docs: update notes

Author: rsapkf

README.md

@@ -4,7 +4,7 @@
 
 [![Netlify Status](https://api.netlify.com/api/v1/badges/8e7bba5d-546f-4672-9b3b-a6efbecf7fa4/deploy-status)](https://app.netlify.com/sites/rsapkf-notes/deploys)
 
-These are companion notes to my [dotfiles](https://github.com/rsapkf/config) and [links](https://github.com/rsapkf/42/) where I keep short snippets of code, shell scripts, tricks and tips to remember stuff.
+These are companion notes to my [dotfiles](https://github.com/rsapkf/config) and [links](https://github.com/rsapkf/42/) where I keep short snippets of code, shell scripts, tricks and tips related to common Linux utilities/concepts, mathematical topics, Martin Gardner's mathemagic tricks, poems that I like, notes from books I've read, etc.
 
 Notes are in the [`docs/`](https://github.com/rsapkf/73/tree/main/docs) directory.
 

docs/linux/awk.md

@@ -47,15 +47,14 @@ awk -F: '{print $1;}' /etc/passwd # Equivalent to above
 awk 'BEGIN { FS=":"; print "User\t\tUID\t\tGID\t\tHome\t\tShell\n--------------"; }
 {print $1,"\t\t",$3,"\t\t",$4,"\t\t",$6,"\t\t",$7;}
 END { print "---------\nFile Complete" }' /etc/passwd
-
 ```
 
 ## Field searching
 
 ```shell
 awk '$2 ~ /^bin/' <file> # Print lines where the second field starts with `bin`
-awk '$2 !~ /^sa/' <file> # The second field does _not_ start with `bin`
-awk '$2 !~ /^sa/ && $1 < 5' <file> # The second field does not start with `bin` _and_ the value of the first column is less than 5
+awk '$2 !~ /^bin/' <file> # The second field does _not_ start with `bin`
+awk '$2 !~ /^bin/ && $1 < 5' <file> # The second field does not start with `bin` _and_ the value of the first column is less than 5
 ```
 
 ## Scripting

docs/linux/curl.md

@@ -15,8 +15,9 @@ curl -L https://github.com/robots.txt # Follow redirects
 curl https://api.github.com/repos/3b1b/manim # Basic GET request to the GitHub API
 curl -i https://github.com/robots.txt # Include HTTP-header in the output
 curl -I https://github.com/robots.txt # Fetch HTTP-header only
+curl -C - -O http://example.org/archive.tar.gz # Resume interrrupted download
 
-# -u, --user: Specify the user name and password to use for server authentication
+# -u, --user: Specify the user name and password for server authentication
 curl -u <username:password> https://api.example.com/endpoint # Basic authentication
 curl -u <username> https://api.example.com/endpoint # Prompt for password; Avoid having password in history
 
@@ -34,18 +35,18 @@ curl -d @data.json <URL> # Read data to POST from file
 
 # -H, --header: Specify headers
 curl -H "Content-Type: application/json" -H "Authorization: <token>" -d @data.json <URL>
-# Can be used multiple times to add/replace/remove multiple headers.
+# Can be used multiple times to add/replace/remove multiple headers
 
 # -b, --cookie: Pass the data to the HTTP server as a cookie
 curl -b "name=Fry" <URL>
 curl --dump-header <file> <URL> # Save cookies received from the server to file
 curl -b <file> <URL> # Use cookies from file
-curl -c <file> <URL> # Save the incoming cookies using netscape cookie format
+curl -c <file> <URL> # Save the incoming cookies using Netscape cookie format
 
 # -x, --proxy: Use the specified HTTP proxy and port number (default: 1080)
 curl -u <username:password> -x <proxyhost[:port]> <URL>
 
-# -U, --proxy-user: Specify the user name and password to use for proxy authentication
+# -U, --proxy-user: Specify the user name and password for proxy authentication
 curl -U <username:password> -x <proxyhost[:port]> <URL>
 
 # -v, --verbose: Make the fetching more verbose/talkative

docs/linux/find.md

@@ -19,33 +19,31 @@ find \! -name "query_to_avoid"
 ```shell
 find -type <type_descriptor> <query> # Basic usage
 # Common type descriptors:
-# f: file
-# d: directory
-# l: symlink
-# c: character device
-# b: block device
+# f: File
+# d: Directory
+# l: Symlink
+# c: Character device
+# b: Block device
 
 find /usr -type f -name "*.conf" # Search for files ending in `.conf` in `/usr` directory
 find /usr -type f -and -name "*.conf" # Equivalent to above; `-and` combines two queries
 find -name query_1 -or -name query_2 # `-or` returns results that match either expression
-
 ```
 
 ### Find by size
 
