School of Earth, Energy & Environmental Sciences

Flow-of-control statments



Last revision August 6, 2004

if and foreach are the basic flow-of-control statements. There are more advanced ones named switch and while which are similar to the statements of the same name in the C language.

if

The if command allows you to conditionally execute a command or set of commands depending upon whether some expression is true. There are two forms.

  • if ( logical_expression ) command ...
    This form will execute command (which can have a long list of arguments) if the logical__expression is true. This expression can be one of the logical or file testing expressions described above. For example, you can test if a file whose name is stored in the shell's built-in variable $1 (first argument to the shell script) exists as a plain file, and if so, make a backup copy of it with:
      if ( -f $1 ) cp $1 $1.bak

    This simple cp command will not work if given a directory to copy, which is why there is the test for a "plain" file.

  • if ( logical_expression ) then
      block of commands - any number of lines
      to be executed if logical_expression is "true"
      (or has non-zero value).
    else
      another block of commands - any number of lines
      to be executed if logical_expression is "false"
      (or has value 0).
    endif

    This form allows you to execute more than one command if the expression is true. The then keyword must follow the logical_expression test on the same line, and the endif keyword must be on a line by itself to end the entire if command.

    The else statement is optional. If you use this, the else keyword must be on a line by itself. The following lines up to the endif are executed if the expression was false.

    The "blocks of commands" may in turn contain additional nested if commands. Just be sure that each if has a matching endif statement enclosed in the same block.

foreach

This statement allows you to execute a loop, like a do statement in Fortran or a for statement in C.

The syntax is:

    foreach name (wordlist)
      block of statements to be executed
    end

foreach and end are keywords. The end statement must be on a line by itself.

name is the name of a variable that you create. This is the "loop index".

(wordlist) is a list of "words", meaning any character strings separated by blanks. The parentheses are required. The "word list" can be a list of actual constant values, or the results of variable substitution, arithmetic expressions, or command substitution.

When the foreach statement is encountered in a shell script, the wordlist is evaluated (necessary variable and command substitutions and expressions are done) and stored. Then the new variable name is set equal to the first word in the list and the block of statements is executed. When the end statement is reached, the script goes back to the foreach line and sets the variable name equal to the next word in the list and executes the block of statements again. This is done over and over until all words in the wordlist have been used up.

This type of looping statement is good for repetitively executing the same commands for a set of arguments. For example, you could check all the arguments given to the shell to see if they are plain files, and if so, make backup copies, using these statements in a shell script:

    foreach file ( $* )
    if (-f $file) cp $file $file.bak
    end

The foreach command can also be used interactively from the terminal. In this form, you will get question mark prompts (?) to enter your commands. When done, type end after one of these prompts. For example, you could type the following commands interactively at your terminal to make backup copies of all Fortran programs in your current directory:

    foreach file ( *.f )
    ? if (-f $file) cp $file $file.bak
    ? end

Comments or Questions?