 'usage: howmany filename [-N]'
    return 1
fi
filename=$1
howmany=${2:-10}
sort -nr $filename | head -$howmany

The return statement informs any calling program that needs to know whether it ran successfully or not.

As an example of the = and != operators, we can add the shell script front end to a C compiler to our solution for Task 4-2. Recall that we are given a filename ending in .c (the source code file), and we need to construct a filename that is the same but ends in .o (the object code file). The modifications we will make have to do with other types of files that can be passed to a C compiler.

5.1.3.2 About C Compilers

Before we get to the shell code, it is necessary to understand a few things about C compilers. We already know that they translate C source code into object code. Actually, they are part of compilation systems that also perform several other tasks. The term "compiler" is often used instead of "compilation system," so we'll use it in both senses.

We're interested here in two tasks that compilers perform other than compiling C code: they can translate assembly language code into object code, and they can link object code files together to form an executable program.

Assembly language works at a level that is close to the bare computer; each assembly statement is directly translatable into a statement of object code-as opposed to C or other higher-level languages, in which a single source statement could translate to dozens of object code instructions. Translating a file of assembly language code into object code is called, not surprisingly, assembling the code.

Although many people consider assembly language to be quaintly old-fashioned - like a typewriter in this age of WYSIWYG word processing and desktop publishing-some programmers still need to use it when dealing with precise details of computer hardware. It's not uncommon for a program to consist of several files' worth of code in a higher-level language (such as C) and a few low-level routines in assembly language.

The other task we'll worry about is called linking. Most real-world programs, unlike those assigned for a first-year programming class, consist of several files of source code, possibly written by several different programmers. These files are compiled into object code; then the object code must be combined to form the final, runnable program, known as an executable. The task of combining is often called "linking": each object code component usually contains references to other components, and these references must be resolved or "linked" together.

C compilation systems are capable of assembling files of assembly language into object code and linking object code files into executables. In particular, a compiler calls a separate assembler to deal with assembly code and a linker (also known as a "loader," "linking loader," or "link editor") to deal with object code files. These separate tools are known in the UNIX world as as and ld, respectively. The C compiler itself is invoked with the command cc.

We can express all of these steps in terms of the suffixes of files passed as arguments to the C compiler. Basically, the compiler does the following:

  1. If the argument ends in .c it's a C source file; compile into a .o object code file.

  2. If the argument ends in .s, it's assembly language; assemble into a .o file.

  3. If the argument ends in .o, do nothing; save for the linking step later.

  4. If the argument ends in some other suffix, print an error message and exit. [7]

    [7] For the purposes of this example. We know this isn't strictly true in real life.

  5. Link all .o object code files into an executable file called a.out. This file is usually renamed to something more descriptive.

Step 3 allows object code files that have already been compiled (or assembled) to be re-used to build other executables. For example, an object code file that implements an interface to a CD-ROM drive could be useful in any program that reads from CD-ROMS.

Figure 5.1 should make the compilation process clearer; it shows how the compiler processes the C source files a.c and b.c, the assembly language file c.s, and the already-compiled object code file d.o. In other words, it shows how the compiler handles the command cc a.c b.c c.s d.o.

Figure 5.1: Files produced by a C compiler

Figure 5.1

Here is how we would begin to implement this behavior in a shell script. Assume that the variable filename holds the argument in question, and that ccom is the name of the program that actually compiles a C source file into object code. Assume further that ccom and as (assembler) take arguments for the names of the source and object files:

if [[ $filename = *.c ]]; then
    objname=${filename%.c}.o
    ccom $filename $objname
elif [[ $filename = *.s ]]; then
    objname=${filename%.s}.o
    as $filename $objname
elif [[ $filename != *.o ]]; then
    print "error: $filename is not a source or object file."
    return 1
fi
further processing...

Recall from the previous chapter that the expression ${filename%.c}.o deletes .c from filename and appends .o; ${filename%.s}.o does the analogous thing for files ending in .s.

The "further processing" is the link step, which we will see when we complete this example later in the chapter.

5.1.3.3 File Attribute Checking

The other kind of operator that can be used in conditional expressions checks a file for certain properties. There are 21 such operators. We will cover those of most general interest here; the rest refer to arcana like sticky bits, sockets, and file descriptors, and thus are of interest only to systems hackers. Refer to Appendix B, Reference Lists for the complete list. Table 5.2 lists those that we will examine.

Table 5.2: File Attribute Operators
Operator True if...
-a filefile exists
-d filefile is a directory
-f file

file is a regular file (i.e., not a directory or other special type of file)

-r fileYou have read permission on file
-s filefile exists and is not empty
-w fileYou have write permission on file
-x file

You have execute permission on file, or directory search permission if it is a directory

-O fileYou own file
-G fileYour group ID is the same as that of file
file1 -nt file2file1 is newer than file2[8]
file1 -ot file2file1 is older than file2

[8] Specifically, the -nt and -ot operators compare modification times of two files.

Before we get to an example, you should know that conditional expressions inside [[ and ]] can also be combined using the logical operators && and ||, just as we saw with plain shell commands above, in the section entitled "Combinations of Exit Statuses." It's also possible to combine shell commands with conditional expressions using logical operators, like this:

if command && [[ condition ]]; then
    ...

Chapter 7 contains an example of this combination.

