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LinuxCommand

Learning the shell

Writing shell scripts

Script library

SuperMan pages

Who, What, Where, Why

Flow Control - Part 3

by William Shotts, Jr.

Now that you learned about positional parameters, it is time to cover the remaining flow control statement, for. Like while and until, for is used to construct loops. for works like this:

 

for variable in words; do
    statements
done

In essence, for assigns a word from the list of words to the specified variable, executes the statements, and repeats this over and over until all the words have been used up. Here is an example:

 

#!/bin/bash

for i in word1 word2 word3; do
    echo $i
done

In this example, the variable i is assigned the string "word1", then the statement echo $i is executed, then the variable i is assigned the string "word2" and the statement echo $i is executed, and so on, until all the words in the list of words have been assigned.

The interesting thing about for is the many ways you can construct the list of words. All kinds of substitutions can be used. In the next example we will construct the list of words from a command:

 

#!/bin/bash

count=0
for i in $(cat ~/.bash_profile); do
    count=$((count + 1))
    echo "Word $count ($i) contains $(echo -n $i | wc -c) characters"
done

Here we take the file .bash_profile and count the number of words in the file and the number of characters in each word.

So what's this got to do with positional parameters? Well, one of the features of for is that it can use the positional parameters as the list of words:

 

#!/bin/bash

for i in $@; do
    echo $i
done

The shell variable $@ contains the list of command line arguments. This technique is a very common approach to processing a list of files on the command line. Here is a another example:

 

#!/bin/bash

for filename in $@; do
    result=
    if [ -f $filename ]; then
        result="$filename is a regular file"
    else
        if [ -d $filename ]; then
            result="$filename is a directory"
        fi
    fi
    if [ -w $filename ]; then
        result="$result and it is writable"
    else
        result="$result and it is not writable"
    fi
    echo "$result"
done

Try this script. Give it a list of files or a wildcard like "*" to see it work.

Here is another example script. This one compares the files in two directories and lists which files in the first directory are missing from the second.

 

#!/bin/bash

# cmp_dir - program to compare two directories

# Check for required arguments
if [ $# -ne 2 ]; then
    echo "usage: $0 directory_1 directory_2" 1>&2
    exit 1
fi

# Make sure both arguments are directories
if [ ! -d $1 ]; then
    echo "$1 is not a directory!" 1>&2
    exit 1
fi

if [ ! -d $2 ]; then
    echo "$2 is not a directory!" 1>&2
    exit 1
fi

# Process each file in directory_1, comparing it to directory_2
missing=0
for filename in $1/*; do
    fn=$(basename "$filename")
    if [ -f "$filename" ]; then
        if [ ! -f "$2/$fn" ]; then
            echo "$fn is missing from $2"
            missing=$((missing + 1))
        fi
    fi
done
echo "$missing files missing"

Now on to the real work. We are going to improve the home_space function in our script to output more information. You will recall that our previous version looked like this:

 

function home_space
{
    # Only the superuser can get this information
    
    if [ "$(id -u)" = "0" ]; then
    echo "<h2>Home directory space by user</h2>"
    echo "<pre>"
    echo "Bytes Directory"
        du -s /home/* | sort -nr
    echo "</pre>"
    fi
    
}   # end of home_space

Here is the new version:

 

function home_space
{
    echo "<h2>Home directory space by user</h2>"
    echo "<pre>"
    format="%8s%10s%10s   %-s\n"
    printf "$format" "Dirs" "Files" "Blocks" "Directory"
    printf "$format" "----" "-----" "------" "---------"
    if [ $(id -u) = "0" ]; then
        dir_list="/home/*"
    else
        dir_list=$HOME
    fi
    for home_dir in $dir_list; do
        total_dirs=$(find $home_dir -type d | wc -l)
        total_files=$(find $home_dir -type f | wc -l)
        total_blocks=$(du -s $home_dir)
        printf "$format" $total_dirs $total_files $total_blocks 
    done
    echo "</pre>"
    
}   # end of home_space

This improved version introduces a new command printf which is used to produce formatted output according to the contents of a format string. printf comes from the C programming language and has been implemented in many other programming languages including C++, perl, awk, java, PHP, and of course, bash. You can read more about printf format strings at:

We also introduce the find command. find is used to search for files or directories that meet specific criteria. In the home_space function, we use find to list the directories and regular files in each home directory. Using the wc command, we count the number of files and directories found.

The really interesting thing about home_space is how we deal with the problem of superuser access. You will notice that we test for the superuser with id and according to the outcome of the test, we assign different strings to the variable dir_list, which becomes the list of words for the for loop that follows. This way, if an ordinary user runs the script, only his/her home directory will be listed.

Another function that can use a for loop is our unfinished system_info function. We can build it like this:

 

function system_info
{
    # Find any release files in /etc
    
    if ls /etc/*release 1>/dev/null 2>&1; then
        echo "<h2>System release info</h2>"
        echo "<pre>"
        for i in /etc/*release; do
        
            # Since we can't be sure of the
            # length of the file, only
            # display the first line.
            
            head -n 1 $i
        done
        uname -orp  
        echo "</pre>"
    fi
        
}   # end of system_info

In this function, we first determine if there are any release files to process. The release files contain the name of the vendor and the version of the distribution. They are located in the /etc directory. To detect them, we perform an ls command and throw away all of its output. We are only interested in the exit status. It will be true if any files are found.

Next, we output the HTML for this section of the page since we now know that there are release files to process. To process the files, we start a for loop to act on each one. Inside the loop, we use the head command to return the first line of each file.

Finally, we use the uname command with the "o", "r", and "p" options to obtain some additional information from the system.

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© 2000-2002, William Shotts, Jr. Verbatim copying and distribution of this entire article is permitted in any medium, provided this copyright notice is preserved.