Trap bash exit signal
Answer:
To trap your script before exit, you can use the trap in bash, e.g.
#!/bin/bash
set -eu
function bye {
echo "Good bye"
}
trap bye EXIT
sleep 1000
# Quit the script by pressing Ctrl+C
Jul 022014
Jun 282014
Writing more robust shell script
Answer:
You should always start your bash shell script with the line set -eu
The meaning is:
- -e: cause the script to exit if an error
- -u: cause the script to exit if any variable is not set
Jul 292013
How to sum a particular column of a CSV file with awk
Answer:
Assume you have a CSV file like the following (only contain number)
1,2,3 2,3,4 3,4,5 ...
How do you sum the nth column of this file? It is easy with the use of awk
e.g. Assume you want to sum the 2nd column:
# awk -F "," '{ sum += $2 } END { print sum }' test.txt
Jul 252013
File content redirection without using cat
Answer:
It is not uncommon that people have abused the command `cat` for many purpose, e.g. file I/O redirection
cat data.txt | sed 's/foo/bar/s'You can avoid the cat command by the following commands:
data.txt < sed 's/foo/bar/s'
sed 's/foo/bar/s' < data.txtBoth directions are okay.
In fact, when you better understand the usage of some commands, you might find out that redirection is completely not needed.
e.g.
sed 's/foo/bar/s' data.txt
Apr 232013
How to echo single quote in bash shell
Answer:
To echo single quote (using single quote only), you can try
# echo foo
foo
# echo \'''foo
'foo
