# Bash Quick Reference

*Commands, scripting, pipes, redirection, job control*

> Source: GNU Bash Manual (gnu.org/software/bash/manual) · MIT

## Basics

### echo & Navigation

```
echo "Hello, World!"     # print text
pwd                      # print working directory
cd /path/to/dir          # change directory
cd ..                    # go up one level
cd ~                     # go to home directory
cd -                     # go to previous directory
```

### Listing & Creating

```
ls                # list files
ls -la            # long format, show hidden
ls -lh            # human-readable sizes
mkdir mydir       # create directory
mkdir -p a/b/c    # create nested directories
```

### Copy, Move & Remove

```
cp file.txt copy.txt      # copy file
cp -r dir/ backup/        # copy directory recursively
mv old.txt new.txt        # rename / move
rm file.txt               # delete file
rm -r dir/                # delete directory recursively
rm -rf dir/               # force delete (no prompt)
```

## Variables & Expansion

### Variables

```
name="Alice"              # assign (no spaces!)
echo "$name"              # variable expansion
echo "${name}_file"       # braces for clarity
readonly PI=3.14          # constant
unset name                # delete variable
```

### Special Variables

| Command | Description |
|---------|-------------|
| `$0` | Script name |
| `$1 $2 ...` | Positional arguments |
| `$#` | Number of arguments |
| `$@` | All arguments (separate words) |
| `$*` | All arguments (single string) |
| `$?` | Exit status of last command |
| `$$` | Current process ID |
| `$!` | PID of last background process |

### Command Substitution & Arithmetic

```
files=$(ls)               # capture output
today=$(date +%Y-%m-%d)   # command substitution
count=$((5 + 3))          # arithmetic: 8
echo $((10 / 3))          # integer division: 3
echo $((10 % 3))          # modulo: 1
```

### String Operations

| Command | Description |
|---------|-------------|
| `${#str}` | String length |
| `${str:0:5}` | Substring (offset:length) |
| `${str/old/new}` | Replace first match |
| `${str//old/new}` | Replace all matches |
| `${str^^}` | Uppercase |
| `${str,,}` | Lowercase |

## Conditionals

### if / elif / else

```
if [[ "$name" == "Alice" ]]; then
    echo "Hi Alice"
elif [[ "$name" == "Bob" ]]; then
    echo "Hi Bob"
else
    echo "Who are you?"
fi
```

### Test Operators

| Command | Description |
|---------|-------------|
| `-eq  -ne` | Integer equal / not equal |
| `-lt  -gt` | Integer less / greater than |
| `-le  -ge` | Integer less/greater or equal |
| `==  !=` | String equal / not equal |
| `-z "$str"` | String is empty |
| `-n "$str"` | String is not empty |
| `-f file` | File exists and is regular |
| `-d dir` | Directory exists |
| `-e path` | Path exists (any type) |
| `-r  -w  -x` | Readable / writable / executable |
| `&&  \|\|` | Logical AND / OR |

## Loops

### for Loop

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

for f in *.txt; do
    echo "File: $f"
done
```

### C-style for Loop

```
for ((i=0; i<5; i++)); do
    echo "$i"
done
```

### while Loop

```
count=0
while [[ $count -lt 5 ]]; do
    echo "$count"
    ((count++))
done
```

### Loop Control

| Command | Description |
|---------|-------------|
| `break` | Exit the loop |
| `continue` | Skip to next iteration |

## Functions

### Defining & Calling

```
greet() {
    echo "Hello, $1!"   # $1 = first arg
    return 0             # exit status
}
greet "Alice"            # Hello, Alice!
```

### Local Variables & Return Values

```
add() {
    local sum=$(($1 + $2))
    echo "$sum"          # "return" via stdout
}
result=$(add 3 5)        # capture: 8
```

## Pipes & Redirection

### Pipes

```
ls -l | grep ".txt"      # pipe output
cat log | sort | uniq    # chain commands
cmd1 | tee out.txt       # pipe + save to file
```

### Redirection

| Command | Description |
|---------|-------------|
| `cmd > file` | Redirect stdout (overwrite) |
| `cmd >> file` | Redirect stdout (append) |
| `cmd < file` | Redirect stdin from file |
| `cmd 2> file` | Redirect stderr |
| `cmd 2>&1` | Redirect stderr to stdout |
| `cmd &> file` | Redirect stdout + stderr |
| `cmd << EOF` | Here document (inline input) |
| `/dev/null` | Discard output: `cmd > /dev/null` |

## File Operations

### Viewing Files

```
cat file.txt             # print entire file
head -n 10 file.txt      # first 10 lines
tail -n 10 file.txt      # last 10 lines
tail -f log.txt          # follow (live updates)
less file.txt            # paginated viewer
```

### Counting & Finding

```
wc -l file.txt           # count lines
wc -w file.txt           # count words
wc -c file.txt           # count bytes
find . -name "*.txt"     # find by name
find . -type d           # find directories
find . -mtime -7         # modified in last 7 days
```

### Other File Commands

| Command | Description |
|---------|-------------|
| `touch file` | Create file / update timestamp |
| `stat file` | File metadata (size, dates) |
| `file img.png` | Detect file type |
| `diff a.txt b.txt` | Compare two files |
| `sort file.txt` | Sort lines |
| `uniq` | Remove adjacent duplicates |
| `cut -d: -f1` | Extract fields by delimiter |
| `tr 'a-z' 'A-Z'` | Translate / replace characters |

## Text Processing

### grep

```
grep "error" log.txt       # search for pattern
grep -i "error" log.txt    # case-insensitive
grep -r "TODO" src/        # recursive search
grep -n "func" file.go     # show line numbers
grep -c "error" log.txt    # count matches
grep -v "debug" log.txt    # invert match
```

### sed

```
sed 's/old/new/' file      # replace first per line
sed 's/old/new/g' file     # replace all
sed -i 's/old/new/g' file  # edit in place
sed -n '5,10p' file        # print lines 5-10
sed '/pattern/d' file      # delete matching lines
```

### awk

```
awk '{print $1}' file      # print first field
awk -F: '{print $1}' file  # custom delimiter
awk '$3 > 100' file        # filter by field value
awk '{sum+=$1} END{print sum}' file  # sum column
```

## Permissions

### chmod

```
chmod 755 script.sh    # rwxr-xr-x
chmod +x script.sh     # add execute
chmod -w file.txt      # remove write
chmod u+x,g-w file     # user +exec, group -write
```

### Permission Reference

| Command | Description |
|---------|-------------|
| `r (4)` | Read |
| `w (2)` | Write |
| `x (1)` | Execute |
| `u / g / o` | User / Group / Others |
| `755` | Owner: rwx, Group/Other: r-x |
| `644` | Owner: rw-, Group/Other: r-- |

### Ownership

```
chown user file.txt         # change owner
chown user:group file.txt   # change owner + group
chown -R user:group dir/    # recursive
```