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Errors and Signals and Traps (Oh, My!) - Part 2

by William Shotts, Jr.

Errors are not the only way that a script can terminate unexpectedly. You also have to be concerned with signals. Consider the following program:

#!/bin/bash

echo "this script will endlessly loop until you stop it"
while true; do
	: # Do nothing
done

After you launch this script it will appear to hang. Actually, like most programs that appear to hang, it is really stuck inside a loop. In this case, it is waiting for the true command to return a non-zero exit status, which it never does. Once started, the script will continue until bash receives a signal that will stop it. You can send such a signal by typing ctrl-c which is the signal called SIGINT (short for SIGnal INTerrupt).

Cleaning up after yourself

OK, so a signal can come along and make your script terminate. Why does it matter? Well, in many cases it doesn't matter and you can ignore signals, but in some cases it will matter.

Let's take a look at another script:

#!/bin/bash

# Program to print a text file with headers and footers

TEMP_FILE=/tmp/printfile.txt

pr $1 > $TEMP_FILE

echo -n "Print file? [y/n]: "
read
if [ "$REPLY" = "y" ]; then
	lpr $TEMP_FILE
fi

This script processes a text file specified on the command line with the pr command and stores the result in a temporary file. Next, it asks the user if they want to print the file. If the user types "y", then the temporary file is passed to the lpr program for printing (you may substitute less for lpr if you don't actually have a printer attached to your system.)

Now, I admit this script has a lot of design problems. While it needs a file name passed on the command line, it doesn't check that it got one, and it doesn't check that the file actually exists. But the problem I want to focus on here is the fact that when the script terminates, it leaves behind the temporary file.

Good practice would dictate that we delete the temporary file $TEMP_FILE when the script terminates. This is easily accomplished by adding the following to the end of the script:

rm $TEMP_FILE

This would seem to solve the problem, but what happens if the user types ctrl-c when the "Print file? [y/n]:" prompt appears? The script will terminate at the read command and the rm command is never executed. Clearly, we need a way to respond to signals such as SIGINT when the ctrl-c key is typed.

Fortunately, bash provides a method to perform commands if and when signals are received.

trap

The trap command allows you to execute a command when a signal is received by your script. It works like this:

trap arg signals

"signals" is a list of signals to intercept and "arg" is a command to execute when one of the signals is received. For our printing script, we might handle the signal problem this way:

#!/bin/bash

# Program to print a text file with headers and footers

TEMP_FILE=/tmp/printfile.txt

trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM

pr $1 > $TEMP_FILE

echo -n "Print file? [y/n]: "
read
if [ "$REPLY" = "y" ]; then
	lpr $TEMP_FILE
fi
rm $TEMP_FILE

Here we have added a trap command that will execute "rm $TEMP_FILE" if any of the listed signals is received. The three signals listed are the most common ones that you will encounter, but there are many more that can be specified. For a complete list, type "trap -l". In addition to listing the signals by name, you may alternately specify them by number.

A clean_up function

While the trap command has solved the problem, we can see that it has some limitations. Most importantly, it will only accept a single string containing the command to be performed when the signal is received. You could get clever and use ";" and put multiple commands in the string to get more complex behavior, but frankly, it's ugly. A better way would be to create a function that is called when you want to perform any actions at the end of your script. In my scripts, I call this function clean_up.

#!/bin/bash

# Program to print a text file with headers and footers

TEMP_FILE=/tmp/printfile.txt

function clean_up {

	# Perform program exit housekeeping
	rm $TEMP_FILE
	exit
}

trap clean_up SIGHUP SIGINT SIGTERM

pr $1 > $TEMP_FILE

echo -n "Print file? [y/n]: "
read
if [ "$REPLY" = "y" ]; then
	lpr $TEMP_FILE
fi
clean_up

The use of a clean up function is a good idea for your error handling routines too. After all, when your program terminates (for whatever reason), you should clean up after yourself. Here is finished version of our program with improved error and signal handling:

#!/bin/bash

# Program to print a text file with headers and footers

# Usage: printfile file

# Create a temporary file name that gives preference
# to the user's local tmp directory and has a name
# that is resistant to "temp race attacks"

if [ -d "~/tmp" ]; then
	TEMP_DIR=~/tmp
else
	TEMP_DIR=/tmp
fi
TEMP_FILE=$TEMP_DIR/printfile.$$.$RANDOM
PROGNAME=$(basename $0)

function usage {

	# Display usage message on standard error
	echo "Usage: $PROGNAME file" 1>&2
}

function clean_up {

	# Perform program exit housekeeping
	# Optionally accepts an exit status
	rm -f $TEMP_FILE
	exit $1
}

function error_exit {

	# Display error message and exit
	echo "${PROGNAME}: ${1:-"Unknown Error"}" 1>&2
	clean_up 1
}

trap clean_up SIGHUP SIGINT SIGTERM

if [ $# != "1" ]; then
	usage
	error_exit "one file to print must be specified"
fi
if [ ! -f "$1" ]; then
	error_exit "file $1 cannot be read"
fi

pr $1 > $TEMP_FILE || error_exit "cannot format file"

echo -n "Print file? [y/n]: "
read
if [ "$REPLY" = "y" ]; then
	lpr $TEMP_FILE || error_exit "cannot print file"
fi
clean_up

Creating safe temporary files

In the program above, there a number of steps taken to help secure the temporary file used by this script. It is a Unix tradition to use a directory called /tmp to place temporary files used by programs. Everyone may write files into this directory. This naturally leads to some security concerns. If possible, avoid writing files in the /tmp directory. The preferred technique is to write them in a local directory such as ~/tmp (a tmp subdirectory in the user's home directory.) If you must write files in /tmp, you must take steps to make sure the file names are not predictable. Predictable file names allow an attacker to create symbolic links to other files that the attacker wants you to overwrite.

A good file name will help you figure out what wrote the file, but will not be entirely predictable. In the script above, the following line of code created the temporary file $TEMP_FILE:

TEMP_FILE=$TEMP_DIR/printfile.$$.$RANDOM

The $TEMP_DIR variable contains either /tmp or ~/tmp depending on the availability of the directory. It is common practice to embed the name of the program into the file name. We have done that with the string "printfile". Next, we use the $$ shell variable to embed the process id (pid) of the program. This further helps identify what process is responsible for the file. Surprisingly, the process id alone is not unpredictable enough to make the file safe, so we add the $RANDOM shell variable to append a random number to the file name. With this technique, we create a file name that is both easily identifiable and unpredictable.


© 2000-2014, William E. Shotts, Jr. Verbatim copying and distribution of this entire article is permitted in any medium, provided this copyright notice is preserved.

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