33.3. Shell Wrappers

#!/bin/bash

# This is a simple script that removes blank lines from a file.
# No argument checking.
#
# You might wish to add something like:
#
# E_NOARGS=65
# if [ -z "$1" ]
# then
#  echo "Usage: `basename $0` target-file"
#  exit $E_NOARGS
# fi


# Same as
#    sed -e '/^$/d' filename
# invoked from the command line.

sed -e /^$/d "$1"
#  The '-e' means an "editing" command follows (optional here).
#  '^' is the beginning of line, '$' is the end.
#  This match lines with nothing between the beginning and the end,
#+ blank lines.
#  The 'd' is the delete command.

#  Quoting the command-line arg permits
#+ whitespace and special characters in the filename.

#  Note that this script doesn't actually change the target file.
#  If you need to do that, redirect its output.

exit 0

#!/bin/bash

#  "subst", a script that substitutes one pattern for
#+ another in a file,
#+ i.e., "subst Smith Jones letter.txt".

ARGS=3         # Script requires 3 arguments.
E_BADARGS=65   # Wrong number of arguments passed to script.

if [ $# -ne "$ARGS" ]
# Test number of arguments to script (always a good idea).
then
  echo "Usage: `basename $0` old-pattern new-pattern filename"
  exit $E_BADARGS
fi

old_pattern=$1
new_pattern=$2

if [ -f "$3" ]
then
    file_name=$3
else
    echo "File \"$3\" does not exist."
    exit $E_BADARGS
fi


#  Here is where the heavy work gets done.

# -----------------------------------------------
sed -e "s/$old_pattern/$new_pattern/g" $file_name
# -----------------------------------------------

#  's' is, of course, the substitute command in sed,
#+ and /pattern/ invokes address matching.
#  The "g", or global flag causes substitution for *every*
#+ occurence of $old_pattern on each line, not just the first.
#  Read the literature on 'sed' for an in-depth explanation.

exit 0    # Successful invocation of the script returns 0.

#!/bin/bash
#  Generic shell wrapper that performs an operation
#+ and logs it.

# Must set the following two variables.
OPERATION=
#         Can be a complex chain of commands,
#+        for example an awk script or a pipe . . .
LOGFILE=
#         Command-line arguments, if any, for the operation.


OPTIONS="$@"


# Log it.
echo "`date` + `whoami` + $OPERATION "$@"" >> $LOGFILE
# Now, do it.
exec $OPERATION "$@"

# It's necessary to do the logging before the operation.
# Why?

#!/bin/bash
# pr-ascii.sh: Prints a table of ASCII characters.

START=33   # Range of printable ASCII characters (decimal).
END=125

echo " Decimal   Hex     Character"   # Header.
echo " -------   ---     ---------"

for ((i=START; i<=END; i++))
do
  echo $i | awk '{printf("  %3d       %2x         %c\n", $1, $1, $1)}'
# The Bash printf builtin will not work in this context:
#     printf "%c" "$i"
done

exit 0


#  Decimal   Hex     Character
#  -------   ---     ---------
#    33       21         !
#    34       22         "
#    35       23         #
#    36       24         $
#
#    . . .
#
#   122       7a         z
#   123       7b         {
#   124       7c         |
#   125       7d         }


#  Redirect the output of this script to a file
#+ or pipe it to "more":  sh pr-asc.sh | more

#!/bin/bash

# Adds up a specified column (of numbers) in the target file.

ARGS=2
E_WRONGARGS=65

if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
then
   echo "Usage: `basename $0` filename column-number"
   exit $E_WRONGARGS
fi

filename=$1
column_number=$2

#  Passing shell variables to the awk part of the script is a bit tricky.
#  One method is to strong-quote the Bash-script variable
#+ within the awk script.
#     $'$BASH_SCRIPT_VAR'
#      ^                ^
#  This is done in the embedded awk script below.
#  See the awk documentation for more details.

# A multi-line awk script is invoked by:  awk ' ..... '


# Begin awk script.
# -----------------------------
awk '

{ total += $'"${column_number}"'
}
END {
     print total
}     

' "$filename"
# -----------------------------
# End awk script.


#   It may not be safe to pass shell variables to an embedded awk script,
#+  so Stephane Chazelas proposes the following alternative:
#   ---------------------------------------
#   awk -v column_number="$column_number" '
#   { total += $column_number
#   }
#   END {
#       print total
#   }' "$filename"
#   ---------------------------------------


exit 0

#!/bin/bash

# Shell commands may precede the Perl script.
echo "This precedes the embedded Perl script within \"$0\"."
echo "==============================================================="

perl -e 'print "This is an embedded Perl script.\n";'
# Like sed, Perl also uses the "-e" option.

echo "==============================================================="
echo "However, the script may also contain shell and system commands."

exit 0

#!/bin/bash
# bashandperl.sh

echo "Greetings from the Bash part of the script."
# More Bash commands may follow here.

exit 0
# End of Bash part of the script.

# =======================================================

#!/usr/bin/perl
# This part of the script must be invoked with -x option.

print "Greetings from the Perl part of the script.\n";
# More Perl commands may follow here.

# End of Perl part of the script.