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Bash

Bash syntax is vast and flexible, so let's break it down into fundamental components and dig deep into each.

1. Shebang (#!)

A script starts with a shebang to specify the interpreter:

# !/bin/bash
  • #! tells the system which interpreter to use.
  • /bin/bash is the path to the Bash shell.

Alternate shebangs:

  • #!/usr/bin/env bash (More portable since it searches for bash in $PATH)
  • #!/bin/sh (POSIX shell; may be linked to dash on some systems)

2. Variables & Expansion

Bash allows variables that can hold strings, numbers, or command outputs.

2.1 Assigning Variables

name="Alice"
age=25
  • No spaces around =
  • Strings don’t need quotes unless they contain spaces

2.2 Referencing Variables

echo "Hello, $name!"
echo "Next year you will be $((age + 1))"
  • $var expands the variable
  • $((expression)) does arithmetic expansion

2.3 Braces {} in Variable Expansion

greeting="Hello"
echo "${greeting}, World!"
  • {} prevents ambiguity (${greeting}World instead of $greetingWorld)

2.4 Command Substitution

date_today=$(date)
echo "Today is: $date_today"
  • $(command) executes and replaces output
  • Legacy: `command` (backticks) but harder to nest

3. Quoting & Escaping

Bash treats quotes differently:

Quote Type Effect
" " Expands variables but keeps spaces
' ' Literal text, no expansion
\ Escapes a single character

Example:

name="Alice"
echo "Hello, $name"  # Expands to "Hello, Alice"
echo 'Hello, $name'  # Prints "Hello, $name"
echo "File path: /home/\$USER"  # Escape `$` to prevent expansion

4. Arrays & Associative Arrays

Bash supports indexed arrays and associative arrays.

4.1 Indexed Arrays

fruits=("apple" "banana" "cherry")
echo "${fruits[0]}"  # apple
echo "${fruits[@]}"  # all elements
echo "${fruits[@]}" # length
  • [@] expands all elements
  • # before array gives the count

4.2 Associative Arrays (Bash 4+)

declare -A capitals
capitals["France"]="Paris"
capitals["Germany"]="Berlin"

echo "${capitals["France"]}"  # Paris

5. Conditionals (if, test and [ ])

5.1 Basic If-Else

if [ "$name" == "Alice" ]; then
    echo "Hello, Alice!"
else
    echo "Who are you?"
fi
  • [ "$name" == "Alice" ] uses test
  • [[ "$name" == "Alice" ]] is preferred (more features)
  • then must be on a new line or after ;

5.2 Numeric Comparisons

if [ $age -gt 18 ]; then
    echo "You are an adult."
fi
  • -eq (equal), -ne (not equal)
  • -gt (greater than), -lt (less than)

5.3 File Checks

if [ -f "/etc/passwd" ]; then echo "File exists"; fi
if [ -d "/home/user" ]; then echo "Directory exists"; fi
  • -f (file exists)
  • -d (directory exists)
  • -e (any file exists)

6. Loops (for, while, until)

6.1 For Loop

for fruit in apple banana cherry; do
    echo "I like $fruit"
done

or using a numeric range:

for i in {1..5}; do
    echo "Number $i"
done

or using seq:

for i in $(seq 1 5); do echo $i; done

6.2 While Loop

counter=1
while [ $counter -le 5 ]; do
    echo "Count: $counter"
    ((counter++))  # Arithmetic expansion
done

6.3 Until Loop (opposite of while)

num=1
until [ $num -gt 5 ]; do
    echo "Number: $num"
    ((num++))
done

7. Functions

greet() {
    echo "Hello, $1!"
}
greet "Alice"
  • $1, $2, etc. are positional parameters

Returning values:

add() {
    echo $(( $1 + $2 ))
}
result=$(add 3 5)
echo "Sum: $result"

8. Special Variables

Variable Description
$0 Script name
$1, $2, ... Positional arguments
$@ All arguments (as separate words)
$* All arguments (as a single word)
$# Number of arguments
$$ PID of the script
$? Exit status of the last command
$! PID of the last background process

Example:

echo "Script name: $0"
echo "First argument: $1"
echo "All args: $@"

