1. Looping with `for`

With for we can loop a list of words:
```
for i in A B C D; do echo $i; done
```
```
for i in {A..D}; do echo $i; done
```
```
for i in *.sh; do echo "$i"; done
```
In these cases it is using shell expansions.

The last file expansion may fail if there are no files like that, in which case shell will return just *.sh (instead of a list of matching files). To protect against this, we can rewrite the last example like this:
```
for i in *.sh; do [[ -e "$i" ]] && echo "$i"; done
```

Let's see an example that finds the longest word in a file:

longest-word.sh

#!/bin/bash

# longest-word: find longest string in a file

while [[ -n "$1" ]]; do
    if [[ -r "$1" ]]; then
        max_word=
        max_len=0
        for i in $(strings "$1"); do
            len="$(echo -n "$i" | wc -c)"
            if (( len > max_len )); then
                max_len="$len"
                max_word="$i"
            fi
        done
        echo "$1: '$max_word' ($max_len characters)"
    fi
    shift
done

vim longest-word.sh

Actually it can take one or more files as parameters and process each of them. This is implemented by the while loop:

while [[ -n "$1" ]]; do
    if [[ -r "$1" ]]; then
        # . . . . .
        # . . . . .
    fi
    shift
done

The if checks that the file is readable (otherwise it is skipped).

The list of words of the file is produced by the command strings "$1". We use a command substitution to use this list in the for statement:

for i in $(strings "$1"); do

Notice that we are not surrounding $(strings "$1") with double quotes, otherwise it will be treated as a single string, which is not what we want.

Let's try it:

echo *

./longest-word.sh *

Let's see a slightly modified version of the previous example:

longest-word2.sh

#!/bin/bash

# longest-word2: find longest string in a file

for i; do
    if [[ -r "$i" ]]; then
        max_word=
        max_len=0
        for j in $(strings "$i"); do
            len="$(echo -n "$j" | wc -c)"
            if (( len > max_len )); then
                max_len="$len"
                max_word="$j"
            fi
        done
        echo "$i: '$max_word' ($max_len characters)"
    fi
done

vim longest-word2.sh

The modification consists on replacing the while loop with a for loop like this:

for i; do
    if [[ -r "$i" ]]; then
        # . . . . .
        # . . . . .
    fi
done

And since we are using the variable i for the outer loop, in the inner loop we have replaced i with j.

When we omit the list of words, for will use by default the positional parameters.

echo *

./longest-word2.sh *

for has also another form, which is similar to that of C (and many other languages):
```
for ((i=0; i<5; i=i+1)); do echo $i; done
```
As you know, the construct ((...)) is used for arithmetic expressions, and inside it we don't use a $ in front of the variables.

Let's also make a small modification to the program sys_info.sh that we saw in the last lesson:

sys_info.sh

#!/bin/bash
# Program to output a system information page

PROGNAME="$(basename "$0")"

usage () {
    cat <<- _EOF_
	Usage:
	    $PROGNAME
	        Output the report to the stdout.
	
	    $PROGNAME (-f | --file) <file>
	        Output the report to the given file.
	
	    $PROGNAME (-i | --interactive)
	        Get the output file interactively from the keyboard.
	
	    $PROGNAME (-h | --help)
	        Display this help message.
	_EOF_
    return
}

main () {
    # global aux vars
    interactive=''
    filename=''

    process_options "$@"
    interactive_mode
    output_html_page
}

process_options () {
    # process command line options
    while [[ -n "$1" ]]; do
        case "$1" in
            -f | --file)
                shift
                filename="$1"
                ;;
            -i | --interactive)
                interactive=1
                ;;
            -h | --help)
                usage
                exit
                ;;
            *)
                usage >&2
                exit 1
                ;;
        esac
        shift
    done
}

interactive_mode () {
    # interactive mode
    if [[ -n "$interactive" ]]; then
        while true; do
            read -p "Enter name of output file: " filename
            if [[ -e "$filename" ]]; then
                read -p "'$filename' exists. Overwrite? [y/n/q] > "
                case "$REPLY" in
                    Y|y)
                        break
                        ;;
                    Q|q)
                        echo "Program terminated."
                        exit
                        ;;
                    *)
                        continue
                        ;;
                esac
            elif [[ -z "$filename" ]]; then
                continue
            else
                break
            fi
        done
    fi
}

output_html_page () {
    # output html page
    if [[ -n "$filename" ]]; then
        if touch "$filename" && [[ -f "$filename" ]]; then
            write_html_page > "$filename"
        else
            echo "$PROGNAME: Cannot write file '$filename'" >&2
            exit 1
        fi
    else
        write_html_page
    fi
}

write_html_page () {
    local TITLE="System Information Report For $HOSTNAME"
    local CURRENT_TIME=$(date +"%x %r %Z")
    local TIMESTAMP="Generated on $CURRENT_TIME, by $USER"

    cat <<- _EOF_
	<html>
	    <head>
	        <title>$TITLE</title>
	    </head>
	    <body>
	        <h1>$TITLE</h1>
	        <p>$TIMESTAMP</p>
	        $(report_uptime)
	        $(report_disk_space)
	        $(report_home_space)
	    </body>
	</html>
	_EOF_
}

report_uptime() {
   cat <<- _EOF_
	<h2>System Uptime</h2>
	<pre>$(uptime)</pre>
	_EOF_
    return
}

report_disk_space() {
   cat <<- _EOF_
	<h2>Disk Space Utilization</h2>
	<pre>$(df -h .)</pre>
	_EOF_
    return
}

report_home_space () {
    local format="%8s%10s%10s\n"
    local i dir_list total_files total_dirs total_size user_name
    if [[ "$(id -u)" -eq 0 ]]; then
        dir_list=/home/*
        user_name="All Users"
    else
        dir_list="$PWD"
        user_name="$USER"
    fi
    echo "<h2>Home Space Utilization ($user_name)</h2>"
    for i in $dir_list; do
        total_files="$(find "$i" -type f | wc -l)"
        total_dirs="$(find "$i" -type d | wc -l)"
        total_size="$(du -sh "$i" | cut -f 1)"
        echo "<H3>$i</H3>"
        echo "<pre>"
        printf "$format" "Dirs" "Files" "Size"
        printf "$format" "----" "-----" "----"
        printf "$format" "$total_dirs" "$total_files" "$total_size"
        echo "</pre>"
    done
    return
}

# call the main function
main "$@"

vim sys_info.sh

Only report_home_space () (the last function) has been modified. It provides more detail for each user’s home directory and includes the total number of files and subdirectories in each. We also use some local variables and use printf (instead of echo) to format some of the output.

./sys_info.sh > report.html

lynx report.html

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