Add shell chapters, update functions lesson, handle missing Welcome.md

tutorials/learnshell.org/en/Regular Expressions.md

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+Tutorial
+--------
+Regular Expressions (Regex) are powerful patterns used to search and manipulate text. In Shell, the most common tool for regex is `grep`. This tutorial covers basic syntax.
+
+In general, `grep` has the following syntax:
+
+    grep [options] 'pattern' [file...]
+
+### Some grep options
+
+* `-E` - Extended regex syntax.
+* `-i` - Ignore case distinctions.
+* `-v` - Invert match (select non-matching lines).
+* `-w` - Match whole words only.
+
+### Basic Regex symbols
+
+* `.` - Matches any single character.
+* `[012]` - Matches `0`, `1`, or `2`.
+* `[0-9]` - Matches any character from `0` to `9` inclusive (any digit).
+* `[^0-9]` - Matches any character EXCEPT `0` to `9` (negation).
+* `a|b` - Matches `a` or `b` (requires `grep -E`).
+* `^` - Matches the start of a line.
+* `$` - Matches the end of a line.
+* `*` - Matches ZERO or more of the preceding element.
+* `+` - Matches ONE or more of the preceding element (requires `grep -E`).
+* `{n}` or `{n,m}` - Matches the preceding element exactly `n` times, or between `n` and `m` times (requires `grep -E`).
+
+### Escaping and Extended Regex (`-E`)
+In standard `grep`, meta-characters like `+`, `?`, `|`, `(`, `)`, `{`, `}` are treated as literal text. To use their special powers, you must escape them with a backslash (e.g., `\+`, `\|`). Using `grep -E` allows you to use these symbols directly without escaping. You only need to escape literal special characters (like a real period `\.`).
+
+### Examples
+
+    # Specific word search
+    echo "Find the error here" | grep -w "error"
+
+    # Simplified IPv4 search (matching 1 to 3 digits separated by literal dots)
+    echo "Server IP: 192.168.1.10" | grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}"
+
+    # Using options: filter out lines with "debug" (case-insensitive)
+    echo -e "Info\nDEBUG\nError" | grep -iv "debug"
+
+    # Simplified example: find 'success' or 'failure' followed by a message
+    echo -e "success: operation finished\nfailure: disk full\ninfo: ok" | grep -E "(success|failure): [a-z ]+"
+
+Exercise
+--------
+You are given a server log snippet. Use `grep` to find all `ERROR` entries (even if the word is misspelled with multiple `R`s) that occurred exactly during the 18:xx hour. Make sure your regex only matches valid minutes.
+
+Tutorial Code
+-------------
+    #!/bin/bash
+    log_text="17:45 INFO Server started
+    18:05 ERRRRRROR Failed to connect to DB
+    18:22 INFO User login
+    18:59 EROR Timeout exception
+    18:99 ERRROR Invalid time log
+    19:01 ERROR Disk full" 
+
+    # write your code here
+
+Expected Output
+---------------
+18:05 ERRRRRROR Failed to connect to DB
+18:59 EROR Timeout exception
+
+Solution
+--------
+    #!/bin/bash
+    log_text="17:45 INFO Server started
+    18:05 ERRRRRROR Failed to connect to DB
+    18:22 INFO User login
+    18:59 EROR Timeout exception
+    18:99 ERRROR Invalid time log
+    19:01 ERROR Disk full" 
+
+    # write your code here
+    echo "$log_text" | grep -E "18:[0-5][0-9] ER+OR"

tutorials/learnshell.org/en/Shell Functions.md

@@ -8,6 +8,11 @@ Like other programming languages, the shell may have functions. A function is a
       command...
     }
 
+    # alternative syntax is also possible
+    function_name() {
+      command...
+    }
+
 Functions are called simply by writing their names. A function call is equivalent to a command. Parameters may be passed to a function, by specifying them after the function name. The first parameter is referred to in the function as $1, the second as $2 etc.
 
