You can find these examples in the subdirectory examples/6-loops1 beneath our class data area on hills.

Example 1:

Write a program normalize to fix non-Unix filenames. Given a directory, normalize changes the name of any visible item in the directory by deleting spaces and the other shell special characters * [ ] ? and ; from the name.

If changing the name of an object would cause a name collision, an error message is output instead that includes the name of the object skipped. Before it exits, normalize outputs a count of the names changed.

Here are some things to consider about normalize:

• argument checking:
• there must be a single argument
• it must be a directory
• the directory must be writable
• the loop should be a for loop, since you can create a list to process. The list should be generated using a wildcard that matches every [visible] name in the directory. Then, for each item in the directory
• use basename to extract the filename 
• change the filename by deleting any illegal character (use tr -d)
• if the name changed
• check if the changed name exists
• if not, rename the object

the appropriate else parts of the if statements are used for error messages.

You could simplify this code by looking only for filenames that contained an illegal character, but the wildcard needed for this is kind of ugly.

Example run:

-bash$ ls tmp
a[*bc?]      abc          hello;again  my           my resume    resume?
-bash$ normalize tmp
normalize: ERROR - cannot change 'a[*bc?]', 'abc' exists
3 items changed
-bash$ ls tmp
a[*bc?]     abc         helloagain  my          myresume    resume

Example 2:

If we have addints in a file (like it is on the examples area), we can source it (using the . operator) to place the function in our login shell, then run the function as a command:

Make sure that the error message goes to standard error. The user may want to capture standard output (the sum) using command-substitution.

Sample problem for Example 2

The third field of the colon-delimited file Depts contains an integer. Add together each integer in this field and place the sum in the variable sum

$ cut -d: -f3 ../../samples/Data/Depts
489
14
1
78
4532
5
17
893
833
622
552

Example 3

Write a function to ask the user a question and retrieve an alphabetic response. The function's name will be getalpha(). It will take a single argument - the question to ask. getalpha() will repeatedly ask the question and retrieve the user's response until the response consists of alphabetic characters and spaces only. If an invalid response is received, getalpha() will output a reasonable error message before reasking the question. 

Once a valid response is retrieved, getalpha() will output it to standard output. This must be the only output sent to standard output by getalpha, since the caller may want to use command-substitution to place the result in a variable or another command. The function must be simple and free of side-effects.

Again, place your function in a file and read it into your shell using the . operator for testing.

Here is a sample of getalpha running:

$ getalpha "How are you"
How are you:o.k.
Response may consist of alphabetic characters and spaces only.
How are you:ok
ok

This last line was the standard output of getalpha. If we had used command-substitution:

Example 4

Write a shell script mycat which acts like an improved cat program. It has one option: -b, followed by any number of file paths. Here is what it does

• it cats each file given to it on the commandline
• if the file is not a text file (as identified by the file program), it refuses to cat it. Instead it outputs an error message

If the -b option was given: 

• the beginning of each file is announced with a banner that includes the file's name and
• it outputs the number of files successfully output, after the last file has been cat-ed.

Here are examples of mycat running on two text files, a directory, and an executable. Note that it quietly skips the directory and the executable. (Maybe the design should specify a message?)

Here are some design considerations:

• the -b option must be first, so it's presence can be checked first. If it is present, set a flag and discard it using shift. 
• assuming there are still arguments, enter a loop that
• checks the current argument. If not readable or it's not a file, just continue. 
• if the -b option was present, output a banner and count the file
• cat the file

There are several problems that are open questions

• during the loop you may want to use continue. Remember, you must shift each time through the loop or your loop will be infinite. Check the solution to see an easy way to handle this.
• what should you do if -b does not come first? This is an easy issue - we will do what all standard programs do - treat it as a file. (filenames that start with a dash are difficult to deal with, but the filename is legal)
• what should you do if an option is used other than -b? Normally, you would check for this and give an error. Our version will just treat it as a file.
• what should you do about non-readable arguments or arguments that are not files? Again, our version will silently skip them. We could alter the design to output a message, perhaps tied to the -b flag.

Answers

The code for each of these examples is in the examples/6-examples directory on our class public data area on hills.

Example 1:

This example has some tricky code for certain parts. Let's discuss it:

• the loop should be a for loop, since you can create a list to process. The list should be generated using a wildcard that matches every [visible] name in the directory. 

• Then, for each item in the directory
• use basename to extract the filename 

• change the filename by deleting any illegal character (use tr -d)

• if the name changed

• check if the changed name exists
• if not, rename the object 

To test normalize, create a directory with some strange names and run normalize on the directory, as in the example above.

Example 2:

Here's the solution from the examples directory:

Example 3

The getalpha program has a couple of unspecified issues. Did you think about them?

• Do you allow an empty response? The sample one didn't.
• What should you do about an empty question? This is a difficult issue, since it is a program error rather than a user error, and it would have to be answered in the context of the application. For us, we will not check that we have a question. This will result in an empty question if one was not provided.

The sample solution is below and is also on hills.

Remember, the : (null command) can replace the command true to control a while loop!

Example 4

Here is the code for mycat. It looks like a lot, but there is really only 16 lines of new code here! The rest is comments, and from our existing error() and fatal() functions

#!/bin/bash
#
# mycat [-b] files...
#
# mycat cats each file given on the commandline that is a
#  readable text file (as identified by file)
#
# the -b option separates files with a banner and outputs
# a count of the files cat-ed
#
# i have added the error and fatal functions from normalize.
# (merging code is very easy with vi)
#
error() {
    echo "$(basename "$0"): ERROR - $*" >&2
    errors=true
}
fatal() {
    error "$*"
    echo "syntax: $(basename "$0") [-b] files..." >&2
    exit 1
}

bflag=false
[ "$1" = -b ] && bflag=true && shift
[ $# -eq 0 ] && fatal "no files given"
filecount=0
while [ $# -gt 0 ]; do
    # get the current file and prepare for the next
    # (so that we can use continue later)
    thisfile=$1
    shift
    # only try for readable items
    [ ! -r "$thisfile" ] && continue
    # check to see if the description (not the filename)
    # output by file contains the word text
    if file "$thisfile" | grep -q ':.*text' ; then
        [ "$bflag" = true ] && \
            echo "************************* $thisfile ***************************"
        # at this point, cat shouldn't give an error
        # but if it does, we'll just let it output the message
        cat "$thisfile" && ((filecount=filecount+1))
    fi
done
if [ "$bflag" = true ] ; then
    echo "*****************************************************************"
    echo "$filecount files cat-ed"
fi