More Bash
Here are some more, hopefully useful hacks for bash scripts.
Default options
It is often recommended to set a few options in bash scripts as default:
set -euo pipefail
What that means:
-e: Immediately exit the bash script if a command fails.-u: Immediately exit the bash script if an undefined variable is called.-o pipefail: In case of chained (“piped”) command (e. g.grep something /var/log/something.log | sort) fails, return the error code of the failed command for the chain/pipe.
This is sometimes also referred to as unofficial “Bash strict mode”. But it comes with caveats!
Redirect output
# Disable stderr
exec 2>/dev/null
# Disable stdout
exec 1>/dev/null
# Disable both
exec 2>&1 1>/dev/null
# Redirect all output to some kind of log file
exec 1> /tmp/output.log
exec 2>&1
The internal field separator (IFS)
This variable defines how the bash splits strings. Useful when used in combination with read, for example:
IFS='|' read key value <<< "someKey|someValue"; echo "key: $key; value: $value"
Default values
# If nothing is provided as first argument, use someFile.txt as filename
filename=${1:-someFile.txt}
Multiline text snippets
cat << EOF
This is some
input over
multiple lines
EOF
mail -s "Alarm!" admin@example.com << EOF
Someone tried to hack your server!
Kind Regards,
The friendly hacker
EOF
Note: EOF is just some marker, you can call it whatever you want. EOF (end of file) or EOT (end of transmission) is often used.