 ```shell
 find -size <number><suffix> # Basic usage
 # Common suffixes
-# c: bytes
-# k: kilobytes
-# M: megabytes
-# G: gigabytes
+# c: Bytes
+# k: Kilobytes
+# M: Megabytes
+# G: Gigabytes
 # b: 512-byte blocks
 
 find /usr -size 50c # Find all files in `/usr` exactly 50 bytes in size
 find /usr -size -50c # Less than 50 bytes in size
 find /usr -size +600M # More than 600 megabytes in size
-
 ```
 
 ### Find by time
@@ -80,7 +78,7 @@ find / -perm 644
 find / -perm -644 # Files with at least these permisions
 ```
 
-### Filter by depth
+### Find by depth
 
 ```shell
 find -mindepth <num>
@@ -89,9 +87,25 @@ find -maxdepth <num>
 
 ### Execute commands on `find` results
 
+Other commands can be executed on results returned by `find` using the `-exec`/`-delete` options or by piping the output to the [`xargs`](./xargs) command or [GNU Parallel](<https://en.wikipedia.org/wiki/Parallel_(software)>).
+
 ```shell
 find <find_parameters> -exec <command_and_options> {} \;
 find . -type d -perm 755 -exec chmod 700 {} \; # Change directory permissions from 755 to 700
+find . -name "*.json" -delete # Delete files ending in `.json`
+
+# Useful options
+# -exec command {} +: Build command by appending each selected file name at the end, and then execute it
+# -execdir: Run command from the subdirectory containing the matched file
+
+find ./docs -type f -print | xargs rm
+# Find all files in `./docs` and remove them
+
+find . -name "*.foo" -print0 | xargs -0 grep bar
+# Use the null character to delimit file names (necessary for dealing with filenames with `,` or space)
+
+find . -name "*.foo" -print0 | parallel -0 grep bar
+# Equivalent to above
 ```
 
 ## `locate`
@@ -110,3 +124,118 @@ locate -i readme.md # Case-insensitive search
 locate -c *.md # Display number of found entries
 locate -S # Print statistics about each used database
 ```
+
+## `whereis`
+
+`whereis` command can be used to efficiently locate the binary, source, and manual page files for a command.
+
+```shell
+> whereis rg
+rg: /usr/bin/rg /usr/share/man/man1/rg.1.gz
+```
+
+## `which`
+
+`which` searches for the binary for a command in your `PATH`.
+
+```shell
+> which rg
+/usr/bin/rg
+```
+
+## `fd`
+
+### Basic usage
+
+```shell
+fd [options] <pattern> <path> # Basic usage
+fd # List all files in current directory recursively, similar to `ls -R`
+
+fd netfl # Basic search; Recursively search current directory for the pattern `netfl`
+fd '^x.*rc$' # Regex search; Search for entries that start with `x` and end with `rc`
+fd '^x.*rc$' /etc # Search in `/etc` directory
+fd -g libc.so /usr # Glob-based search; Find all `libc.so` files in `usr`
+
+# Useful options
+# -H, --hidden: Search in hidden directories
+# -I, --no-ignore: Search directories and show files that match `.gitignore` patterns
+# -p, --full-path: Search in full paths instead of just filenames
+# -l, --list-details: Use a long listing format with file metadata
+# -L, --follow: Follow symbolic links
+```
+
+### Find by type
+
+```shell
+fd -t <filetype> <pattern>
+# Common filetypes:
+# f: File
+# d: Directory
+# l: Symlink
+# x: Executable
+# e: Empty
+# s: Socket
+# p: Pipe
+```
+
+### Find by size
+
+```shell
+fd -s <size> <pattern>
+```
+
+### Find by time
+
+```shell
+fd --changed-within <date|dur> <pattern> # Filter by file modification time (newer than)
+fd --changed-before <date|dur> <pattern> # Filter by file modification time (older than)
+```
+
+### Find by depth
+
+```shell
+fd -d <depth> <pattern> # Set maximum search depth
+```
+
+### Find by extension
+
+```shell
+fd -e json # Find files with `.json` extension
+fd -e json <pattern> # Find json files that contain the pattern
+```
+
+### Exclude files and directories
+
+```shell
+fd -E '*.zip' <pattern> # Exclude zip files
+fd -H -E .git <pattern> # Search in hidden dirs but exclude matches from `.git`
+# You can create a `.fdignore` file (similar to `.gitignore`) to make exclude-patterns permanent.
+```
+
+### Execute commands on `fd` results
+
+```shell
+# -x, --exec <cmd>: Execute a command for each search result
+# -X, --exec-batch <cmd>: Execute a command with all search results at once
+
+fd -e zip -x unzip # Recursively find and unzip all zip archives
+fd -e h -e cpp -x clang-format -i # Find all `*.h` and `*.cpp` files and auto-format them inplace with `clang-format -i`
+
+fd -g 'test_*.py' -X vim # Find all `test_*.py` files and open them with `vim`
+# https://vimhelp.org/usr_07.txt.html#07.2
+
+fd -H '^\.DS_Store$' -tf -X rm # Recursively remove all `.DS_Store` files
+```
+
+#### Placeholder tokens
+
+```shell
+fd -e jpg -x convert {} {.}.png # Convert all `*.jpg` files to `*.png` files
+# `{}` will be replaced by the path of the search result (`docs/images/party.jpg`)
+
+# Other placeholder tokens
+# {.}: Like {}, but without the file extension (`docs/images/party`)
+# {/}: The basename of the search result (`party.jpg`)
+# {//}: The parent of the discovered path (`docs/images`)
+# {/.}: The basename without the extension (`party`)
+```

