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--- linux [2021/01/16 11:14] – created roger
+++ linux [2023/01/11 10:33] (current) – [Print the first column of a string] roger
@@ Line 3: / Line 3: @@
 Useful Linux commands, with no particular order
-==== Check which program is using a particular port ====
+===== Check which program is using a particular port =====
 <code bash>sudo lsof -i:port</code>
-==== Kill a particular PID ====
+===== Kill a particular PID =====
 <code bash>sudo kill -9 PID</code>
-==== Check the current folder size ====
+===== Check the current folder size =====
 <code bash>du -hs .</code>
-==== Show filesystem information ====
+===== Show filesystem information =====
 <code bash>df -h</code>
+===== SSH =====
+More info: https://www.ssh.com/ssh/tunneling/example/
+==== Copying local keys to a remote server ====
+<code bash>ssh-copy-id user@server</code>
+==== Create a remote tunnel into localhost ====
+<code bash>ssh -L local-port:localhost:remote-port user@server -N</code>
+Note: ''AllowTcpForwarding'' and ''PermitOpen'' have to be enabled in ''/etc/ssh/sshd_config''
+==== Forward a local port to a remote host (like ngrok) ====
+<code bash>ssh -N -T -R local-port:localhost:remote-port user@server</code>
+  * -N      Do not execute a remote command.  This is useful for just forwarding ports.
+  * -T      Disable pseudo-terminal allocation.
+  * -R      Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the local side.
+==== SCP ====
+<code bash>scp -r /folder/to/upload user@server:/destination/on/server</code>
+===== tar & untar =====
+==== tar ====
+<code bash>tar -czvf file.tar.gz file/or/folder/to/tar/</code>
+  * -c create
+  * -z gzip
+  * -v verbose
+  * -f filename
+==== untar ====
+<code bash>tar -xvf file.tar.gz</code>
+  * -x extract
+  * -v verbose
+  * -f filename
+===== gpg encrypt/decrypt =====
+==== Encrypt ====
+<code bash>gpg -c --cipher-algo AES256 file/to/encrypt</code>
+You will be asked to enter a password
+==== Decrypt ====
+<code bash>gpg file/to/decrypt</code>
+You will be asked to enter a password
+===== Download an entire website using wget =====
+More: https://gist.github.com/mikecrittenden/fe02c59fed1aeebd0a9697cf7e9f5c0c
+<code bash>
+wget \
+     --mirror \ # Makes (among other things) the download recursive.
+     --page-requisites \ # Get all assets/elements (CSS/JS/images).
+     --adjust-extension \ # Save files with .html on the end.
+     --span-hosts \ # Include necessary assets from offsite as well.
+     --convert-links \ # Update links to still work in the static version.
+     --restrict-file-names=windows \ # Modify filenames to work in Windows as well.
+     --domains yoursite.com \ # Do not follow links outside this domain.
+     --no-parent \ # Don't follow links outside the directory you pass in.
+         yoursite.com/whatever/path # The URL to download
+</code>
+===== Create a X.509 sha256 self signed certificate =====
+<code bash>
+openssl req \
+  -x509 \
+  -newkey rsa:4096 \
+  -sha256 \
+  -keyout mykeyname.key \
+  -out mycertname.pem \
+  -days 365
+  -nodes # only if you need no password
+</code>
+===== Find =====
+==== Find a specific file in the specified folder ====
+You can use:
+<code bash>
+find mypath -type f -name "myfile.extension"
+</code>
+  * ''mypath'': a path in the OS to perform the search.
+  * ''-type'': type of file to look for. Most common is ''f'', which means "Regular file".
+  * ''-name'': the name of the file you are looking for. You can also use wildcards, for example: ''*.json'' to find all the JSON files in the current directory.
+===== xargs =====
+''xargs'' is used to create new commands from the output of another command. For example, if I'm performing a ''find'' command, I could use ''xargs'' to issue a new command for each line of the output of ''find''.
+<code bash>
+find . -type f -name \"*.json\" | xargs --verbose -I % sh -c 'cat % | jq -c || exit 255'
+</code>
+What's happening?
+  * First, we are performing a [[linux#find|find]] command. The output is going to look like this:
+<code bash>
+./folder/this_is_a_json.json
+./another.json
+</code>
+  * Then, we pipe the output of the find command to ''xargs''. The ''-I %'' means "replace string", so ''xargs'' is going to replace any ''%'' it finds with the value of the current line it is processing.
+  * ''xargs'' is going to run ''sh -c "cat % | jq -c || exit 255"'' (remember its going to replace ''%'' with the value of the current line it is processing) on each of the lines resulting from the ''find'' command. ''jq'' is a program that is used to format JSON. If ''jq'' fails (eg, the JSON is malformed), it's going to return ''exit 255'', to stop the execution of ''xargs''.
+This command in particular is very usefull to check if all the JSON files in a repository are well formated in a CI/CD step.
+===== ncdu =====
+Ncdu is a disk usage analyzer with an ncurses interface. It is designed to find space hogs on a remote server where you don’t have an entire graphical setup available, but it is a useful tool even on regular desktop systems. Ncdu aims to be fast, simple and easy to use, and should be able to run in any minimal POSIX-like environment with ncurses installed.
+Usage:
+<code bash>
+ncdu -x /
+</code>
+Where ''/'' is the filesystem you want to check
+===== awk =====
+==== Print the first column of a string ====
+<code bash>
+echo "This is a string" | awk '{print ($1)}'
+# Output: "This"
+</code>
+==== Print a full string in lowercase or uppercase ====
+<code bash>
+echo "This Is A CaPiTaLiZeD String" | awk '{print tolower($0)}'
+# Output: "this is a capitalized string"
+echo "This Is A CaPiTaLiZeD String" | awk '{print toupper($0)}'
+# Output: "THIS IS A CAPITALIZED STRING"
+</code>