How to Use Unrar with Stdin and Bash Pipes

This article explains how the unrar command-line utility handles standard input (stdin) within Bash scripts. While many Linux command-line tools seamlessly process streamed data through pipes, unrar has specific architectural limitations regarding archive extraction from standard input. Below, you will learn why direct piping fails with archive data, what unrar actually supports via stdin, and the standard workarounds used in Bash automation.

The Limitation: Piping Archive Data

Standard unrar does not support reading RAR archive files directly from stdin. Running a command such as:

cat archive.rar | unrar x -

or attempting to read from the standard input stream directly:

cat archive.rar | unrar x /dev/stdin

will fail.

The RAR container format requires random access (seeking) to navigate headers, metadata, and compressed file tables. Because standard input pipes in Unix are unidirectional, non-seekable streams (FIFOs), unrar cannot rewind or jump to arbitrary byte offsets. Consequently, the utility expects a regular file path on a seekable filesystem.

Process substitution similarly fails for extraction:

# This will fail with an error like "Cannot open /dev/fd/63"
unrar x <(curl -s https://example.com/archive.rar)

What Unrar Can Accept via Stdin

While archive files themselves cannot be piped, unrar does read from stdin for interactive prompts and passwords.

1. Passing Passwords via Stdin

You can pipe a password into unrar to avoid exposing secrets in process tables (ps aux):

echo "SecretPassword123" | unrar x -p archive.rar

The -p flag without an attached password forces unrar to read the password from standard input.

2. Handling Overwrite Prompts

In automated environments, interactive prompts can be suppressed using flags rather than pipes. Use -o+ to overwrite existing files, -o- to skip them, or -y to assume "yes" to all queries:

unrar x -y -p"SecretPassword123" archive.rar

Workarounds for Bash Scripts

When downloading or streaming an archive from an external source (such as curl, wget, or standard output from another utility), you must write the stream to a temporary file before invoking unrar.

Using Temporary Files with mktemp

The standard, POSIX-compliant solution is creating a secure temporary file and cleaning it up on script exit:

#!/usr/bin/env bash
set -euo pipefail

# Create a secure temporary file
TEMP_RAR=$(mktemp --suffix=.rar)

# Ensure the temporary file is deleted when the script exits
trap 'rm -f "$TEMP_RAR"' EXIT

# Stream data into the file
curl -sL "https://example.com/files/backup.rar" > "$TEMP_RAR"

# Extract the archive
unrar x -inul "$TEMP_RAR" /path/to/destination/

Alternative: Using 7-Zip (7z)

If your pipeline strictly requires streaming without intermediate disk writes, the p7zip implementation of 7-Zip (7z) supports extracting certain streamable archives from stdin via the -si flag:

cat archive.rar | 7z x -si -an

However, note that non-solid or multi-volume RAR archives may still encounter stream limitations depending on how the original archive was created. For production Bash scripts, buffering to a temporary file remains the most reliable method for handling RAR archives.