Error Checking and Handling
- The following topics
- Error checking
- Error handling
- Error Prevention
- The exit status (exit codes or return codes) is the way Bash communicates the successful or unsuccessful termination of a process to its parent.
- Linux command execution process
- Lets use mktemp command
mktemp --help
- Lets write a script with exit codes for success & failure
#!/bin/bash
# Run a command with will always work
mktemp
# storing the exit code of mktemp
mktemp_ec=$?
# Run a command which always fails
mkdir /home/ubuntu
mkdir_ec=$?
echo "mktemp returned success exit code which ${mktemp_ec}"
echo "mkdir returned failure exit code which ${mkdir_ec}"
- now execute this script
- Return code of 0 is success any other return code is failure
- For the standard exit codes and their meanings refer here
Test Shorthand
- We use test command in lot of scripts. Lets explore test
- Directory checking
# full command
test -d <dirpath>
# short hand
[ -d <dirpath> ]
- Using test we can do comparisons using -gt, -ne, -eq, -lt
- To check the file existence use test -f
- So far we looked at how to check for directories, files and conditions which can act as error checks. if we combine this checks with conditional statements we can avoid errors.
- For handling errors also we need a conditional statements
if-then-exit
- The basic idea is to test for a condition (IF), and if that condition is true, we do something (THEN) and if condition is false we return failure exit codes (exit)
- We want to write a script which prints content of file
- lets write a basic if condition
if <condition> then
statements
fi
- Using this lets write the script as shown below
#!/bin/bash
FILE=/tmp/random.txt
# Check if the file exits
if [[ -f ${FILE} ]]; then
cat ${FILE}
exit 0
fi
exit 1
- Now execute the script for negative test (/tmp/random.txt doesnot exist)
