Linux CURL Command

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Introduction

Curl command or simply CURL is a command-line utility that allows you to transfer data from or to a server using different protocols such as HTTP, FTP, IMAP, POP3, SMTP, etc. In a shell script, curl can be used to automate the process of fetching data from remote servers and storing it locally, or sending data to a remote server.

Table Of Contents

1. Introduction
2. Syntax
3. Examples
   • An example code snippet that uses curl in a shell script to download a file from a remote server
4. Example to get and use Authentication Token

Syntax

The basic syntax for using curl command in a shell script is as follows:

curl [options] [URL]

Here, [options] are the various command-line options available for curl, and [URL] is the URL of the remote server to fetch or send data to.

Options: Curl has a large number of options available for different use cases. Here are a few commonly used options:

1. -o: Specifies the name of the file to which the data should be saved. For example, curl -o data.txt https://example.com/data.txt will save the contents of https://example.com/data.txt to a file named data.txt.
2. -O: Saves the output with the same name as the remote file. For example, curl -O https://example.com/data.txt will save the contents of https://example.com/data.txt to a file named data.txt.
3. -d: Sends data to the remote server using HTTP POST. For example, curl -d "param1=value1&param2=value2" https://example.com/post will send the data “param1=value1&param2=value2” to https://example.com/post.
4. -H: Sets a header for the request. For example, curl -H "Authorization: Bearer TOKEN" https://example.com/api will set the “Authorization” header to “Bearer TOKEN” for the request.
5. -s: Silent mode. Disables progress and error messages.
6. -u: Sets the username and password for the request. For example, curl -u username:password https://example.com/api will set the username to “username” and the password to “password” for the request.

Examples

1. Downloading a file:

#!/bin/bash

curl -o data.txt https://example.com/data.txt

This script will download the contents of https://example.com/data.txt and save it to a file named data.txt.

2. Uploading a file:

#!/bin/bash

curl -F "file=@/path/to/local/file.txt" https://example.com/upload

This script will upload the file /path/to/local/file.txt to https://example.com/upload.

3. Sending data using HTTP POST:

#!/bin/bash

curl -d "param1=value1&param2=value2" https://example.com/post

This script will send the data “param1=value1&param2=value2” to https://example.com/post using HTTP POST.

An example code snippet that uses curl in a shell script to download a file from a remote server

#!/bin/bash

# Set the URL of the file to download
URL="https://example.com/data.txt"

# Set the output file name
OUTPUT="data.txt"

# Use curl to download the file and save it to the specified output file
curl -o $OUTPUT $URL

# Check if the download was successful
if [ $? -eq 0 ]; then
    echo "Download completed successfully"
else
    echo "Download failed"
fi

In this example, the script sets the URL of the file to download and the name of the output file. It then uses the curl command with the -o option to download the file and save it to the specified output file.

The script then checks the exit status of the curl command using the $? variable. If the exit status is 0, it prints a message indicating that the download was successful. If the exit status is not 0, it prints a message indicating that the download failed.

Example to get and use Authentication Token

A code snippet that uses curl in a shell script to obtain an authentication token from a server and use it in subsequent requests:

#!/bin/bash

# Set the URL of the authentication endpoint
AUTH_URL="https://example.com/auth"

# Set the credentials to use for authentication
USERNAME="user"
PASSWORD="pass"

# Use curl to authenticate and obtain a token
TOKEN=$(curl -s -X POST -H "Content-Type: application/json" -d '{"username": "'"$USERNAME"'", "password": "'"$PASSWORD"'"}' $AUTH_URL | jq -r '.token')

# Check if the token was obtained successfully
if [ -z "$TOKEN" ]; then
    echo "Failed to obtain authentication token"
    exit 1
fi

# Set the URL of the resource to access
RESOURCE_URL="https://example.com/resource"

# Use curl with the token to access the resource
curl -s -H "Authorization: Bearer $TOKEN" $RESOURCE_URL

In this example, the script first sets the URL of the authentication endpoint and the credentials to use for authentication. It then uses curl to send a POST request to the authentication endpoint with the specified credentials, and use jq to extract the token from the JSON response.

The script then checks if the token was obtained successfully, and if so, sets the URL of the resource to access. It then uses curl with the -H option to set the Authorization header to Bearer $TOKEN, and sends a request to access the resource.

Also, see the example code shell-scripting-examples in our GitHub repository. See complete examples in our GitHub repositories.

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