Answer:

If you want to change the case of filename of all files in a directory, e.g. change to all UPPER case, you can use:

# rename 'y/a-z/A-Z/' *

Answer:

If you know Perl well, there is no need to use awk to do string processing such as string splitting, it is easy with Perl also, e.g.

# echo "foo,bar,k3"  | perl -F',' -lane 'print $F[1]'
bar

Answer:

To show only the filename of Linux command, you can use

# find /dir -type f -printf "%f\n"
file1
file2
file3
...

Answer:

To replace all newline in a file by command, you can use the command

# tr '\n' ',' < input.txt > output.txt

Answer:

To check the SSL cert expiration date of a domain, you can use the following command:

# echo | openssl s_client -connect www.google.com:443 2>/dev/null | openssl x509 -noout -dates
notBefore=Dec 11 12:02:58 2013 GMT
notAfter=Apr 10 00:00:00 2014 GMT

Answer:

Assume you have a huge list of file, e.g.

line1
line2
line3
..

You want to "feed every line" of this file to an external program that read from stdin, to parallel it, you can use the gnu parallel, e.g.

cat input.txt | parallel -j16 -N1 --pipe myprogram

Answer:

To show all the listening ports in Mac OSX, try

# lsof -iTCP -sTCP:LISTEN

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.txt

Both 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

Answer:

Assume you server is listening on port 80, you can check how many established connection to this port with the following command:

netstat -an | grep :80 | grep ESTABLISHED | wc -l
4919

Answer:

Use the -L or --copy-links option, e.g.

rsync -avuz --copy-links -e ssh /home/john 192.168.1.5:/data/backup/