9. Redirection & Pipes

9.1 Standard Streams

  • 0 = stdin
  • 1 = stdout
  • 2 = stderr

Redirect output:

ls > files.txt   # Save stdout to file
ls >> files.txt  # Append stdout to file
ls 2> errors.txt # Save stderr to file
ls 2>&1          # Merge stderr into stdout

Deep Dive

In Bash, standard streams are the primary way processes handle input and output. There are three standard streams, each with an associated file descriptor (FD):

Stream File Descriptor (FD) Purpose
Standard Input 0 Input to a program (usually from the keyboard)
Standard Output 1 Normal output from a program (displayed on the terminal by default)
Standard Error 2 Error messages from a program (also displayed on the terminal by default)

1. Standard Input (stdin) – FD 0

  • Receives input for a program (default: keyboard)
  • Can be redirected from a file or piped from another command.

Example 1: Using stdin from a file

cat < myfile.txt

Here, < redirects myfile.txt as input to cat.

Example 2: Using stdin with read

read name
echo "Hello, $name!"
  • This waits for user input and stores it in $name.

Example 3: Using << (Heredoc) to feed input

cat << EOF
This is
multiline input.
EOF

2. Standard Output (stdout) – FD 1

  • Used for normal program output (default: terminal)
  • Can be redirected to a file or another command.

Example 1: Redirect stdout to a file (>)

ls > output.txt
  • Saves ls output to output.txt, overwriting it.

Example 2: Append stdout to a file (>>)

ls >> output.txt
  • Appends ls output to output.txt instead of overwriting.

Example 3: Pipe stdout to another command (|)

ls | grep ".txt"
  • Sends the output of ls to grep, filtering for .txt files.

3. Standard Error (stderr) – FD 2

  • Used for error messages (default: terminal)
  • Can be redirected separately from stdout.

Example 1: Redirect stderr to a file (2>)

ls non_existent_file 2> errors.log
  • Captures error messages in errors.log.

Example 2: Redirect stderr and stdout to different files

ls file.txt non_existent_file > output.txt 2> errors.txt
  • Normal output goes to output.txt, errors go to errors.txt.

Example 3: Redirect stderr and stdout to the same file (&> or 2>&1)

ls file.txt non_existent_file &> all_output.txt
# Equivalent to:
ls file.txt non_existent_file > all_output.txt 2>&1
  • Merges stdout and stderr into all_output.txt.

4. Combining and Suppressing Output

Discarding output (send to /dev/null)

ls non_existent_file 2> /dev/null  # Hide errors
ls file.txt > /dev/null 2>&1       # Hide all output

Using stderr in a pipeline

find / -name "myfile" 2>/dev/null | grep "myfile"
  • Suppresses errors and only passes valid results to grep.

Summary of Redirection Operators

Operator Purpose
> Redirect stdout (overwrite)
>> Redirect stdout (append)
2> Redirect stderr (overwrite)
2>> Redirect stderr (append)
&> or > file 2>&1 Redirect both stdout and stderr
< Redirect stdin
<< Heredoc (inline multi-line input)
|
/dev/null Discard output

Would you like a deeper dive into any specific part?

9.2 Piping

ls | grep "txt"
  • | passes stdout of ls to grep

10. Process Management

Run commands in the background:

sleep 60 &
echo "Background process PID: $!"
  • jobs → Lists background jobs
  • fg %1 → Brings job 1 to foreground
  • kill 1234 → Terminates process with PID 1234

11. Brace Expansion & Globbing

11.1 Brace Expansion

echo {A,B,C}      # A B C
echo {1..5}       # 1 2 3 4 5
echo file{1..3}.txt  # file1.txt file2.txt file3.txt

11.2 Globbing (Wildcards)

Pattern Meaning
* Matches anything (file* matches file1, file2)
? Matches a single character (file?.txt matches file1.txt but not file10.txt)
[abc] Matches a, b, or c

Example:

ls *.txt
ls file?.txt
ls file[1-3].txt

Final Notes

  • Bash has built-in commands (echo, cd, pwd, test) and external commands (ls, grep, awk).
  • Use man bash or help for built-in documentation.
  • Debug scripts with bash -x script.sh.