Book Excerpt: A Practical Guide to Linux Commands, Editors, and Shell Programming

The next example uses if control structures to expand the abbreviations used in some of the first fields. As long as gawk does not change a record, it leaves the entire record—including any separators—intact. Once it makes a change to a record, gawk changes all separators in that record to the value of the output field separator. The default output field separator is a SPACE.

$ cat separ_demo
        {
        if ($1 ~ /ply/)  $1 = "plymouth"
        if ($1 ~ /chev/) $1 = "chevrolet"
        print
        }

$ gawk -f separ_demo cars
plymouth fury 1970 73 2500
chevrolet malibu 1999 60 3000
ford    mustang 1965    45      10000
volvo   s80     1998    102     9850
ford    thundbd 2003    15      10500
chevrolet malibu 2000 50 3500
bmw     325i    1985    115     450
honda   accord  2001    30      6000
ford    taurus  2004    10      17000
toyota  rav4    2002    180     750
chevrolet impala 1985 85 1550
ford    explor  2003    25      9500

Stand-alone script

Instead of calling gawk from the command line with the –f option and the name of the program you want to run, you can write a script that calls gawk with the commands you want to run. The next example is a stand-alone script that runs the same program as the previous example. The #!/bin/gawk –f command (page 280) runs the gawk utility directly. To execute it, you need both read and execute permission to the file holding the script (page 278).

$ chmod u+rx separ_demo2
$ cat separ_demo2
#!/bin/gawk -f
        {
        if ($1 ~ /ply/)  $1 = "plymouth"
        if ($1 ~ /chev/) $1 = "chevrolet"
        print
        }

$ ./separ_demo2 cars
plymouth fury 1970 73 2500
chevrolet malibu 1999 60 3000
ford    mustang 1965    45      10000
...

OFS variable

You can change the value of the output field separator by assigning a value to the OFS variable. The following example assigns a TAB character to OFS, using the backslash escape sequence \t. This fix improves the appearance of the report but does not line up the columns properly.

$ cat ofs_demo
BEGIN       {OFS = "\t"}
        {
        if ($1 ~ /ply/)  $1 = "plymouth"
        if ($1 ~ /chev/) $1 = "chevrolet"
        print
        }

$ gawk -f ofs_demo cars
plymouth        fury    1970    73      2500
chevrolet       malibu  1999    60      3000
ford    mustang 1965    45      10000
volvo   s80     1998    102     9850
ford    thundbd 2003    15      10500
chevrolet       malibu  2000    50      3500
bmw     325i    1985    115     450
honda   accord  2001    30      6000
ford    taurus  2004    10      17000
toyota  rav4    2002    180     750
chevrolet       impala  1985    85      1550
ford    explor  2003    25      9500

printf

You can use printf to refine the output format. The following example uses a backslash at the end of two program lines to quote the following NEWLINE. You can use this technique to continue a long line over one or more lines without affecting the outcome of the program.

$ cat printf_demo
BEGIN       {
    print "                                 Miles"
    print "Make       Model       Year      (000)       Price"
    print \
    "--------------------------------------------------"
    }
    {
    if ($1 ~ /ply/)  $1 = "plymouth"
    if ($1 ~ /chev/) $1 = "chevrolet"
    printf "%-10s %-8s    %2d   %5d     $ %8.2f\n",\
        $1, $2, $3, $4, $5
    }
$ gawk -f printf_demo cars
                                 Miles
Make       Model       Year      (000)       Price
--------------------------------------------------
plymouth   fury        1970      73     $  2500.00
chevrolet  malibu      1999      60     $  3000.00
ford       mustang     1965      45     $ 10000.00
volvo      s80         1998     102     $  9850.00
ford       thundbd     2003      15     $ 10500.00
chevrolet  malibu      2000      50     $  3500.00
bmw        325i        1985     115     $   450.00
honda      accord      2001      30     $  6000.00
ford       taurus      2004      10     $ 17000.00
toyota     rav4        2002     180     $   750.00
chevrolet  impala      1985      85     $  1550.00
ford       explor      2003      25     $  9500.00

Redirecting output

The next example creates two files: one with the lines that contain chevy and one with the lines that contain ford.

$ cat redirect_out
/chevy/         {print > "chevfile"}
/ford/          {print > "fordfile"}
END         {print "done."}

$ gawk -f redirect_out cars
done.

$ cat chevfile
chevy   malibu  1999    60      3000
chevy   malibu  2000    50      3500
chevy   impala  1985    85      1550

The summary program produces a summary report on all cars and newer cars. Although they are not required, the initializations at the beginning of the program represent good programming practice; gawk automatically declares and initializes variables as you use them. After reading all the input data, gawk computes and displays the averages.

$ cat summary
BEGIN       {
        yearsum = 0 ; costsum = 0
        newcostsum = 0 ; newcount = 0
        }
        {
        yearsum += $3
        costsum += $5
        }
$3 > 2000 {newcostsum += $5 ; newcount ++}
END     {
        printf "Average age of cars is %4.1f years\n",\
            2006 - (yearsum/NR)
        printf "Average cost of cars is $%7.2f\n",\
            costsum/NR
            printf "Average cost of newer cars is $%7.2f\n",\
                newcostsum/newcount
        }

$ gawk -f summary cars
Average age of cars is 13.1 years
Average cost of cars is $6216.67
Average cost of newer cars is $8750.00

The following gawk command shows the format of a line from a Linux passwd file that the next example uses:

$ awk '/mark/ {print}' /etc/passwd
mark:x:107:100:ext 112:/home/mark:/bin/tcsh

FS variable

The next example demonstrates a technique for finding the largest number in a field. Because it works with a Linux passwd file, which delimits fields with colons (:), the example changes the input field separator (FS) before reading any data. It reads the passwd file and determines the next available user ID number (field 3). The numbers do not have to be in order in the passwd file for this program to work.

The pattern ($3 > saveit) causes gawk to select records that contain a user ID number greater than any previous user ID number it has processed. Each time it selects a record, gawk assigns the value of the new user ID number to the saveit variable. Then gawk uses the new value of saveit to test the user IDs of all subsequent records. Finally gawk adds 1 to the value of saveit and displays the result.

$ cat find_uid
BEGIN               {FS = ":"
                saveit = 0}
$3 > saveit              {saveit = $3}
END             {print "Next available UID is " saveit + 1}

$ gawk -f find_uid /etc/passwd
Next available UID is 1092