     function function_B {

tutorials/learnshell.org/en/Special Commands sed,awk,grep,sort.md

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+Tutorial
+--------
+
+Unix-like systems encourage a pipeline-based workflow: use small, specialized tools and connect them with `|` so the output of one command becomes the input of the next.
+
+    command1 | command2 | command3
+
+This tutorial covers the most common text-processing tools used in shell pipelines.
+
+### 1. `grep` — search and filter text
+
+This utility was described in the previous chapter.
+
+### 2. `sort` — order lines
+
+`sort` arranges lines in a specific order.
+
+Useful options:
+
+* `-n` — Numeric sort.
+* `-r` — Reverse order.
+* `-u` — Remove duplicate lines.
+
+Examples:
+
+    printf "10\n2\n7\n" | sort
+    10
+    2
+    7
+
+    printf "10\n2\n7\n" | sort -n
+    2
+    7
+    10
+
+    printf "a\na\nb\n" | sort -u
+    a
+    b
+
+### 3. `awk` — extract and transform columns
+
+`awk` is a small text-processing programming language. It treats each input line as a record and splits it into fields, usually separated by spaces or tabs. Because it is a full language, it supports variables, conditions, loops, arithmetic, and pattern-based actions.
+
+Common fields:
+
+* `$1` — first column
+* `$2` — second column
+* `$0` — whole line
+
+Examples:
+
+    echo "Alice 24" | awk '{print $1}'
+    Alice
+
+    echo "Alice 24" | awk '{print $2, $1}'
+    24 Alice
+
+    echo "Alice 24" | awk '{print $1 ":" $2}'
+    Alice:24
+
+    echo "Alice 24" | awk '$2 >= 18 {print $1, "is an adult"}'
+    Alice is an adult
+
+    echo -e "Alice 24\nBob 16\nCarol 31" | awk '{sum += $2} END {print sum / NR}'
+    23.6667
+
+### 4. `sed` — stream editor
+
+`sed` is commonly used for search-and-replace, deletion, insertion, and line-based text transformations.
+
+General syntax:
+
+    sed [options] 'script' [file...]
+
+A common substitute command looks like this:
+
+    sed 's/old/new/g'
+
+Examples:
+
+    echo "cat cat" | sed 's/cat/dog/'
+    dog cat
+
+    echo "apple apple" | sed 's/apple/orange/g'
+    orange orange
+
+    echo -e "keep\nremovethis\nkeep" | sed '/removethis/d'
+    keep
+    keep
+
+    echo -e "one\ntwo\nthree" | sed -n '2p'
+    two
+
+    echo "abcdc" | sed 'y/abc/xyz/'
+    xyzdz
+
+### 5. `head` and `tail` — limit output
+
+These tools let you inspect only part of a file or stream.
+
+* `head -n 5` — first 5 lines
+* `tail -n 3` — last 3 lines
+
+Examples:
+
+    seq 1 10 | head -n 3
+    1
+    2
+    3
+
+    seq 1 10 | tail -n 3
+    8
+    9
+    10
+
+### 6. `tr` — translate or delete characters
+
+`tr` works on characters rather than lines.
+
+Examples:
+
+    echo "Hello" | tr '[:lower:]' '[:upper:]'
+    HELLO
+
+    echo "a b c" | tr -d ' '
+    abc
+
+### Example pipeline
+
+This pipeline filters apple rows, extracts the numeric column, and sorts it numerically:
+
+    echo "green-apple 5
+    red-apple 3
+    banana 2
+    fuji-apple 8" | grep "apple" | awk '{print $2}' | sort -n
+
+Result:
+
+    3
+    5
+    8
+
+Exercise
+--------
+
+You are given a product list. Use a pipeline to:
+
+1. keep only lines containing `berry`,
+2. extract the price column,
+3. sort the prices from low to high,
+4. print only the most expensive berry price.
+
+Tutorial Code
+-------------
+    #!/bin/bash
+    data="blueberry 5
+    strawberry 3
+    banana 2
+    raspberry 8
+    gooseberry 6
+    apple 10"
+
+    # write your code here
+
+Expected Output
+---------------
+8
+
+Solution
+--------
+    #!/bin/bash
+    data="blueberry 5
+    strawberry 3
+    banana 2
+    raspberry 8
+    gooseberry 6
+    apple 10"
+
+    # write your code here
+    echo "$data" | grep "berry" | awk '{print $2}' | sort -n | tail -n 1

tutorials/learnshell.org/es/Welcome.md

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+# Bienvenido
+
+Bienvenido al tutorial interactivo de Programación Shell de learnshell.org.
+
+Tanto si eres un programador experimentado como si no, este sitio web está pensado para cualquier persona que quiera aprender programación con intérpretes de shell Unix/Linux.
+
+Te invitamos a unirte a nuestro grupo en Facebook para preguntas, discusiones y novedades.
+
+Solo haz clic en el capítulo por el que quieras comenzar y sigue las instrucciones. ¡Buena suerte!
+
+### Aprende lo básico
+
+- [[Hello, World!]]
+- [[Variables]]
+- [[Passing Arguments to the Script]]
+- [[Arrays]]
+- [[Basic Operators]]
+- [[Basic String Operations]]
+- [[Decision Making]]
+- [[Loops]]
+- [[Array-Comparison]]
+- [[Shell Functions]]
+
+### Tutoriales avanzados
+
+- [[Special Variables]]
+- [[Bash trap command]]
+- [[File Testing]]
+- [[Input Parameter Parsing]]
+- [[Pipelines]]
+- [[Process Substitution]]
+- [[Regular Expressions]]
+- [[Special Commands  sed,awk,grep,sort]]
+
+### Tutoriales para contribuir
+
+Lee más aquí: [[Contributing Tutorials]]