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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
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
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
Constant in Ruby
Answer:
In Ruby, a class constant has a name starting with an uppercase character, e.g.
Foo = 'bar'
puts FooWhere you try to re-define the constant, a warning message will be shown, e.g.
Foo = 'bar'
Foo = '123'
(irb):18: warning: already initialized constant FooGlobal and Local variable in Ruby
Answer:
In Ruby, global variable has a name beginning with $, e.g.
$foo = 'bar'
puts $fooIn contrast, local variable has a name start with a lower case letter or an underscore character, e.g.
foo = 'bar'
puts foo