Using ex Scripts (Learning the vi Editor, Sixth Edition)

7.4.1. Looping in a Shell Script

You may know that the shell is a programming language as well as a command-line interpreter. To invoke ex on a number of files, we use a simple type of shell script command called the for loop. A for loop allows you to apply a sequence of commands for each argument given to the script. (The for loop is probably the single most useful piece of shell programming for beginners. You'll want to remember it even if you don't write any other shell programs.)

Here's the syntax of a for loop:

for variable in list
  do
  	  command(s)
  done

For example:

for file in $*
  do
	  ex - $file < exscript
  done

(The command doesn't need to be indented; we indented it for clarity.) After we create this shell script, we save it in a file called correct and make it executable with the chmod command. (If you aren't familiar with the chmod command and the procedures for adding a command to your UNIX search path, see Learning the UNIX Operating System, published by O'Reilly & Associates.) Now type:

$ correct sect1 sect2

The for loop in correct will assign each argument (each file in the list specified by $*, which stands for all arguments) to the variable file and execute the ex script on the contents of that variable.

It may be easier to grasp how the for loop works with an example whose output is more visible. Let's look at a script to rename files:

for file in $*
do
	  mv $file $file.x
done

Assuming this script is in an executable file called move, here's what we can do:

$ ls
ch01 ch02 ch03 move
$ move ch??
$ ls
ch01.x ch02.x ch03.x move

With creativity, you could rewrite the script to rename the files more specifically:

for nn in $*
do
  	  mv ch$nn sect$nn
done

With the script written this way, you'd specify numbers instead of filenames on the command line:

$ ls
ch01 ch02 ch03 move
$ move 01 02 03
$ ls
sect01 sect02 sect03 move

The for loop need not take $* (all arguments) as the list of values to be substituted. You can specify an explicit list as well. For example:

for variable in a b c d

will assign variable to a, b, c, and d in turn. Or you can substitute the output of a command. For example:

for variable in `grep -l "Alcuin" *`

will assign variable in turn to the name of each file in which grep finds the string Alcuin.

If no list is specified:

for variable

the variable will be assigned to each command-line argument in turn, much as it was in our initial example. This is actually not equivalent to:

for variable in $*

but to:

for variable in "$@"

which has a slightly different meaning. The symbol $* expands to $1, $2, $3, etc., but the four-character sequence "$@" expands to "$1", "$2", "$3", etc. Quotation marks prevent further interpretation of special characters.

Let's return to our main point and our original script:

for file in $*
do
	  ex - $file < exscript
done

It may seem a little inelegant to have to use two scripts—the shell script and the ex script. And in fact, the shell does provide a way to include an editing script inside a shell script.

7.4. Using ex Scripts

7.4.1. Looping in a Shell Script

7.4.2. Here Documents

7.4.3. Sorting Text Blocks: A Sample ex Script

7.4.4. Comments in ex Scripts

7.4.5. Beyond ex