Linux Ask! is a Q & A web site specific for Linux related questions. Questions are collected, answered and audited by experienced Linux users.
How to perform checksum on a file?
Answer:
You can use the md5sum or shasum commands.
Example
#md5sum test.txt
f1d41356dcabb8d8acefed069b22b0da test.txt
#shasum test.txt
809970aec0109c5464dd36a3a9a5a9e79f838313 test.txt
md5sum is faster if you perform the command on a very large file, but shasum is more secure since the chance of two different files have the same checksum is smaller.
References:
How to print out a newline character using the echo command?
Answer:
If you try
echo "foo\nbar"
It will print out
foo\nbar
But if you want the two words separate by an actual newline, you can use
echo -e "foo\nbar"
Output would be
foo
bar
You can combine the "-e" flag with other control characters such as "\t", "\r", "\v"... as well.
How to reverse print a file?
Answer:
Normally, you can print a file to standard out by using
cat test.txt
It will show something like:
a
b
c
However, if you want to start from the last line, you can use tac (actually it is reverse of the cat command)
tac test.txt
It will give:
c
b
a
Delete files older than x days in Linux?
Answer:
For example, to delete files under /tmp folder, which were modified more than 6 days, use the command
find /tmp -mtime +5 | xargs rm
Run tasks in parallel with xargs
Answer:
xargs provided a very simple to parallel tasks, e.g. setting P=3 means run 3 processes in parallel
seq 1 10 | xargs -n 1 -P 3 echo
Result:
1
2
3
4
5
7
6
9
8
10
Try to run the command several times, you might notice that number's order might be changed.
Why? Since they are running in parallel and some numbers finished first and some later!
