Deleting files with rm and getting rm: invalid option –Try ‘rm –help’ for more information.
In Linux, trying to delete a file its name starts with dash(‘-‘) or double dashes(‘–‘) will fails, as dash or ‘-‘ is interpreted as an option by the rm and most linux commands.
Here are two files with filenames starting with ‘-‘ and ‘–‘ and the typical rm command deletion attempt fails with an error –
$ ls -1
-tempfile1
--tempfile2
$ rm -tempfile1
rm: invalid option -- 't'
Try 'rm ./-tempfile1' to remove the file '-tempfile1'.
Try 'rm --help' for more information.
$ rm --tempfile2
rm: unrecognized option '--tempfile2'
Try 'rm ./--tempfile2' to remove the file '--tempfile2'.
Try 'rm --help' for more information.
There are several ways of addressing this – you can precede the file name with ./ OR pass double dash after rm to end all option processing.
In Unix, everything is a file. In this particular case though we will be demonstrating how to create a text file. Of course, these are some of the many ways of creating a file
touch
Just touch it! – touch command followed by some file name and a file will magically appear.
$ ls linuxfreelancer
ls: cannot access 'linuxfreelancer': No such file or directory
$ touch linuxfreelancer
$ $ ls -l linuxfreelancer
-rw-rw-r-- 1 daniel daniel 0 Feb 8 16:53 linuxfreelancer
vi
vi or any text editor. In fact, any process which writes to a file.
$ vi linuxfreelancer
Save with “:wq” in vi to save the file.
cat
Write multi-line text with “Here Document” syntax in bash.
$ cat <<EOF>linuxfreelancer
my blog is at https://www.linuxfreelancer.com
EOF
$ cat linuxfreelancer
my blog is at https://www.linuxfreelancer.com
$
echo
The echo command with some redirection –
$ echo 'My blog is at https://linuxfreelancer.com' > linuxfreelancer
$ cat linuxfreelancer
My blog is at https://linuxfreelancer.com
Redirection
You can redirect the output of any command to a new file
$ tee linuxfreelancer
Writing for my blog linuxfreelancer.com
Writing for my blog linuxfreelancer.com
...Crl+X
$ cat linuxfreelancer
Writing for my blog linuxfreelancer.com
$
Why does the tee command repeat what I typed? that is what it does – it reads from standard input and write to standard output and file at the same time.
How to exclude files from being added to docker image
TL;DR – use .dockerignore file, Docker’s equivalent of .gitignore for git
When building docker images, minimizing the size of the image is the goal. During building docker images with Dockerfile, especially with in a git repository, we might unintentionally add all the files into the docker image.
It is not uncommon to have something like “ADD . /app” in your Dockerfile. There are two ways to prevent this –
Explicitly add only the files you need to Dockfile
Use .dockerignore file
A typical .dockerignore files in a git repo directory might look like this –
daniel@hidmo:/tmp/myapp$ ls -al
total 56
drwxr-xr-x 4 daniel daniel 4096 Dec 17 17:51 .
drwxrwxrwt 20 root root 4096 Dec 17 17:51 ..
-rwxr-xr-x 1 daniel daniel 338 Dec 17 17:41 build-image.sh
drwxr-xr-x 2 daniel daniel 4096 Dec 17 17:41 .cache
-rw-r--r-- 1 daniel daniel 245 Dec 17 17:41 Dockerfile
-rw-r--r-- 1 daniel daniel 102 Dec 17 17:51 .dockerignore
drwxr-xr-x 8 daniel daniel 4096 Dec 17 17:52 .git
-rw-r--r-- 1 daniel daniel 6 Dec 17 17:51 .gitignore
-rw-r--r-- 1 daniel daniel 133 Dec 17 17:41 README
-rw-r--r-- 1 daniel daniel 181 Dec 17 17:41 requirements.txt
-rw-r--r-- 1 daniel daniel 7871 Dec 17 17:41 web.py
-rw-r--r-- 1 daniel daniel 7871 Dec 17 17:50 web.pyc
daniel@hidmo:/tmp/myapp$ cat .dockerignore
# Exclude files from being added to docker image
.git
.gitignore
.cache
.pyc
Dockerfile
README
readme
Linux – query a specific name server with nslookup or dig
By default, nslookup in Linux will use the name servers configured in /etc/resolv.conf. To check against a specific dns server, add the dns server IP address or name at the end of the nslookup command.
Below is an example to query Cloudflare name server 1.1.1.1 –
daniel@linux:/$ nslookup -type=MX gmail.com 1.1.1.1
Server: 1.1.1.1
Address: 1.1.1.1#53
Non-authoritative answer:
gmail.com mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 5 gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
For dns related debugging though, dig (under “dnsutils” package) is more feature rich. For troubleshooting the “dig +trace” command is handy in spotting failure points. Here is a useful link on how to use dig to troubleshoot dns issues – https://linuxfreelancer.com/troubleshooting-dns-dig-tracing
It provides nicely colored help page, with plenty of examples in a CLI. Here are some sample runs I did.