You can also negate the truth value of a conditional expression by preceding it with an exclamation point (!), so that ! expr evaluates to true only if expr is false. Furthermore, you can make complex logical expressions of conditional operators by grouping them with parentheses. [9]

[9] It turns out that this is true outside of the [[/]] construct as well. As we will see in Chapter 8, Process Handling the construct (statement list) runs the statement list in a subshell, whose exit status is that of the last statement in the list. However, there is no equivalent of the negation (!) operator outside of the [[/]] construct, although there will be in future releases.

Here is how we would use two of the file operators to embellish (yet again) our pushd function. Instead of having cd determine whether the argument given is a valid directory-i.e., by returning with a bad exit status if it's not-we can do the checking ourselves. Here is the code:

function pushd {                # push current directory onto stack
    dirname=$1
    if [[ -d $dirname && -x $dirname ]]; then
        cd $dirname
        DIRSTACK="$dirname ${DIRSTACK:-$PWD}"
        print "$DIRSTACK"
    else
        print "still in $PWD."
    fi
}

The conditional expression evaluates to true only if the argument $1 is a directory (-d) and the user has permission to change to it (-x). [10] Notice that this conditional also handles the case where the argument is missing: $dirname is null, and since the null string isn't a valid directory name, the conditional will fail.

[10] Remember that the same permission flag that determines execute permission on a regular file determines search permission on a directory. This is why the -x operator checks both things depending on file type.

Here is a more comprehensive example of the use of file operators.

Task 5.1

Write a script that prints essentially the same information as ls -l but in a more user-friendly way.

Although this task requires relatively long-winded code, it is a straightforward application of many of the file operators:

if [[ ! -a $1 ]]; then
    print "file $1 does not exist."
    return 1
fi
if [[ -d $1 ]]; then
    print -n "$1 is a directory that you may "
    if [[ ! -x $1 ]]; then
        print -n "not "
    fi
    print "search."
elif [[ -f $1 ]]; then
    print "$1 is a regular file."
else
    print "$1 is a special type of file."
fi
if [[ -O $1 ]]; then
    print 'you own the file.'
else
    print 'you do not own the file.'
fi
if [[ -r $1 ]]; then
    print 'you have read permission on the file.'
fi
if [[ -w $1 ]]; then
    print 'you have write permission on the file.'
fi
if [[ -x $1 && ! -d $1 ]]; then
    print 'you have execute permission on the file.'
fi

We'll call this script fileinfo. Here's how it works:

  • The first conditional tests if the file given as argument does not exist (the exclamation point is the "not" operator; the spaces around it are required). If the file does not exist, the script prints an error message and exits with error status.

  • The second conditional tests if the file is a directory. If so, the first print prints part of a message; remember that the -n option tells print not to print a LINEFEED at the end. The inner conditional checks if you do not have search permission on the directory. If you don't have search permission, the word "not" is added to the partial message. Then, the message is completed with "search." and a LINEFEED.

  • The elif clause checks if the file is a regular file; if so, it prints a message.

  • The else clause accounts for the various special file types on recent UNIX systems, such as sockets, devices, FIFO files, etc. We assume that the casual user isn't interested in details of these.

  • The next conditional tests to see if the file is owned by you (i.e., if its owner ID is the same as your login ID). If so, it prints a message saying that you own it.

  • The next two conditionals test for your read and write permission on the file.

  • The last conditional checks if you can execute the file. It checks to see if you have execute permission and that the file is not a directory. (If the file were a directory, execute permission would really mean directory search permission.)

As an example of fileinfo's output, assume that you do an ls -l of your current directory and it contains these lines:

-rwxr-xr-x   1 billr    other        594 May 28 09:49 bob
-rw-r-r-     1 billr    other      42715 Apr 21 23:39 custom.tbl
drwxr-xr-x   2 billr    other         64 Jan 12 13:42 exp
-r-r-r-      1 root     other        557 Mar 28 12:41 lpst

custom.tbl and lpst are regular text files, exp is a directory, and bob is a shell script. Typing fileinfo bob produces this output:

bob is a regular file.
you own the file.
you have read permission on the file.
you have write permission on the file.
you have execute permission on the file.

Typing fileinfo custom.tbl results in this:

custom.tbl is a regular file.
you own the file.
you have read permission on the file.
you have write permission on the file.

Typing fileinfo exp results in this:

exp is a directory that you may search.
you own the file.
you have read permission on the file.
you have write permission on the file.

Finally, typing fileinfo lpst produces this:

lpst is a regular file.
you do not own the file.
you have read permission on the file.

Chapter 7 contains an example of the -nt test operator.

5.1.4 Integer Conditionals

The shell also provides a set of arithmetic tests. These are different from character string comparisons like < and >, which compare lexicographic values of strings, not numeric values. For example, "6" is greater than "57" lexicographically, just as "p" is greater than "ox," but of course the opposite is true when they're compared as integers.

The integer comparison operators are summarized in Table 5.3. FORTRAN programmers will find their syntax slightly familiar.

Table 5.3: Arithmetic Test Operators
TestComparison
-ltLess than
-leLess than or equal
-eqEqual
-geGreater than or equal
-gtGreater than
-neNot equal

You'll find these to be of the most use in the context of the integer variables we'll see in the next chapter. They're necessary if you want to combine integer tests with other types of tests within the same conditional expression.

However, the shell has a separate syntax for conditional expressions that involve integers only. It's considerably more efficient, so you should use it in preference to the arithmetic test operators listed above. Again, we'll cover the shell's integer conditionals in the next chapter.


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