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Exit CodesExit codes are a number between 0 and 255, which is returned by any Unix command when it returns control to its parent process.Other numbers can be used, but these are treated modulo 255, so exit -10 is
equivalent to exit 246 , and exit 257 is equivalent to exit 1 .
These can be used within a shell script to change the flow of execution depending on the success or failure of commands executed. This was briefly introduced in Variables - Part II. Here we shall look in more detail in the available interpretations of exit codes.
Success is traditionally represented with We shall look at three different methods for checking error status, and discuss the pros and cons of each approach. Firstly, the simple approach: #!/bin/sh # First attempt at checking return codes USERNAME=`grep "^${1}:" /etc/passwd|cut -d":" -f1` if [ "$?" -ne "0" ]; then echo "Sorry, cannot find user ${1} in /etc/passwd" exit 1 fi NAME=`grep "^${1}:" /etc/passwd|cut -d":" -f5` HOMEDIR=`grep "^${1}:" /etc/passwd|cut -d":" -f6` echo "USERNAME: $USERNAME" echo "NAME: $NAME" echo "HOMEDIR: $HOMEDIR" This script works fine if you supply a valid username in /etc/passwd .
However, if you enter an invalid code, it does not do what you might at first
expect - it keeps running, and just shows:
USERNAME: NAME: HOMEDIR:Why is this? As mentioned, the $? variable is set to the return code
of the last executed command. In this case, that is cut . cut
had no problems which it feels like reporting - as far as I can tell from testing it, and
reading the documentation, cut returns zero whatever happens! It was fed an
empty string, and did its job - returned the first field of its input, which just happened
to be the empty string.
So what do we do? If we have an error here,
#!/bin/sh # Second attempt at checking return codes grep "^${1}:" /etc/passwd > /dev/null 2>&1 if [ "$?" -ne "0" ]; then echo "Sorry, cannot find user ${1} in /etc/passwd" exit 1 fi USERNAME=`grep "^${1}:" /etc/passwd|cut -d":" -f1` NAME=`grep "^${1}:" /etc/passwd|cut -d":" -f5` HOMEDIR=`grep "^${1}:" /etc/passwd|cut -d":" -f6` echo "USERNAME: $USERNAME" echo "NAME: $NAME" echo "HOMEDIR: $HOMEDIR" This fixes the problem for us, though at the expense of slightly longer code. That is the basic way which textbooks might show you, but it is far from being all there is to know about error-checking in shell scripts. This method may not be the most suitable to your particular command-sequence, or may be unmaintainable. Below, we shall investigate two alternative approaches. As a second approach, we can tidy this somewhat by putting the test into a seperate function, instead of littering the code with lots of 4-line tests: #!/bin/sh # A Tidier approach check_errs() { # Function. Parameter 1 is the return code # Para. 2 is text to display on failure. if [ "${1}" -ne "0" ]; then echo "ERROR # ${1} : ${2}" # as a bonus, make our script exit with the right error code. exit ${1} fi } ### main script starts here ### grep "^${1}:" /etc/passwd > /dev/null 2>&1 check_errs $? "User ${1} not found in /etc/passwd" USERNAME=`grep "^${1}:" /etc/passwd|cut -d":" -f1` check_errs $? "Cut returned an error" echo "USERNAME: $USERNAME" check_errs $? "echo returned an error - very strange!" This allows us to test for errors 3 times, with customised error messages, without having to write 3 individual tests. By writing the test routine once. we can call it as many times as we wish, creating a more intelligent script, at very little expense to the programmer. Perl programmers will recognise this as being similar to the die command in Perl.As a third approach, we shall look at a simpler and cruder method. I tend to use this for building Linux kernels - simple automations which, if they go well, should just get on with it, but when things go wrong, tend to require the operator to do something intelligent (ie, that which a script cannot do!): #!/bin/sh cd /usr/src/linux && \ make dep && make bzImage && make modules && make modules_install && \ cp arch/i386/boot/bzImage /boot/my-new-kernel && cp System.map /boot && \ echo "Your new kernel awaits, m'lord."This script runs through the various tasks involved in building a Linux kernel (which can take quite a while), and uses the &&
operator to check for success. To do this with if would involve:
#!/bin/sh cd /usr/src/linux if [ "$?" -eq "0" ]; then make dep if [ "$?" -eq "0" ]; then make bzImage if [ "$?" -eq "0" ]; then make modules if [ "$?" -eq "0" ]; then make modules_install if [ "$?" -eq "0" ]; then cp arch/i386/boot/bzImage /boot/my-new-kernel if [ "$?" -eq "0" ]; then cp System.map /boot/ if [ "$?" -eq "0" ]; then echo "Your new kernel awaits, m'lord." fi fi fi fi fi fi fi fi ... which I, personally, find pretty difficult to follow.
The #!/bin/sh cp /foo /bar && ( echo Success ; echo Success part II ) || ( echo Failed ; echo Failed part II )This code will either echo Success Success part IIor Failed Failed part IIdepending on whether or not the cp command was succesful. Look carefully at this;
the construct is command && command-to-execute-on-success || command-to-execute-on-failureOnly one command can be in each part, though the ( ) brackets make a subshell, which
is treated as a single command by the top-level shell.
This method is handy for simple success / fail scenarios, but if you want to
check on the status of the
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