Curl cheat sheet
daniel@hidmo:/tmp$ curl cheat.sh/curl
# Download a single file
curl http://path.to.the/file
# Download a file and specify a new filename
curl http://example.com/file.zip -o new_file.zip
# Download multiple files
curl -O URLOfFirstFile -O URLOfSecondFile
# Download all sequentially numbered files (1-24)
curl http://example.com/pic[1-24].jpg
# Download a file and follow redirects
curl -L http://example.com/file
# Download a file and pass HTTP Authentication
curl -u username:password URL
# Download a file with a Proxy
curl -x proxysever.server.com:PORT http://addressiwantto.access
# Download a file from FTP
curl -u username:password -O ftp://example.com/pub/file.zip
# Get an FTP directory listing
curl ftp://username:password@example.com
# Resume a previously failed download
curl -C - -o partial_file.zip http://example.com/file.zip
# Fetch only the HTTP headers from a response
curl -I http://example.com
# Fetch your external IP and network info as JSON
curl http://ifconfig.me/all/json
# Limit the rate of a download
curl --limit-rate 1000B -O http://path.to.the/file
# POST to a form
curl -F "name=user" -F "password=test" http://example.com
# POST JSON Data
curl -H "Content-Type: application/json" -X POST -d '{"user":"bob","pass":"123"}' http://example.com
# POST data from the standard in / share data on sprunge.us
curl -F 'sprunge=&lt;-' sprunge.us
Python lists cheat list
daniel@hidmo:/tmp$ curl cheat.sh/python/list
# python - Why does += behave unexpectedly on lists?
#
# The general answer is that += tries to call the __iadd__ special
# method, and if that isn't available it tries to use __add__ instead.
# So the issue is with the difference between these special methods.
#
# The __iadd__ special method is for an in-place addition, that is it
# mutates the object that it acts on. The __add__ special method returns
# a new object and is also used for the standard + operator.
#
# So when the += operator is used on an object which has an __iadd__
# defined the object is modified in place. Otherwise it will instead try
# to use the plain __add__ and return a new object.
#
# That is why for mutable types like lists += changes the object's
# value, whereas for immutable types like tuples, strings and integers a
# new object is returned instead (a += b becomes equivalent to a = a +
# b).
#
# For types that support both __iadd__ and __add__ you therefore have to
# be careful which one you use. a += b will call __iadd__ and mutate a,
# whereas a = a + b will create a new object and assign it to a. They
# are not the same operation!
>>> a1 = a2 = [1, 2]
>>> b1 = b2 = [1, 2]
>>> a1 += [3] # Uses __iadd__, modifies a1 in-place
>>> b1 = b1 + [3] # Uses __add__, creates new list, assigns it to b1
>>> a2
[1, 2, 3] # a1 and a2 are still the same list
>>> b2
[1, 2] # whereas only b1 was changed
# For immutable types (where you don't have an __iadd__) a += b and a =
# a + b are equivalent. This is what lets you use += on immutable types,
# which might seem a strange design decision until you consider that
# otherwise you couldn't use += on immutable types like numbers!
#
# [Scott Griffiths] [so/q/2347265] [cc by-sa 3.0]
Golang concurrency cheat sheet
daniel@hidmo:/tmp$ curl cheat.sh/go/concurrency
/*
* go - When should I use concurrency in golang?
*
* Not an expert in Go (yet) but I'd say:
*
* Whenever it is easiest to do so.
*
* The beauty of the concurrency model in Go is that it is not
* fundamentally a multi-core architecture with checks and balances where
* things usually break - it is a multi-threaded paradigm that not only
* fits well into a multi-core architecture, it also fits well into a
* distributed system architecture.
*
* You do not have to make special arrangements for multiple goroutines
* to work together harmoniously - they just do!
*
* Here's an example of a naturally concurrent algorithm - I want to
* merge multiple channels into one. Once all of the input channels are
* exhausted I want to close the output channel.
*
* It is just simpler to use concurrency - in fact it doesn't even look
* like concurrency - it looks almost procedural.
*/
/*
Multiplex a number of channels into one.
*/
func Mux(channels []chan big.Int) chan big.Int {
// Count down as each channel closes. When hits zero - close ch.
var wg sync.WaitGroup
wg.Add(len(channels))
// The channel to output to.
ch := make(chan big.Int, len(channels))
// Make one go per channel.
for _, c := range channels {
go func(c &lt;-chan big.Int) {
// Pump it.
for x := range c {
ch &lt;- x
}
// It closed.
wg.Done()
}(c)
}
// Close the channel when the pumping is finished.
go func() {
// Wait for everyone to be done.
wg.Wait()
// Close.
close(ch)
}()
return ch
}
/*
* The only concession I have to make to concurrency here is to use a
* sync.WaitGroup as a counter for concurrent counting.
*
* Note that this is not purely my own work - I had a great deal of help
* with this here (https:stackoverflow.com/q/19192377/823393).
*
* [OldCurmudgeon] [so/q/19747950] [cc by-sa 3.0]
*/
curl – use variables to show response times and other parameters
curl is a tool to interact with a server for transferring data. Although it supports various protocols, it is most commonly used with HTTP/S. It is sort of a browser for CLI folks and a go to tool when writing scripts to interact with servers.
It is best to add the variables in a file and use curl to reference the file for better formatting. Here I have added several http request and response variables I am interested in, such as num_connects, size_download, size_header, time_namelookup, time_pretransfer etc.
As far as time related parameters, listed below are the ones you will most likely use –
time_appconnect The time, in seconds, it took from the start until the SSL/SSH/etc connect/handshake to the remote host was completed. (Added in 7.19.0)
time_connect The time, in seconds, it took from the start until the TCP connect to the remote host (or proxy) was completed.
time_namelookup The time, in seconds, it took from the start until the name resolving was completed.
time_pretransfer The time, in seconds, it took from the start until the file transfer was just about to begin. This includes all pre-transfer commands and negotiations that are specific to the particular protocol(s) involved.
time_redirect The time, in seconds, it took for all redirection steps including name lookup, connect, pretransfer and transfer before the final transaction was started. time_redirect shows the complete execution time for multiple redirections. (Added in 7.12.3)
time_starttransfer The time, in seconds, it took from the start until the first byte was just about to be transferred. This includes time_pretransfer and also the time the server needed to calculate the result.
time_total The total time, in seconds, that the full operation lasted.