docs/linux/fzf.md

@@ -0,0 +1,50 @@
+---
+id: fzf
+title: fzf
+---
+
+`fzf` is a command line fuzzy finder. It can search through the entries in a list of files, command history, processes, hostnames, bookmarks, git commits, etc.
+
+`fzf` provides 3 keyboard shortcuts:
+
+- **`CTRL-T`** to paste the selected **files and directories** onto the command-line.
+
+  By default, `fzf` uses `find` to search for files. This can be configured setting `FZF_CTRL_T_COMMAND` in `.bashrc`/`,zshrc`:
+
+  ```shell title="~/.zshrc"
+  # Set fd as the default source for fzf
+  export FZF_DEFAULT_COMMAND='fd -type f --follow --hidden --exclude .git'
+
+  # Apply the command to CTRL-T as well
+  export FZF_CTRL_T_COMMAND="$FZF_DEFAULT_COMMAND"
+
+  # Preview contents of files
+  export FZF_CTRL_T_OPTS="--preview '(highlight -O ansi -l {} 2> /dev/null || cat {} || tree -C {}) 2> /dev/null | head -200'"
+
+  # Set color scheme to Dracula
+  export FZF_DEFAULT_OPTS=$FZF_DEFAULT_OPTS'
+  --color=dark
+  --color=fg:-1,bg:-1,hl:#5fff87,fg+:-1,bg+:-1,hl+:#ffaf5f
+  --color=info:#af87ff,prompt:#5fff87,pointer:#ff87d7,marker:#ff87d7,spinner:#ff87d7
+  '
+  ```
+
+- **`CTRL-R`** to paste the selected **command from history** onto the command-line.
+
+  Pressing `CTRL_R` again will show the commands in chronological order.
+
+- **`ALT-C`** to **cd** into the selected directory.
+
+  ```shell title="~/.zshrc"
+  # Use fd here as well
+  export FZF_ALT_C_COMMAND="fd -t d"
+
+  # Preview files
+  export FZF_ALT_C_OPTS="--preview 'tree -C {} | head -200'"
+  ```
+
+To start the finder in a tmux split pane:
+
+```shell title="~/.bashrc"
+export FZF_TMUX=1
+```

docs/linux/git.md

@@ -25,11 +25,11 @@ git commit --amend -m "New commit message" # Edit the last commit message
 - Get a list of the deleted files in history
 
   ```shell
-    ## without commit hashes
-    git log --diff-filter=D --summary | grep delete
+  ## without commit hashes
+  git log --diff-filter=D --summary | grep delete
 
-    ## with commit hashes
-    git log --diff-filter=D --summary | grep -E 'delete|^commit\s+\S+'
+  ## with commit hashes
+  git log --diff-filter=D --summary | grep -E 'delete|^commit\s+\S+'
   ```
 
 - Amend older or multiple commit messages

docs/linux/grep.md

@@ -30,7 +30,7 @@ grep "^[[:upper:]]" <file> # Equivalent to above; Using POSIX classes
 
 ### Extended regular expressions
 
-Along with basic regular expressions (BREs, which is the default), `grep` also supports [extended regular expressions (EREs)](https://en.wikipedia.org/wiki/Regular_expression#Standards) by using the `-E` flag or by calling the `egrep` command.
+Along with basic regular expressions (BREs, which is the default), `grep` also supports [extended regular expressions (EREs)](https://en.wikipedia.org/wiki/Regular_expression#Standards) by using the `-E` option or by calling the `egrep` command.
 
 ```shell
 grep "\(group\)" <file> # Grouping