Shell Scripting¶

A shell script is a file containing a series of commands that the shell interprets and executes.

Shell scripts can automate repetitive tasks, manage system operations, or run complex scripts involving multiple commands.
Platform: Linux/Unix, macOS
Interpreter: Various (e.g., sh, bash, zsh, ksh)
File Extension: Usually none, but can use .sh
Syntax: Varies slightly depending on the shell

Basic Syntax¶

Shebang¶

The first line of a shell script often specifies the interpreter using the shebang (#!).
This line tells the system to use the Bash shell to execute the script.
```
#!/bin/bash
```

Comments¶

Comments begin with # and extend to the end of the line. They are ignored by the shell.
```
# This is a comment
```

Variable¶

Variables store data that can be referenced and manipulated.
```
NAME="John Doe"
echo "Hello, $NAME"
```
Use export to make variables available to subprocesses.
```
export NAME="John Doe"
```

Quoting¶

Quoting prevents the shell from interpreting special characters.

Double Quotes ("): Preserve the literal value of all characters except $, ', and \.
```
echo "Hello, $NAME"
```
Single Quotes ('): Preserve the literal value of all characters within the quotes.
```
echo 'Hello, $NAME'
```
Backslash (\): Escapes a single character.
```
echo "Hello, \$NAME"
```

Control Structures¶

Conditional Statements¶

if-elif-else-then-else

if [ "$NAME" = "John" ]; then
    echo "Hello, John"
elif [ "$NAME" = "Doe" ]; then
    echo "Hello, Doe"
else
    echo "You are not John or Doe"
fi

Loops¶

for loop

for i in 1 2 3 4 5; do
    echo "Iteration $i"
done

while Loop

[ $COUNT -le 5 ] uses the -le (less than or equal to) operator to compare COUNT with 5.

COUNT=1
while [ $COUNT -le 5 ]; do
    echo "Iteration $COUNT"
    COUNT=$((COUNT + 1))
done

Functions¶

Basic Definition

my_function() {
    echo "This is a function"
}

my_function

Functions can accept parameters.

greet() {
    echo "Hello, $1"
}

greet "John"

Input and Output¶

Reading Input¶

echo "Enter your name:"
read NAME
echo "Hello, $NAME"

Output to file¶

Redirecting Output

echo "This is a test" > output.txt
cat output.txt

Append to a file

echo "This is another test" >> output.txt

Example¶

Hello World¶

#!/bin/bash
# This is a simple hello world script
echo "Hello, World!"

Backup¶

#!/bin/bash
# Backup script

SOURCE_DIR="/path/to/source"
DEST_DIR="/path/to/backup"

if [ -d "$SOURCE_DIR" ]; then
    cp -r "$SOURCE_DIR" "$DEST_DIR"
    echo "Backup completed successfully."
else
    echo "Source directory does not exist."
fi

Same example used in Batch¶

$HOME is used instead of %USERPROFILE% for the user's home directory.
[ ! -d "$SOURCE_DIR" ] checks if the source directory does not exist.
cp -r "$SOURCE_DIR" "$DEST_DIR" copies the directory recursively.
$? checks the exit status of the last command.

export PATH="$DEST_DIR:$DEST_DIR/Scripts:$PATH" modifies the PATH variable.

#!/bin/bash

# Set source and destination directories
SOURCE_DIR="/c/Program Files/ArcGIS/Pro/bin/Python/envs/arcgispro-py3"
DEST_DIR="$HOME/.local/share/ESRI/conda/envs/arcgispro-py3-addins"

# Check if the source directory exists
if [ ! -d "$SOURCE_DIR" ]; then
    echo "Source directory does not exist: $SOURCE_DIR"
    exit 1
fi

# Copy the source directory to the destination
echo "Copying from $SOURCE_DIR to $DEST_DIR..."
cp -r "$SOURCE_DIR" "$DEST_DIR"
if [ $? -ne 0 ]; then
    echo "Failed to copy the directory."
    exit 1
fi

echo "Directory copied successfully."

# Check if the constructed path exists
if [ -d "$DEST_DIR" ]; then
    # Set environment variables to mimic activation
    export CONDA_DEFAULT_ENV="arcgispro-py3-addins"
    export CONDA_PREFIX="$DEST_DIR"

    # Add the environment's Scripts directory to the PATH
    export PATH="$DEST_DIR:$DEST_DIR/Scripts:$PATH"

    # Installing required libraries from requirements.txt...
    pip install -r requirements.txt
    if [ $? -eq 0 ]; then
        echo "Libraries installed successfully."
    else
        echo "Failed to install libraries."
        exit 1
    fi
else
    echo "Failed to copy the directory."
    exit 1
fi

# Pause the script (similar to 'pause' in batch files)
read -p "Press any key to continue..."

Usage
1. Save the script to a file, for example, script.sh.
2. Make the script: executable: chmod +x script.sh
  - The chmod +x command is used to change the permissions of a file to make it executable. The +x option adds execute permissions to the file.
3. Run the script: ./script.sh